Common LUT Format (CLF)  A Common File Format for LookUp Tables¶
Introduction¶
LookUp Tables (LUTs) are a common implementation for transformations from one set of color values to another. With a large number of product developers providing software and hardware solutions for LUTs, there is an explosion of unique vendorspecific LUT file formats, which are often only trivially different from each other. This can create workflow problems when a LUT being used on a production is not supported by one or more of the applications being used. Furthermore, many LUT formats are designed for a particular use case only and lack the quality, flexibility, and metadata needed to meet modern requirements.
The Common LUT Format (CLF) can communicate an arbitrary chain of color operators (also called processing nodes) which are sequentially processed to achieve an end result. The set of available operator types includes matrices, 1D LUTs, 3D LUTs, ASCCDL, log and exponential shaper functions, and more. Even when 1D or 3D LUTs are not present, CLF can be used to encapsulate any supported color transforms as a text file conforming to the XML schema.
Scope¶
This document introduces a humanreadable text file format for the interchange of color transformations using an XML schema. The XML format supports LookUp Tables of several types: 1D LUTs, 3D LUTs, and 3×1D LUTs, as well as additional transformation needs such as matrices, range rescaling, and “shaper LUTs.” The document defines what is a valid CLF file. Though it is not intended as a tutorial for users to create their own files, LUT creators will find it useful to understand the elements and attributes available for use in a CLF file. The document is also not intended to provide guidance to implementors on how to optimize their implementations, but does provide a few notes on the subject. This document assumes the reader has knowledge of basic color transformation operators and XML.
References¶
The following standards, specifications, articles, presentations, and texts are referenced in this text:
 IETF RFC 3066: IETF (Internet Engineering Task Force). RFC 3066: Tags for the Identification of Lan guages, ed. H. Alvestrand. 2001 IEEE DRAFT Standard P123
 Academy S2014002, Academy Color Encoding System – Versioning System
 Academy TB2014002, Academy Color Encoding System Version 1.0 User Experience Guidelines
 ASC Color Decision List (ASC CDL) Transfer Functions and Interchange Syntax. ASCCDL Release1.2. Joshua Pines and David Reisner. 20090504.
Specification¶
General¶
A Common LUT Format (CLF) file shall be written using Extensible Markup Language (XML) and adhere to a defined XML structure. A CLF file shall have the file extension '.clf
'.
The top level element in a CLF file defines a ProcessList
which represents a sequential set of color transformations. The result of each individual color transformation feeds into the next transform in the list to create a daisy chain of transforms.
An application reads a CLF file and initializes a transform engine to perform the operations in the list. The transform engine reads as input a stream of code values of pixels, performs the calculations and/or interpolations, and writes an output stream representing a new set of code values for the pixels.
In the sequence of transformations described by a ProcessList
, each ProcessNode
performs a transform on a stream of pixel data, and only one input line (input pixel values) may enter a node and only one output line (output pixel values) may exit a node. A ProcessList
may be defined to work on either 1 component or 3component pixel data, however all transforms in the list must be appropriate, especially in the 1component case (blackandwhite) where only 1D LUT operations are allowed. Implementation may process 1component transforms by applying the same processing to R, G, and B.
The file format does not provide a mechanism to assign color transforms to either image sequences or image regions. However, the XML structure defining the LUT transform, a ProcessList, may be encapsulated in a larger XML structure potentially providing that mechanism. This mechanism is beyond the scope of this document.
Each CLF file shall be completely selfcontained requiring no external information or metadata. The full content of a color transform must be included in each file and a color transform may not be incorporated by reference to another CLF file. This restriction ensures that each CLF file can be an independent archival element.
Each ProcessList shall be given a unique ID for reference.
The data for LUTs shall be an ordered array that is either all floats or all integers. When three RGB color components are present, it is assumed that these are red, green, and blue in that order. There is only one order for how the data array elements are specified in a LUT, which is in general from black to white (from the minimum input value position to the maximum input value position). Arbitrary ordering of list elements is not supported in the format (see XML Elements for details).
Note
For 3D LUTs, the indexes to the cube are assumed to have regular spacing across the range of input values. To accommodate irregular spacing, a "halfDomain
" 1D LUT or Log node should be used as a shaper function prior to the 3D LUT.
XML Structure¶
General¶
A CLF file shall contain a single occurrence of the XML root element known as the ProcessList. The ProcessList element shall contain one or more elements known as ProcessNodes. The order and number of process nodes is determined by the designer of the CLF file.
An example of the overall structure of a simple CLF file is thus:
<ProcessList id="123">
<Matrix id="1">
data & metadata
</Matrix>
<LUT1D id="2">
data & metadata
</LUT1D>
<Matrix id="3">
data & metadata
</Matrix>
</ProcessList>
XML Version and Encoding¶
A CLF file shall include a starting line that declares XML version number and character encoding. This line is mandatory once in a file and looks like this:
<?xml version="1.0" encoding="UTF8"?>
Comments¶
The file may also contain XML comments that may be used to describe the structure of the file or save information that would not normally be exposed to a database or to a user. XML comments are enclosed in brackets like so:
<! This is a comment >
Language¶
It is often useful to identify the natural or formal language in which text strings of XML documents are written. The special attribute named xml:lang may be inserted in XML documents to specify the language used in the contents and attribute values of any element in an XML document. The values of the attribute are language identifiers as defined by IETF RFC 3066. In addition, the empty string may be specified. The language specified by xml:lang applies to the element where it is specified (including the values of its attributes), and to all elements in its content unless overridden with another instance of xml:lang. In particular, the empty value of xml:lang can be used to override a specification of xml:lang on an enclosing element, without specifying another language.
White Space¶
Particularly when creating CLF files containing certain elements (such as Array
, LUT1D
, or LUT3D
) it is desirable that single lines per entry are maintained so that file contents can be scanned more easily by a human reader. There exist some difficulties with maintaining this behavior as XML has some nonspecific methods for handling whitespace. Especially if files are rewritten from an XML parser, white space will not necessarily be maintained. To maintain line layout, XML style sheets may be used for reviewing and checking the CLF file’s entries.
Newline Control Characters¶
Different end of line conventions, including <CR>
, <LF>
, and <CRLF>
, are utilized between Mac, Unix, and Windows systems. Different newline characters may result in the collapse of values into one long line of text. To maintain intended linebreaks, CLF specifies that the ‘newline’ string (i.e. the byte(s) to be interpreted as ending each line of text) shall be the single code value \(10_{10} = 0\textrm{A}_{16}\) (ASCII ‘Line Feed’ character), also indicated <LF>
.
Note
Parsers of CLF files may choose to interpret Microsoft’s <CR><LF>
or olderMacOS’ <CR>
newline conventions, but CLF files should only be generated with the <LF>
encoding.
Note
<LF>
is the newline convention native to all *nix operating systems (including Linux and modern macOS).
XML Elements¶
ProcessList¶
Description:
The ProcessList
is the root element for any CLF file and is composed of one or more ProcessNodes. A ProcessList
is required even if only one ProcessNode
will be present.
