Understanding RawTherapee color output

[daviejones]

Thanks for the help @JackH and @gaaned92.

@gaaned92

There is no XYZ color model (Color model - Wikipedia) at the input of RT. Input can be either RGB color model for demosaiced photo or camera space for raw.

I was trying for the latter: give RT values in camera raw space, and since ColorMatrix1=identity, they’d be interpreted as XYZ.D65 values (since the DNG spec defines ColorMatrix1 as translating between XYZ and camera space).

Anyway, since I couldn’t get the DNG fields to work as I expected, I created a custom ICC profile containing my color matrix, and fortunately that works.

In case it helps someone else, I’ve outlined my steps below, which allow you to:

• control the color matrix that RawTherapee uses for interpreting raw CFA data, and
• independently verify RawTherapee’s color math.

Controlling the color matrix that RawTherapee uses to interpret color filter array data

• Create a custom ICC profile

  • Compile the iccFromXml tool from the RefIccMAX project:
    https://github.com/InternationalColorConsortium/RefIccMAX

  • Create the ICC profile in XML. Here’s a template:

    <?xml version="1.0" encoding="UTF-8"?>
    <IccProfile>
        <Header>
            <PreferredCMMType></PreferredCMMType>
            <CreationDateTime>2020-01-01T00:00:00</CreationDateTime>
            <ProfileVersion>4.3.0.0</ProfileVersion>
            <ProfileDeviceClass>scnr</ProfileDeviceClass>
            <DataColourSpace>RGB </DataColourSpace>
            <PCS>XYZ </PCS>
            <DeviceAttributes ReflectiveOrTransparency="reflective" GlossyOrMatte="glossy" MediaPolarity="positive" MediaColour="colour"/>
            <ProfileFlags EmbeddedInFile="false" UseWithEmbeddedDataOnly="false"/>
            <RenderingIntent>Perceptual</RenderingIntent>
            <PCSIlluminant> <XYZNumber X="0.96422" Y="1" Z="0.8249"/> </PCSIlluminant>
        </Header>
    
        <Tags>
            <profileDescriptionTag> <multiLocalizedUnicodeType>
              <LocalizedText LanguageCountry="enUS">Description</LocalizedText>
            </multiLocalizedUnicodeType> </profileDescriptionTag>
    
            <copyrightTag> <multiLocalizedUnicodeType>
              <LocalizedText LanguageCountry="enUS">Copyright</LocalizedText>
            </multiLocalizedUnicodeType> </copyrightTag>
    
            <mediaWhitePointTag> <XYZArrayType> <XYZNumber X="1" Y="1" Z="1"/> </XYZArrayType> </mediaWhitePointTag>
    
            <redColorantTag> <XYZArrayType> <XYZNumber X="1" Y="0" Z="0"/> </XYZArrayType> </redColorantTag>
            <greenColorantTag> <XYZArrayType> <XYZNumber X="0" Y="1" Z="0"/> </XYZArrayType> </greenColorantTag>
            <blueColorantTag> <XYZArrayType> <XYZNumber X="0" Y="0" Z="1"/> </XYZArrayType> </blueColorantTag>
    
            <redTRCTag> <curveType> <Curve> </Curve> </curveType> </redTRCTag>
            <greenTRCTag> <curveType> <Curve> </Curve> </curveType> </greenTRCTag>
            <blueTRCTag> <curveType> <Curve> </Curve> </curveType> </blueTRCTag>
        </Tags>
    </IccProfile>
    

  • Insert the color matrix into the redColorantTag / greenColorantTag / blueColorantTag fields. (Each field is a column of the matrix.)

  • Convert the XML ICC profile into its binary represenation:

    iccFromXml profile.xml profile.icc
    

• Convert CFA data to a DNG image

  • Save the CFA data as a monochrome TIFF, but name it img.dng

  • Strip metadata from the image:

    exiftool -overwrite_original -all= img.dng

  • Add DNG fields:

    exiftool                                            \
    -DNGVersion="1.4.0.0"                               \
    -DNGBackwardVersion="1.4.0.0"                       \
    -IFD0:BlackLevel=0                                  \
    -IFD0:WhiteLevel=65535                              \
    -PhotometricInterpretation="Color Filter Array"     \
    -SamplesPerPixel=1                                  \
    -IFD0:CFARepeatPatternDim="2 2"                     \
    -IFD0:CFAPattern2="1 0 2 1"                         \
    -overwrite_original                                 \
    img.dng
    

• Load img.dng in RawTherapee

  • Disable all image processing (except demosaicing)
  • Input Profile: select the custom ICC profile
  • Output Profile: RTv4_sRGB

Verifying RawTherapee’s color output

Given a 16-bit image filled with [0x7FFF 0x7FFF 0x7FFF]/65535 = [.5 .5 .5], RawTherapee should output sRGB = [0.744 0.726 0.812] = [190 185 207], assuming the ICC profile contains the identity matrix. Other color values (using the identity matrix) can be verified using this color calculator:

• Welcome to Bruce Lindbloom's Web Site
  Set Ref. White = D50, since ICC profiles use D50 as the illuminant

The generalized formula to determine the sRGB output color for any rawColor and colorMatrix is:

srgb = SRGBFromLSRGB(LSRGB_From_XYZ * XYZ_D65_From_XYZ_D50_Bradford * colorMatrix * rawColor)

where:

// From brucelindbloom.com
LSRGB_From_XYZ = [ 3.2404542 -1.5371385 -0.4985314 ; -0.9692660 1.8760108 0.0415560 ; 0.0556434 -0.2040259 1.0572252 ]
XYZ_D65_From_XYZ_D50_Bradford = [ 0.9555766 -0.0230393 0.0631636 ; -0.0282895 1.0099416 0.0210077 ; 0.0122982 -0.0204830 1.3299098 ]

float SRGBFromLSRGB(float x) {
    if (x <= 0.0031308) return 12.92*x;
    return 1.055*pow(x, 1/2.4) - .055;
}

Note that SRGBFromLSRGB() is applied to each component separately.

---

Anyway hopefully that helps someone in the future.

David