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Remove [0,1] clamping from ICC transforms #2224

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4834497
Addressing issue #1915, remove [0, 1] clamping from ICC profiles.
cozdas Dec 18, 2025
5559c43
- Fixing the op tests in the inverse direction
cozdas Dec 18, 2025
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38 changes: 18 additions & 20 deletions src/OpenColorIO/fileformats/FileFormatICC.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,6 @@
#include "ops/gamma/GammaOp.h"
#include "ops/lut1d/Lut1DOp.h"
#include "ops/matrix/MatrixOp.h"
#include "ops/range/RangeOp.h"
#include "Platform.h"
#include "transforms/FileTransform.h"

Expand Down Expand Up @@ -794,13 +793,23 @@ LocalFileFormat::buildFileOps(OpRcPtrVec & ops,
}
}

// The matrix/TRC transform in the ICC profile converts display device code values to the
// CIE XYZ based version of the ICC profile connection space (PCS).
// However, in OCIO the most common use of an ICC monitor profile is as a display color space,
// and in that usage it is more natural for the XYZ to display code value transform to be called
// the forward direction.
// The matrix/TRC transform in the ICC profile converts display device code
// values to the CIE XYZ based version of the ICC profile connection space
// (PCS). However, in OCIO the most common use of an ICC monitor profile is
// as a display color space, and in that usage it is more natural for the
// XYZ to display code value transform to be called the forward direction.

// Curves / ParaCurves operates in the range 0.0 to 1.0 as per ICC specifications.
// The ICC spec states that the TRC tags should clamp to [0,1]. For curves
// that are implemented in the ICC profile as LUTs and most parametric
// curves (which become LUTs in OCIO), this is the case. However, as
// floating-point and HDR workflows become more common, the clamping has
// become a critical roadblock. For example, it is now common to have ICC
// profiles for linear color spaces that need to pass values outside [0,1].
// Therefore, OCIO now implements single entry 'curv' tags and type 0 'para'
// tags without clamping using an ExponentTransform which extends above 1
// and mirrors below 0. (Note that gamma values of 1 do not need to be
// tested for here since they will be omitted as no-ops later by the
// optimizer.)

switch (newDir)
{
Expand All @@ -817,18 +826,12 @@ LocalFileFormat::buildFileOps(OpRcPtrVec & ops,
const GammaOpData::Params greenParams = { cachedFile->mGammaRGB[1] };
const GammaOpData::Params blueParams = { cachedFile->mGammaRGB[2] };
const GammaOpData::Params alphaParams = { cachedFile->mGammaRGB[3] };
auto gamma = std::make_shared<GammaOpData>(GammaOpData::BASIC_FWD,
auto gamma = std::make_shared<GammaOpData>(GammaOpData::BASIC_MIRROR_FWD,
redParams,
greenParams,
blueParams,
alphaParams);

// GammaOp will clamp at 0 so we don't do it in the RangeOp.
CreateRangeOp(ops,
RangeOpData::EmptyValue(), 1,
RangeOpData::EmptyValue(), 1,
TRANSFORM_DIR_FORWARD);

CreateGammaOp(ops, gamma, TRANSFORM_DIR_FORWARD);
}

Expand Down Expand Up @@ -859,18 +862,13 @@ LocalFileFormat::buildFileOps(OpRcPtrVec & ops,
const GammaOpData::Params greenParams = { cachedFile->mGammaRGB[1] };
const GammaOpData::Params blueParams = { cachedFile->mGammaRGB[2] };
const GammaOpData::Params alphaParams = { cachedFile->mGammaRGB[3] };
auto gamma = std::make_shared<GammaOpData>(GammaOpData::BASIC_REV,
auto gamma = std::make_shared<GammaOpData>(GammaOpData::BASIC_MIRROR_REV,
redParams,
greenParams,
blueParams,
alphaParams);

CreateGammaOp(ops, gamma, TRANSFORM_DIR_FORWARD);