Note
The last node of the ProcessList
is expected to be the final output of the LUT. A LUT designer can allow floatingpoint values to be interpreted by applications and thus delay control of the final encoding through user selections.
Note
If needed, a Range
node can be placed at the end of a ProcessList
to control minimum and maximum output values and clamping.
 Attributes:
id
(required) a string to serve as a unique identifier of the
ProcessList
compCLFversion
(required) a string indicating the minimum compatible CLF specification version required to read this file
ThecompCLFversion
corresponding to this version of the specification is be"3.0"
. name
(optional) a concise string used as a text name of the
ProcessList
for display or selection from an application’s user interface inverseOf
(optional) a string for linking to another ProcessList
id
(unique) which is the inverse of this one  Elements:
Description
(optional) a string for comments describing the function, usage, or any notes about the
ProcessList
. AProcessList
can contain zero or moreDescription
elements. InputDescriptor
(optional) an arbitrary string used to describe the intended source code values of the
ProcessList
OutputDescriptor
(optional) an arbitrary string used to describe the intended output target of the
ProcessList
(e.g. target display) ProcessNode
(required) a generic XML element that in practice is substituted with a particular color operator. The
ProcessList
must contain at least oneProcessNode
. TheProcessNode
is described in ProcessNode. Info
(optional) optional element for including additional custom metadata not needed to interpret the transforms. The
Info
element includes:
AppRelease
(optional) a string used for indicating application software release level
Copyright
(optional) a string containing a copyright notice for authorship of the CLF file
Revision
(optional) a string used to track the version of the LUT itself (e.g. an increased resolution from a previous version of the LUT)
ACEStransformID
(optional) a string containing an ACES transform identifier as described in Academy S2014002. If the transform described by the
ProcessList
is the concatenation of several ACES transforms, this element may contain several ACES Transform IDs, separated by white space or line separators. This element is mandatory for ACES transforms and may be referenced from ACES Metadata Files. ACESuserName
(optional) a string containing the userfriendly name recommended for use in product user interfaces as described in Academy TB2014002
CalibrationInfo
(optional) container element for calibration metadata used when making a LUT for a specific device.
CalibrationInfo
can contain the following child elements:DisplayDeviceSerialNum
DisplayDeviceHostName
OperatorName
CalibrationDateTime
MeasurementProbe
CalibrationSoftwareName
CalibrationSoftwareVersion
ProcessNode¶
Description:
A ProcessNode
element represents an operation to be applied to the image data. At least one ProcessNode
element must be included in a ProcessList
. The generic ProcessNode
element contains attributes and elements that are common to and inherited by the specific subtypes of the ProcessNode
element that can substitute for ProcessNode
. All ProcessNode
substitutes shall inherit the following attributes.
 Attributes:
id
(optional) a unique identifier for the
ProcessNode
name
(optional) a concise string defining a name for the
ProcessNode
that can be used by an application for display in a user interface inBitDepth
(required) a string that is used by some ProcessNodes to indicate how array or parameter values have been scaled
outBitDepth
(required) a string that is used by some ProcessNodes to indicate how array or parameter values have been scaled The supported values for both
inBitDepth
andoutBitDepth
are the same:
"8i"
: 8bit unsigned integer"10i"
: 10bit unsigned integer"12i"
: 12bit unsigned integer"16i"
: 16bit unsigned integer"16f"
: 16bit floating point (halffloat)"32f"
: 32bit floating point (single precision)
 Elements:
Description
(optional) an arbitrary string for describing the function, usage, or notes about the ProcessNode. A ProcessNode can contain one or more Descriptions.
Array¶
Description:
The Array
element contains a table of entries with a single line for each grouping of values. This element is used in the LUT1D
, LUT3D
, and Matrix
ProcessNodes. The dim
attribute specifies the dimensions of the array and, depending on context, defines the size of a matrix or the length of a LUT table. The specific formatting of the dim
attribute must match with the type of node in which it is being used. The usages are summarized below but specific requirements for each application of Array
are described when it appears as a child element for a particular ProcessNode
.
 Attributes:
dim
(required)
Specifies the dimension of the LUT or the matrix and the number of color components. The
dim
attribute provides the dimensionality of the indexes, where: 
 4 entries represent the dimensions of a 3D cube and the number of components per entry.
e.g.dim = 17 17 17 3
indicates a 17cubed 3D LUT with 3 color components
 4 entries represent the dimensions of a 3D cube and the number of components per entry.

 2 entries represent the dimensions of a matrix.
e.g.dim = 3 3
indicates a 3×3 matrix
e.g.dim = 3 4
indicates a 3×4 matrix
 2 entries represent the dimensions of a matrix.

 2 entries represent the length of the LUT and the component value (1 or 3).
e.g.dim = 256 3
indicates a 256 element 1D LUT with 3 components (a 3×1D LUT)
e.g.dim = 256 1
indicates a 256 element 1D LUT with 1 component (1D LUT)
 2 entries represent the length of the LUT and the component value (1 or 3).
Substitutes for ProcessNode
¶
General¶
The attributes and elements defined for ProcessNode
are inherited by the substitutes for ProcessNode
. This section defines the available substitutes for the generalized ProcessNode
element.
The inBitDepth
of a ProcessNode
must match the outBitDepth
of the preceding ProcessNode
(if any).
LUT1D
¶
Description:
A 1D LUT transform uses an input pixel value, finds the two nearest index positions in the LUT, and then interpolates the output value using the entries associated with those positions.
This node shall contain either a 1D LUT or a 3x1D LUT in the form of an Array
. If the input to a LUT1D
is an RGB value, the same LUT shall be applied to all three color components.
A 3x1D LUT transform looks up each color component in a separate LUT1D
of the same length. In a 3x1D LUT, by convention, the LUT1D
for the first component goes in the first column of Array
.
The scaling of the array values is based on the outBitDepth
(the inBitDepth
is not considered).
The length of a 1D LUT should be limited to at most 65536 entries, and implementations are not required to support LUT1D
s longer than 65536 entries.
Linear interpolation shall be used for LUT1D
. More information about linear interpolation can be found in Appendix A.
 Elements:
Array
(required) an array of numeric values that are the output values of the 1D LUT.
Array
shall contain the table entries of a LUT in order from minimum value to maximum value.  For a 1D LUT, one value per entry is used for all color channels. For a 3x1D LUT, each line should contain 3 values, creating a table where each column defines a 1D LUT for each color component.
 For RGB, the first column shall correspond to R’s 1D LUT, the second column shall correspond to G’s 1D LUT, and the third column shall correspond to B’s 1D LUT.
 Attributes:
dim
(required)
two integers that represent the dimensions of the array. The first value defines the length of the array and shall equal the number of entries (lines) in the LUT. The second value indicates the number of components per entry and shall equal 1 for a 1D LUT or 3 for a 3x1D LUT.
Example
dim = "1024 3"
indicates a 1024 element 1D LUT with 3 component color (a 3x1D LUT)Example
dim = "256 1"
indicates a 256 element 1D LUT with 1 component color (a 1D LUT)
Note
Array
is formatted differently when it is contained in a LUT3D
or Matrix
element (see Array).
 Attributes:
interpolation
(optional)
a string indicating the preferred algorithm used to interpolate values in the
1DLUT
. This attribute is optional but, if present, shall be set to"linear"
.Note
Previous versions of this specification allowed for implementations to utilize different types of interpolation but did not define what those interpolation types were or how they should be labeled. For simplicity and to ensure similarity across implementations, 1D LUT interpolation has been limited to
"linear"
in this version of the specification. Support for additional interpolation types could be added in future version. halfDomain
(optional)
If this attribute is present, its value must equal
“true”
. When true, the input domain to the node is considered to be all possible 16bit floatingpoint values, and there must be exactly 65536 entries in theArray
element.Note
For example, the unsigned integer 15360 has the same bitpattern (0011110000000000) as the halffloat value 1.0, so the 15360th entry (zeroindexed) in the
Array
element is the output value corresponding to an input value of 1.0. rawHalfs
(optional)
If this attribute is present, its value must equal
“true”
. When true, therawHalfs
attribute indicates that the output array values in the form of unsigned 16bit integers are interpreted as the equivalent bit pattern, half floatingpoint values.Note
For example, to represent the value 1.0, one would use the integer 15360 in the
Array
element because it has the same bitpattern. This allows the specification of exact halffloat values without relying on conversion from decimal text strings.
Examples:
<LUT1D id="lut23" name="4 Value Lut" inBitDepth="12i" outBitDepth="12i">
<Description>1D LUT  Turn 4 grey levels into 4 inverted codes</Description>
<Array dim="4 1">
3
2
1
0
</Array>
</LUT1D>
Example 1. Example of a very simple LUT1D
LUT3D
¶
Description:
This node shall contain a 3D LUT in the form of an Array. In a LUT3D, the 3 color components of the input value are used to find the nearest indexed values along each axis of the 3D cube. The 3component output value is calculated by interpolating within the volume defined by the nearest corresponding positions in the LUT.
LUT3Ds have the same dimension on all axes (i.e. Array dimensions are of the form “n n n 3”). A LUT3D with axial dimensions greater than 128x128x128 should be avoided.
The scaling of the array values is based on the outBitDepth (the inBitDepth is not considered).
 Attributes:
interpolation
(optional)
a string indicating the preferred algorithm used to interpolate values in the 3DLUT. This attribute is optional with a default of
"trilinear"
if the attribute is not present.
Supported values are:"trilinear"
: perform trilinear interpolation"tetrahedral"
: perform tetrahedral interpolation
Note
Interpolation methods are specified in Appendix A.
 Elements:
Array
(required) an array of numeric values that are the output values of the 3D LUT. The
Array
shall contain the table entries for theLUT3D
from the minimum to the maximum input values, with the third component index changing fastest.
 Attributes:
dim
(required)
four integers that reperesent the dimensions of the 3D LUT and the number of color components. The first three values define the dimensions of the LUT and if multiplied shall equal the number of entries actually present in the array. The fourth value indicates the number of components per entry.
4 entries have the dimensions of a 3D cube plus the number of components per entry.Example
dim = "17 17 17 3"
indicates a 17cubed 3D lookup table with 3 component color
Note
Array
is formatted differently when it is contained in a LUT1D
or Matrix
element (see Array).
Examples:
<LUT3D id="lut24" name="green look" interpolation="trilinear" inBitDepth="12i" outBitDepth="16f">
<Description>3D LUT</Description>
<Array dim="2 2 2 3">
0.0 0.0 0.0
0.0 0.0 1.0
0.0 1.0 0.0
0.0 1.0 1.0
1.0 0.0 0.0
1.0 0.0 1.0
1.0 1.0 0.0
1.0 1.0 1.0
</Array>
</LUT3D>
Example 2. Example of a simple LUT3D
Matrix
¶
Description:
This node specifies a matrix transformation to be applied to the input values. The input and output of a Matrix
are always 3component values.
All matrix calculations should be performed in floating point, and input bit depths of integer type should be treated as scaled floats. If the input bit depth and output bit depth do not match, the coefficients in the matrix must incorporate the results of the ‘scale’ factor that will convert the input bit depth to the output bit depth (e.g. input of 10i
with an output of 12i
requires the matrix coefficients already have a factor of \(4095/1023\) applied). Changing the input or output bit depth requires creation of a new set of coefficients for the matrix.
The output values are calculated using roworder convention:
which is equivalent in functionality to the following:
Matrices using an offset calculation will have one more column than rows. An offset matrix may be defined using a 3x4 Array
, wherein the fourth column is used to specify offset terms, \(k_1\), \(k_2\), \(k_3\):
Expanded out, this means that the offset terms \(k_1\), \(k_2\), and \(k_3\) are added to each of the normal matrix calculations:
 Elements:
Array
(required) a table that provides the coefficients of the transformation matrix. The matrix dimensions are either 3x3 or 3x4. The matrix is serialized row by row from top to bottom and from left to right,
i.e., "\(a_{11}\ a_{12}\ a_{13}\ a_{21}\ a_{22}\ a_{23}\ \ldots\)" for a 3x3 matrix.
 Attributes:
dim
(required)
two integers that describe the dimensions of the matrix array. The first value define the number of rows and the second is the number of columns.
2 entries have the dimensions of a matrixExample
dim = "3 3"
indicates a 3x3 matrixExample
dim = "3 4"
indicates a 3x4 matrix
Note
Previous versions of this specification used three integers for the dim
attribute, rather than the current two. In order to facilitate backwards compatibility, implementations should allow a third value for the dim
attribute and may simply ignore it.
Note
Array
is formatted differently when it is contained in a LUT1D or LUT3D element (see Array)
Examples:
<Matrix id="lut28" name="AP0 to AP1" inBitDepth="16f" outBitDepth="16f" >
<Description>3x3 color space conversion from AP0 to AP1</Description>
<Array dim="3 3">
1.45143931614567 0.236510746893740 0.214928569251925
0.0765537733960204 1.17622969983357 0.0996759264375522
0.00831614842569772 0.00603244979102103 0.997716301365324
</Array>
</Matrix>
Example 3. Example of a Matrix
node with dim="3 3 3"
<Matrix id="lut25" name="colorspace conversion" inBitDepth="10i" outBitDepth="10i" >
<Description> 3x4 Matrix , 4th column is offset </Description>
<Array dim="3 4">
1.2 0.0 0.0 0.002
0.0 1.03 0.001 0.005
0.004 0.007 1.004 0.0
</Array>
</Matrix>
Example 4. Example of a Matrix
node
Range
¶
Description:
This node maps the input domain to the output range by scaling and offsetting values. The Range
element can also be used to clamp values.
Unless otherwise specified, the node’s default behavior is to scale and offset with clamping. If clamping is not desired, the style
attribute can be set to "noClamp"
.
To achieve scale and/or offset of values, all of minInValue
, minOutValue
, maxInValue
, and maxOutValue
must be present. In this explicit case, the formula for Range
shall be:
where:
The scaling of minInValue
and maxInValue
depends on the input bitdepth, and the scaling of minOutValue
and maxOutValue
depends on the output bitdepth.
If style="Clamp"
, the output value of \(out\) from the above equation is furthur modified as follows:
where:
\(\mathrm{MIN}(a,b)\) returns \(a\) if \(a < b\) and \(b\) if \(b \leq a\)
The Range
element can also be used to clamp values on only the top or bottom end. In such instances, no offset is applied, and the formula simplifies because only one pair of min or max values are required. (The style
shall not be "noClamp"
for this usecase.)
If only the minimum value pair is provided, then the result shall be clamping at the low end, according to:
Values must be set such that \(\texttt{minOutValue} = \texttt{minInValue} \times bitDepthScale\).
Likewise, if only the maximum values pairs are provided, the result shall be clamping at the high end, according to:
And values must be set such that \(\texttt{maxOutValue} = \texttt{maxInValue} \times bitDepthScale\).
The following formulas are used in the above equations:
Note
The bit depth scale factor intentionally uses \(2^{bitDepth}−1\) and not \(2^{bitDepth}\). This means that the scale factor created for scaling between different bit depths is "noninteger" and is slightly different depending on the bit depths being scaled between. While instinct might be that this scale should be a clean bitshift factor (i.e. \(2\times\) or \(4\times\) scale), testing with a few example values plugged into the formula will show that the resulting noninteger scale is the correct and intended behavior.
At least one pair of either minimum or maximum values, or all four values, must be provided.
 Elements:
minInValue
(optional) The minimum input value. Required if
minOutValue
is present. maxInValue
(optional) The maximum input value. Required if
maxOutValue
is present.
ThemaxInValue
shall be greater than theminInValue
. minOutValue
(optional) The minimum output value. Required if
minInValue
is present. maxOutValue
(optional) The maximum output value. Required if
maxInValue
is present.
ThemaxOutValue
shall be greater than or equal to theminOutValue
.  Attributes:
style
(optional) Describes the preferred handling of the scaling calculation of the
Range
node. If the style attribute is not present, clamping is performed.
The options forstyle
are:
"noClamp"
 If present, scale and offset is applied without clamping (i.e. values below
minOutValue
or abovemaxOutValue
are preserved) "Clamp"
 If present, clamping is applied upon the result of the scale and offset expressed by the result of the nonclamping
Range
equation
Examples:
<Range inBitDepth="10i" outBitDepth="10i">
<Description>10bit full range to SMPTE range</Description>
<minInValue>0</minInValue>
<maxInValue>1023</maxInValue>
<minOutValue>64</minOutValue>
<maxOutValue>940</maxOutValue>
</Range>
Example 5. Using "Range"
for scaling 10bit full range to 10bit SMPTE (legal) range.
Log
¶
Description:
This node contains parameters for processing pixels through a logarithmic or antilogarithmic function. A couple of main formulations are supported. The most basic formula follows a pure logarithm or antilogarithm of either base 2 or base 10. Another supported formula allows for a logarithmic function with a gain factor and offset. This formulation can be used to convert from linear to Cineon. Another style of log formula follows a piecewise function consisting of a logarithmic function with a gain factor, an offset, and a linear segment. This style can be used to implement many common “cameralog” encodings.
Note
The equations for the Log
node assume integer data is normalized to floatingpoint scaling. LogParams
do not change based on the input and output bitdepths.
Note
On occasion it may be necessary to transform a logarithmic function specified in terms of traditional Cineonstyle parameters to the parameters used by CLF. Guidance on how to do this is provided in Appendix B.
 Attributes:
style
(required)
specifies the form of the of log function to be applied
Supported values for ”style” are:"log10"
"antiLog10"
"log2"
"antiLog2"
"linToLog"
"logToLin"
"cameraLinToLog"
"cameraLogToLin"

The formula to be applied for each style is described by the equations below, for all of which:
\[ \texttt{FLT_MIN} = 1.175494 \times 10^{38} \]\(\textrm{MAX}(a, b)\) returns \(a\) if \(a \gt b\) and \(b\) if \(b \geq a\) "log10"
: applies a base 10 logarithm according to
\[ y = log_{10}(\textrm{MAX}(x,\texttt{FLT_MIN})) \]"antiLog10"
: applies a base 10 antilogarithm according to
\[ x = 10^{y} \]"log2"
: applies a base 2 logarithm according to
\[ y = log_{2}(\textrm{MAX}(x,\texttt{FLT_MIN})) \]"antiLog2"
: applies a base 2 antilogarithm according to
\[ x = 2^{y} \]"linToLog"
: applies a logarithm according to
\[ y = \text{logSideSlope} \times \text{log}_\text{base}(\textrm{MAX}(\text{linSideSlope} \times x + \text{linSideOffset}, \texttt{FLT_MIN}))+\text{logSideOffset} \]"logToLin"
: applies an antilogarithm according to
\[ x = \frac{\left(\text{base}^{\frac{y\text{logSideOffset}}{\text{logSideSlope}}}  \text{linSideOffset}\right)}{\text{linSideSlope}} \]"cameraLinToLog"
: applies a piecewise function with logarithmic and linear segments on linear values, converting them to nonlinear values
\[ y = \begin{cases} \text{linearSlope} \times x + \text{linearOffset} & \text{if } x \leq \text{linSideBreak}\\ \text{logSideSlope} \times \text{log}_{\text{base}}(\mathrm{MAX}(\text{linSideSlope} \times x + \text{linSideOffset},\texttt{FLT_MIN})) + \text{logSideOffset} & \text{otherwise} \\ \end{cases} \\ \]Note
The calculation of \(\text{linearSlope}\), and \(\text{linearOffset}\) is described in Solving for
LogParams
"cameraLogToLin"
: applies a piecewise function with logarithmic and linear segments on nonlinear values, converting them to linear values
\[ x = \begin{cases} \frac{(y  \text{linearOffset})}{\text{linearSlope}} & \text{if } y \leq \text{logSideBreak} \\ \frac{\left(\text{base}^{\frac{y\text{logSideOffset}}{\text{logSideSlope}}}  \text{linSideOffset}\right)}{\text{linSideSlope}} & \text{otherwise} \end{cases} \]Note
The calculation of \(\text{logSideBreak}\), \(\text{linearSlope}\), and \(\text{linearOffset}\) is described in Solving for
LogParams
 Elements:
LogParams
(required  if"style"
is not a basic logarithm)
contains the attributes that control the
"linToLog"
,"logToLin"
,"cameraLinToLog"
, or"cameraLogToLin"
functions
This element is required ifstyle
is of type"linToLog"
,"logToLin"
,"cameraLinToLog"
, or"cameraLogToLin"
.
Attributes:"base"
(optional) the base of the logarithmic function
Default is 2. "logSideSlope"
(optional) "slope" (or gain) applied to the log side of the logarithmic segment.
Default is 1. "logSideOffset"
(optional) offset applied to the log side of the logarithmic segment.
Default is 0. "linSideSlope"
(optional) slope of the linear side of the logarithmic segment.
Default is 1. "linSideOffset"
(optional) offset applied to the linear side of the logarithmic segment.
Default is 0. "linSideBreak"
(optional) the breakpoint, defined in linear space, at which the piecewise function transitions between the logarithmic and linear segments.
This is required ifstyle="cameraLinToLog"
or"cameraLogToLin"
"linearSlope"
(optional) the slope of the linear segment of the piecewise function. This attribute does not need to be provided unless the formula being implemented requires it. The default is to calculate using
linSideBreak
such that the linear portion is continuous in value with the logarithmic portion of the curve, by using the value of the logarithmic portion of the curve at the breakpoint. This is described in the following note below. "channel"
(optional) the color channel to which the exponential function is applied. Possible values are
"R"
,"G"
,"B"
. If this attribute is utilized to target different adjustments per channel, then up to threeLogParams
elements may be used, provided that"channel"
is set differently in each. However, the same value of base must be used for all channels. If this attribute is not otherwise specified, the logarithmic function is applied identically to all three color channels.
Solving for LogParams
\(\text{linearOffset}\) is the offset of the linear segment of the piecewise function. This value is calculated using the position of the breakpoint and the linear slope in order to ensure continuity of the two segments. The following steps describe how to calculate \(\text{linearOffset}\).
First, the value of the breakpoint on the logaxis is calculated using the value of \(\text{linSideBreak}\) as input to the logarithmic segment of the piecewise function, as below:
Then, if \(\text{linearSlope}\) was not provided, the value of \(\text{linSideBreak}\) is used again to solve for the derivative of the logarithmic function. The value of \(\text{linearSlope}\) is set to equal the instantaneous slope at the breakpoint, or derivative, as shown below:
Finally, the value of \(\text{linearOffset}\) can be solved for by rearranging the linear segment of of the piecewise function and using the values of \(\text{logSideBreak}\) and \(\text{linearSlope}\), as below:
Examples:
<Log inBitDepth="16f" outBitDepth="16f" style="log10">
<Description>Base 10 Logarithm</Description>
</Log>
Example 6. Log
node applying a base 10 logarithm.
<Log inBitDepth="32f" outBitDepth="32f" style="cameraLinToLog">
<Description>Linear to DJI DLog</Description>
<LogParams base="10" logSideSlope="0.256663" logSideOffset="0.584555"
linSideSlope="0.9892" linSideOffset="0.0108" linSideBreak="0.0078"
linearSlope="6.025"/>
</Log>
Example 7. Log
node applying the DJI DLog formula.
Exponent
¶
Description: This node contains parameters for processing pixels through a power law function. Two main formulations are supported. The first follows a pure power law. The second is a piecewise function that follows a power function for larger values and has a linear segment that is followed for small and negative values. The latter formulation can be used to represent the Rec. 709, sRGB, and CIE L* equations.
 Attributes:
style
(required)
specifies the form of the exponential function to be applied. Supported values are:
"basicFwd"
"basicRev"
"basicMirrorFwd"
"basicMirrorRev"
"basicPassThruFwd"
"basicPassThruRev"
"monCurveFwd"
"monCurveRev"
"monCurveMirrorFwd"
"monCurveMirrorRev"
Each of these supported styles are described in detail below, and for all of which the following definitions apply:
\(g =\)exponent
\(k =\)offset
\(\textrm{MAX}(a, b)\) returns \(a\) if \(a \gt b\) and \(b\) if \(b \geq a\)"basicFwd"
 applies a power law using the exponent value specified in the
ExponentParams
element.
Values less than zero are clamped.
\[ \text{basicFwd}(x) = [\textrm{MAX}(0,x)]^g \]"basicRev"
 applies power law using the exponent value specified in the
ExponentParams
element.
Values less than zero are clamped.
\[ \text{basicRev}(y) = [\textrm{MAX}(0,y)]^{1/g} \]"basicMirrorFwd"
 applies a basic power law using the exponent value specified in the
ExponentParams
element for values greater than or equal to zero and mirrors the function for values less than zero (i.e. rotationally symmetric around the origin):
\[ \text{basicMirrorFwd}(x) = \begin{cases} x^{g} & \text{if } x \geq 0 \\ [6pt] \Big[(x)^{g}\Big] & \text{otherwise} \end{cases} \]"basicMirrorRev"
 applies a basic power law using the exponent value specified in the
ExponentParams
element for values greater than or equal to zero and mirrors the function for values less than zero (i.e. rotationally symmetric around the origin):
\[ \text{basicMirrorRev}(y) = \begin{cases} y^{1/g} & \text{if } y \geq 0 \\[6pt] \Big[(y)^{1/g}\Big] & \text{otherwise} \end{cases} \]"basicPassThruFwd"
 applies a basic power law using the exponent value specified in the
ExponentParams
element for values greater than or equal to zero and passes values less than zero unchanged:
\[ \text{basicPassThruFwd}(x) = \begin{cases} x^{g} & \text{if } x \geq 0 \\[6pt] x & \text{otherwise} \end{cases} \]"basicPassThruRev"
 applies a basic power law using the exponent value specified in the
ExponentParams
element for values greater than or equal to zero and and passes values less than zero un changed:
\[ \text{basicPassThruRev}(y) = \begin{cases} y^{1/g} & \text{if } y \geq 0 \\[6pt] y & \text{otherwise} \end{cases} \]"monCurveFwd"
 applies a power law function with a linear segment near the origin
\[ \text{monCurveFwd}(x) = \begin{cases} \left( \frac{x\:+\:k}{1\:+\:k} \right)^{g} & \text{if } x \geq xBreak \\[8pt] x\:s & \text{otherwise} \end{cases} \] where:
\(xBreak = \dfrac{k}{g1}\)  and, for the \(\text{monCurveFwd}\) (above) and \(\text{monCurveRev}\) (below) equations:
\(s = \left(\dfrac{g1}{k}\right) \left(\dfrac{k g}{(g1)(1+k)}\right)^{g}\) "monCurveRev"
 applies a power law function with a linear segment near the origin
\[ \text{monCurveRev}(y) = \begin{cases} (1 + k)\:y^{(1/g)}  k & \text{if } y \geq yBreak \\[8pt] \dfrac{y}{s} & \text{otherwise} \end{cases} \] where:
\(yBreak = \left(\dfrac{k g}{(g1)(1+k)}\right)^g\) "monCurveMirrorFwd"
 applies a power law function with a linear segment near the origin and mirrors the function for values less than zero (i.e. rotationally symmetric around the origin):
\[ \text{monCurveMirrorFwd}(x) = \begin{cases} \text{monCurveFwd}(x) & \text{if } x \geq 0 \\[8pt] [\text{monCurveFwd}(x)] & \text{otherwise} \end{cases} \]"monCurveMirrorRev"
 applies a power law function with a linear segment near the origin and mirrors the function for values less than zero (i.e. rotationally symmetric around the origin):
\[ \text{monCurveMirrorRev}(y) = \begin{cases} \text{monCurveRev}(y) & \text{if } y \geq 0 \\[8pt] [\text{monCurveRev}(y)] & \text{otherwise} \end{cases} \]
Note
The above equations assume that the input and output bitdepths are floatingpoint. Integer values are normalized to the range \([0.0, 1.0]\).
 Elements:
ExponentParams
(required)
contains one or more attributes that provide the values to be used by the enclosing
Exponent
element.
Ifstyle
is any of the “basic” types, then onlyexponent
is required.
Ifstyle
is any of the “monCurve” types, thenexponent
andoffset
are required. Attributes:
"exponent"
(required) the power to which the value is to be raised
If style is any of the “monCurve” types, the valid range is \([1.0, 10.0]\). The nominal value is 1.0.
Note
When using a “monCurve” style, a value of 1.0 assigned to
exponent
could result in a dividebyzero error. Implementors should protect against this case."offset"
(optional) the offset value to use
If offset is used, the enclosingExponent
element’s style attribute must be set to one of the “monCurve” types. Offset is not allowed whenstyle
is any of the “basic” types.
The valid range is \([0.0, 0.9]\). The nominal value is 0.0.
Note
If zero is provided as a value for
offset
, the calculation of \(xBreak\) or \(yBreak\) could result in a dividebyzero error. Implementors should protect against this case."channel"
(optional) the color channel to which the exponential function is applied.
Possible values are"R"
,"G"
,"B"
.
If this attribute is utilized to target different adjustments per channel, up to threeExponentParams
elements may be used, provided that"channel"
is set differently in each. If this attribute is not otherwise specified, the exponential function is applied identically to all three color channels.
Examples:
<Exponent inBitDepth="32f" outBitDepth="32f" style="basicFwd">
<Description>Basic 2.2 Gamma</Description>
<ExponentParams exponent="2.2"/>
</Exponent>
Example 8. Using Exponent
node for applying a 2.2 gamma.
<Exponent inBitDepth="32f" outBitDepth="32f" style="monCurveFwd">
<Description>EOTF (sRGB)</Description>
<ExponentParams exponent="2.4" offset="0.055" />
</Exponent>
Example 9. Using Exponent
node for applying the intended EOTF found in IEC 6196621:1999 (sRGB).
<Exponent inBitDepth="32f" outBitDepth="32f" style="monCurveRev">
<Description>CIE L*</Description>
<ExponentParams exponent="3.0" offset="0.16" />
</Exponent>
Example 10. Using Exponent
node to apply CIE L* formula.
<Exponent inBitDepth="32f" outBitDepth="32f" style="monCurveRev">
<Description>Rec. 709 OETF</Description>
<ExponentParams exponent="2.2222222222222222" offset="0.099" />
</Exponent>
Example 11. Using Exponent
node to apply Rec. 709 OETF.
ASC_CDL
¶
Description:
This node processes values according to the American Society of Cinematographers’ Color Decision List (ASC CDL) equations. Color correction using ASC CDL is an industrywide method of recording and exchanging basic color correction adjustments via parameters that set particular color processing equations.
The ASC CDL equations are designed to work on an input domain of floatingpoint values of [0 to 1.0] although values greater than 1.0 can be present. The output data may or may not be clamped depending on the processing style used.
If the style
attribute is not specified, the node shall default to "Fwd"
 i.e. the classic implementation of the v1.2 ASCCDL equations.
Note
Equations 4.314.34 assume that \(in\) and \(out\) are scaled to normalized floatingpoint range. If the ASC_CDL
node has inBitDepth
or outBitDepth
that are integer types, then the input or output values must be normalized to or from 01 scaling. In other words, the slope, offset, power, and saturation values stored in the ProcessNode
do not depend on inBitDepth
and outBitDepth
; they are always interpreted as if the bit depths were float.
Attributes:
id
(optional) This should match the id attribute of the ColorCorrection element in the ASC CDL XML format.
style

Determines the formula applied by the operator. The valid options are:
 `"Fwd"
 implementation of v1.2 ASC CDL equation (default)
"Rev"
 inverse equation
"FwdNoClamp"
 similar to the Fwd equation, but without clamping
"RevNoClamp"
 inverse equation, without clamping
The first two implement the math provided in version 1.2 of the ASC CDL specification. The second two omit the clamping step and are intended to provide compatibility with the many applications that take that alternative approach.
Elements:
SOPNode
(optional)
The
SOPNode
is optional, and if present, must contain each of the following subelements:Slope
 three decimal values representing the R, G, and B slope values, which is similar to gain, but changes the slope of the transfer function without shifting the black level established by
offset
Valid values for slope must be greater than or equal to zero (\(\geq\) 0).
The nominal value is 1.0 for all channels. Offset
 three decimal values representing the R, G, and B offset values, which raise or lower overall brightness of a color component by shifting the transfer function up or down while holding the slope constant
The nominal value is 0.0 for all channels. Power
 three decimal values representing the R, G, and B power values, which change the intermediate shape of the transfer function
Valid values for power must be greater than zero (\(\gt\) 0).
The nominal value is 1.0 for all channels.
SatNode
(optional)
The
SatNode
is optional, but if present, must contain one of the following subelement:Saturation
 a single decimal value applied to all color channels
Valid values for saturation must be greater than or equal to zero (\(\geq\) 0).
The nominal value is 1.0.
Note
If either element is not specified, values should default to the nominal values for each element. If using the "noClamp"
style, the result of the defaulting to the nominal values is a noop.
Note
The structure of this ProcessNode
matches the structure of the XML format described in the v1.2 ASC CDL specification. However, unlike the ASC CDL XML format, there are no alternate spellings allowed for these elements.
The math for style="Fwd"
is:
Where:
\(\textrm{CLAMP()}\) clamps the argument to \([0,1]\)
The math for style="FwdNoClamp"
is the same as for "Fwd"
but the two clamp() functions are omitted.
Also, if \((in \times \textrm{slope} + \textrm{offset}) < 0\), then no power function is applied.
The math for style="Rev"
is:
Where:
\(\textrm{CLAMP()}\) clamps the argument to \([0,1]\)
The math for style="RevNoClamp"
is the same as for "Rev"
but the \(\textrm{CLAMP}()\) functions are omitted.
Also, if \(out_{\textrm{SAT}} \lt 0\), then no power function is applied.
Examples:
<ASC_CDL id="cc01234" inBitDepth="16f" outBitDepth="16f" style="Fwd">
<Description>scene 1 exterior look</Description>
<SOPNode>
<Slope>1.000000 1.000000 0.900000</Slope>
<Offset>0.030000 0.020000 0.000000</Offset>
<Power>1.2500000 1.000000 1.000000</Power>
</SOPNode>
<SatNode>
<Saturation>1.700000</Saturation>
</SatNode>
</ASC_CDL>
Example 12. Example of an ASC_CDL
node.
Implementation Notes¶
Bit Depth¶
Processing Precision¶
All processing shall be performed using 32bit floatingpoint values. The values of the inBitDepth
and outBitDepth
attributes shall not affect the quantization of color values.
Note
For some hardware devices, 32bit float processing might not be possible. In such instances, processing should be performed at the highest precision available. Because CLF permits complex series of discrete operations, CLF LUT files are unlikely to run on hardware devices without some form of preprocessing. Any preprocessing to prepare a CLF for more limited hardware applications should adhere to the processing precision requirements.
Input To and Output From a ProcessList¶
Applications often support multiple pixel formats (e.g. 8i, 10i, 16f, 32f, etc.). Often the actual pixel format to be processed may not agree with the inBitDepth
of the first ProcessNode or the outBitDepth
of the last ProcessNode. (Note that the ProcessList
element itself does not contain global inBitDepth
or outBitDepth
attributes.) Therefore, in some cases an application may need to rescale a given ProcessNode
to be appropriate for the actual image data being processed.
For example, if the last ProcessNode in a ProcessList is a LUT1D
with an outBitDepth
of 12i, it indicates that the LUT Array values are scaled relative to 4095. If the application wants to produce floatingpoint pixel values, it should therefore divide the LUT Array values by 4095 before processing the pixels (according to Conversion). Likewise, if the outBitDepth
was instead 32f and the application wants to produce 12i pixel values, it should multiply the LUT Array values by 4095. (Note that in this case, since the result of the computations may exceed 4095 and the application wants to produce 12bit integer output, the application would want to clamp, round, and quantize the value.)
Input To and Output From a ProcessNode¶
In order to ensure the scaling of parameter values of all ProcessNodes in a ProcessList are consistent, the inBitDepth
of each ProcessNode must match the outBitDepth
of the previous ProcessNode (if any).
Please note that an integer inBitDepth
or outBitDepth
of a ProcessNode does not indicate that any clamping or quantization should be done. These attributes are strictly used to indicate the scaling of parameter and array values. As discussed above, processing precision shall be floatingpoint.
Furthermore, because the processing precision is intended to be floatingpoint, the inBitDepth
and outBitDepth
only control the scaling of parameter and array values and do not impose range limits. For example, even if the outBitDepth
of a LUT Array is 12i, it does not mean that the Array values must be limited to \([0,4095]\) or that they must be integer values. It simply means that in order to rescale to 32f that a scale factor of 1/4095 should be used (as per Conversion).
Because processing within a ProcessList should be done at floatingpoint precision, applications may optionally want to rescale the interfaces all ProcessNodes “interior” to a ProcessList to be 32f according to Conversion. As discussed in Input To and Output From a ProcessList, applications may want to rescale the “exterior” interfaces of the ProcessList based on the type of pixel data being processed.
For some applications, it may be easiest to simply rescale all ProcessNodes to 32f input and output bitdepth when parsing the file. That way, the ProcessList may be considered a purely 32f set of operations and the implementation therefore does not need to track or deal with bitdepth differences at the ProcessNode level.
Conversion Between Integer and Normalized Float Scaling¶
As discussed above, the inBitDepth
or outBitDepth
of a ProcessNode may need to be rescaled in order to accommodate the pixel data type being processed by the application.
The scale factor associated with the bitdepths 8i, 10i, 12i, and 16i is \(2^n − 1\), where \(n\) is the bitdepth.
The scale factor associated with the bitdepths 16f and 32f is 1.0.
To rescale Matrix, LUT1D, or LUT3D Array
values when the outBitDepth
changes, the scale factor is equal to \(\frac{\textrm{newScale}}{\textrm{oldScale}}\). For example, to convert from 12i to 10i, multiply array values by \(1023/4095\).
To rescale Matrix Array
values when the inBitDepth
changes, the scale factor is equal to \(\frac{\textrm{oldScale}}{\textrm{newScale}}\). For example, to convert from 32f to 10i, multiply array values by \(1/1023\).
To rescale Range parameters when the inBitDepth
changes, the scale factor for minInValue
and maxInValue
is \(\frac{\textrm{newScale}}{\textrm{oldScale}}\). To rescale Range parameters when the outBitDepth
changes, the scale factor for minOutValue
and maxOutValue
is \(\frac{\textrm{newScale}}{\textrm{oldScale}}\).
Please note that in all cases, the conversion shall be only a scale factor. In none of the above cases should clamping or quantization be applied.
Aside from the specific cases listed above, changes to inBitDepth
and outBitDepth
do not affect the parameter or array values of a given ProcessNode.
If an application needs to convert between different integer pixel formats or between integer and float (or vice versa) on the way into or out of a ProcessList, the same scale factors should be used. Note that when converting from floatingpoint to integer at the application level that values should be clamped, rounded, and quantized.
Required vs Optional¶
The required or optional indicated in parentheses throughout this specification indicate the requirement for an element or attribute to be present for a valid CLF file. In the spirit of a LUT format to be used commonly across different software and hardware, none of the elements or attributes should be considered optional for implementors to support. All elements and attributes, if present, should be recognized and supported by an implementation.
If, due to hardware or software limitations, a particular element or attribute is not able to be supported, a warning should be issued to the user of a LUT that contains one of the offending elements. The focus shall be on the user and maintaining utmost compatibility with the specification so that LUTs can be interchanged seamlessly.
Efficient Processing¶
The transform engine may merge some ProcessNodes in order to obtain better performance. For example, adjacent Matrix
operators may be combined into a single matrix. However, in general, combining operators in a way that preserves accuracy is difficult and should be avoided.
Hardware implementations may need to convert all ProcessNodes into some other form that is consistent with what the hardware supports. For example, all ProcessNodes might need to be combined into a single 3D LUT. Using a grid size of 64 or larger is recommended to preserve as much accuracy as possible. Implementors should be aware that the success of such approximations varies greatly with the nature of the input and output color spaces. For example, if the input color space is scenelinear in nature, it may be necessary to use a “shaper LUT” or similar nonlinearity before the 3D LUT in order to convert values into a more perceptually uniform representation.
Extensions¶
It is recommended that implementors of CLF file readers protect against unrecognized elements or attributes that are not defined in this specification. Unrecognized elements that are not children of the Info
element should either raise an error or at least provide a warning message to the user to indicate that there is an operator present that is not recognized by the reader. Applications that need to add custom metadata should place it under the Info
element rather than at the top level of the ProcessList.
One or more Description
elements in the ProcessList can and should be used for metadata that does not fit into a provided field in the Info
element and/or is unlikely to be recognized by other applications.
Examples¶
<?xml version="1.0" encoding="UTF8"?>
<ProcessList id="ACEScsc.ACES_to_ACEScg.a1.0.3" name="ACES20651 to ACEScg"
compCLFversion="3.0">
<Info>
<ACEStransformID>ACEScsc.ACES_to_ACEScg.a1.0.3</ACEStransformID>
<ACESuserName>ACES20651 to ACEScg</ACESuserName>
</Info>
<Description>ACES20651 to ACEScg</Description>
<InputDescriptor>ACES20651</InputDescriptor>
<OutputDescriptor>ACEScg</OutputDescriptor>
<Matrix inBitDepth="16f" outBitDepth="16f">
<Array dim="3 3">
1.451439316146 0.236510746894 0.214928569252
0.076553773396 1.176229699834 0.099675926438
0.008316148426 0.006032449791 0.997716301365
</Array>
</Matrix>
</ProcessList>
Example 13. ACES20651 to ACEScg
<?xml version="1.0" encoding="UTF8"?>
<ProcessList id="ACEScsc.ACES_to_ACEScct.a1.0.3" name="ACES20651 to ACEScct"
compCLFversion="3.0">
<Description>ACES20651 to ACEScct Log working space</Description>
<InputDescriptor>Academy Color Encoding Specification (ACES20651)</InputDescriptor>
<OutputDescriptor>ACEScct Log working space</OutputDescriptor>
<Info>
<ACEStransformID>ACEScsc.ACES_to_ACEScct.a1.0.3</ACEStransformID>
<ACESuserName>ACES20651 to ACEScct</ACESuserName>
</Info>
<Matrix inBitDepth="16f" outBitDepth="16f">
<Array dim="3 3">
1.451439316146 0.236510746894 0.214928569252
0.076553773396 1.176229699834 0.099675926438
0.008316148426 0.006032449791 0.997716301365
</Array>
</Matrix>
<Log inBitDepth="16f" outBitDepth="16f" style="cameraLinToLog">
<LogParams base="2" logSideSlope="0.05707762557" logSideOffset="0.5547945205"
linSideBreak="0.0078125" />
</Log>
</ProcessList>
Example 14. ACES20651 to ACEScct
<?xml version="1.0" encoding="UTF8"?>
<ProcessList id="5ac02dc71e024f87af46fa5a83d5232d" compCLFversion="3.0">
<Description>CIEXYZ D65 to CIELAB L*, a*, b* (scaled by 1/100, neutrals at
0.0 chroma)</Description>
<InputDescriptor>CIEXYZ, D65 white (scaled [0,1])</InputDescriptor>
<OutputDescriptor>CIELAB L*, a*, b* (scaled by 1/100, neutrals at 0.0
chroma)</OutputDescriptor>
<Matrix inBitDepth="16f" outBitDepth="16f">
<Array dim="3 3">
1.052126639 0.000000000 0.000000000
0.000000000 1.000000000 0.000000000
0.000000000 0.000000000 0.918224951
</Array>
</Matrix>
<Exponent inBitDepth="16f" outBitDepth="16f" style="monCurveRev">
<ExponentParams exponent="3.0" offset="0.16" />
</Exponent>
<Matrix inBitDepth="16f" outBitDepth="16f">
<Array dim="3 3">
0.00000000 1.00000000 0.00000000
4.31034483 4.31034483 0.00000000
0.00000000 1.72413793 1.72413793
</Array>
</Matrix>
</ProcessList>
Example 15. CIE XYZ to CIELAB
Appendices¶
Appendix A: Interpolation¶
When an input value falls between sampled positions in a LUT, the output value must be calculated as a proportion of the distance along some function that connects the nearest surrounding values in the LUT. There are many different types of interpolation possible, but only three types of interpolation are currently specified for use with the Common LUT Format (CLF).
The first interpolation type, linear, is specified for use with a LUT1D
node. The other two, trilinear and tetrahedral interpolation, are specified for use with a LUT3D
node.
Linear Interpolation¶
With a table of the sampled input values in \(inValue[i]\) where \(i\) ranges from \(0\) to \((n1)\), and a table of the corresponding output values in \(outValue[j]\) where \(j\) is equal to \(i\),
index \(i\)  inValue  index \(j\)  outValue  

0  0  0  0  
\(\vdots\)  \(\vdots\)  \(\vdots\)  \(\vdots\)  
\(n  1\)  1  \(n  1\)  1000 
the \(output\) resulting from \(input\) can be calculated after finding the nearest \(inValue[i] < input\).
When \(inValue[i] = input\), the result is evaluated directly.
Trilinear Interpolation¶
Trilinear interpolation implements linear interpolation in threedimensions by successively interpolating each direction.
Note
The convention used for notation is uppercase variables for mesh points and lowercase variables for points on the grid.
Consider a sampled point as depicted in Figure 4. Let \(V(r,g,b)\) represent the value at the point with coordinate \((r,g,b)\). The distance between each node per color coordinate shows the proportion of each mesh point's color coordinate values that contribute to the sampled point.
The general expression for trilinear interpolation can be expressed as:
where:
Expressed in matrix form:
The expression in above can be written as: \(\mathbf{C} = \mathbf{A}\mathbf{V}\).
Trilinear interpolation shall be done according to \(V(r,g,b) = \mathbf{C}^T \mathbf{\Delta} = \mathbf{V}^T \mathbf{A}^T \mathbf{\Delta}\).
Note
The term \(\mathbf{V}^T \mathbf{A}^T\) does not depend on the variable \((r,g,b)\) and thus can be computed in advance for optimization. Each subcube can have the values of the vector \(\mathbf{C}\) already stored in memory. Therefore the algorithm can be summarized as:
 Find the subcube containing the point \((r,g,b)\)
 Select the vector \(\mathbf{C}\) corresponding to that subcube
 Compute \(\Delta_r\), \(\Delta_g\), \(\Delta_b\)
 Return \(V(r,g,b) = \mathbf{C}^T \mathbf{\Delta}\)
Tetrahedral Interpolation}¶
Tetrahedral interpolation subdivides the cubelet defined by the vertices surrounding a sampled point into six tetrahedra by segmenting along the main (and usually neutral) diagonal (Figure 5).
To find the tetrahedron containing the point \((r,g,b)\):
 if \(\Delta_b > \Delta_r > \Delta_g\), then use the first tetrahedron, \(t1\)
 if \(\Delta_b > \Delta_g > \Delta_r\), then use the first tetrahedron, \(t2\)
 if \(\Delta_g > \Delta_b > \Delta_r\), then use the first tetrahedron, \(t3\)
 if \(\Delta_r > \Delta_b > \Delta_g\), then use the first tetrahedron, \(t4\)
 if \(\Delta_r > \Delta_g > \Delta_b\), then use the first tetrahedron, \(t5\)
 else, use the sixth tetrahedron, \(t6\)
The matrix notation is:
Trilinear interpolation shall be done according to:
Note
The vectors \(\mathbf{T}_i \mathbf{V}\) for \(i = 1,2,3,4,5,6\) does not depend on the variable \((r,g,b)\) and thus can be computed in advance for optimization.
Appendix B: Cineonstyle Log Parameters¶
When using a Log
node, it might be desirable to conform an existing logarithmic function that uses Cineon style parameters to the parameters used by CLF. A translation from Cineonstyle parameters to those used by CLF's LogParams
element is quite straightforward using the following steps.
Traditionally, \(\textrm{refWhite}\) and \(\textrm{refBlack}\) are provided as 10bit quantities, and if they indeed are, first normalize them to floating point by dividing by 1023:
where subscript 10\(i\) indicates a 10bit quantity.
The density range is assumed to be:
Then solve the following quantities:
Where \(MIN(x,y)\) returns \(x\) if \(x<y\), otherwise returns \(y\)
The parameters for the LogParams
element are then:
Appendix C: Changes between v2.0 and v3.0¶
 Add
Log
ProcessNode  Add
Exponent
ProcessNode  Revise formulas for defining use of
Range
ProcessNode to clamp at the low or high end. IndexMaps
removed. Use ahalfDomain
LUT to achieve reshaping of input to a LUT. Move
ACEStransform
elements toInfo
element of ProcessList in main spec  Changed syntax for
dim
attribute ofArray
when contained in aMatrix
. Two integers are now used to define the dimensions of the matrix instead of the previous three values which defined the dimensions of the matrix and the number of color components.