CreateRangeOp(ops,
RangeOpData::EmptyValue(), 1,
RangeOpData::EmptyValue(), 1,
TRANSFORM_DIR_FORWARD);
}
break;
}
Expand Down
42 changes: 26 additions & 16 deletions tests/cpu/fileformats/FileFormatICC_tests.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -244,6 +244,8 @@ OCIO_ADD_TEST(FileFormatICC, test_apply)
{
OCIO::ContextRcPtr context = OCIO::Context::Create();
{
// This test uses a profile where the TRC is a 1024 element LUT.

static const std::string iccFileName("icc-test-3.icm");
OCIO::OpRcPtrVec ops;
OCIO_CHECK_NO_THROW(BuildOpsTest(ops, iccFileName, context, OCIO::TRANSFORM_DIR_INVERSE));
Expand Down Expand Up @@ -287,7 +289,8 @@ OCIO_ADD_TEST(FileFormatICC, test_apply)
op->apply(srcImage, 3);
}

// Values outside [0.0, 1.0] are clamped and won't round-trip.
// Currently the LUT-based TRC's clamp the values outside
// [0.0, 1.0], thus those values won't round-trip.
static constexpr float bckImage[] = {
0.0f, 0.0f, 0.3f, 0.0f,
0.4f, 0.5f, 0.6f, 0.5f,
Expand All @@ -301,34 +304,43 @@ OCIO_ADD_TEST(FileFormatICC, test_apply)
}

{
// This test uses a profile where the TRC is
// a parametric curve of type 0 (basic gamma) with
// gamma values {2.174, 2.174, 2.174, 1.0}.

static const std::string iccFileName("icc-test-2.pf");
OCIO::OpRcPtrVec ops;
OCIO_CHECK_NO_THROW(BuildOpsTest(ops, iccFileName, context, OCIO::TRANSFORM_DIR_INVERSE));
OCIO_CHECK_NO_THROW(ops.finalize());
OCIO_CHECK_NO_THROW(ops.optimize(OCIO::OPTIMIZATION_LOSSLESS));
OCIO_REQUIRE_EQUAL(2, ops.size());
OCIO_CHECK_EQUAL("<GammaOp>", ops[0]->getInfo());
OCIO_CHECK_EQUAL("<MatrixOffsetOp>", ops[1]->getInfo());

// apply ops
float srcImage[] = {
const std::array<float, 12> srcImage{
-0.1f, 0.0f, 0.3f, 0.0f,
0.4f, 0.5f, 0.6f, 0.5f,
0.7f, 1.0f, 1.9f, 1.0f };

const float dstImage[] = {
0.012437f, 0.004702f, 0.070333f, 0.0f,
const std::array<float, 12> dstImage{
0.009241f, 0.003003f, 0.070198f, 0.0f,
0.188392f, 0.206965f, 0.343595f, 0.5f,
0.693246f, 0.863199f, 1.07867f , 1.0f };
1.210462f, 1.058761f, 4.003706f, 1.0f };

std::array<float, 12> testImage = srcImage;

for (const auto & op : ops)
{
op->apply(srcImage, 3);
op->apply(testImage.data(), 3);
}

// Compare results
const float error = 2e-5f;

for (unsigned int i = 0; i<12; ++i)
{
OCIO_CHECK_CLOSE(srcImage[i], dstImage[i], error);
OCIO_CHECK_CLOSE(testImage[i], dstImage[i], error);
}

// Invert the processing.
Expand All @@ -337,24 +349,22 @@ OCIO_ADD_TEST(FileFormatICC, test_apply)
OCIO_CHECK_NO_THROW(BuildOpsTest(opsInv, iccFileName, context, OCIO::TRANSFORM_DIR_FORWARD));
OCIO_CHECK_NO_THROW(opsInv.finalize());
OCIO_CHECK_NO_THROW(opsInv.optimize(OCIO::OPTIMIZATION_LOSSLESS));
OCIO_REQUIRE_EQUAL(2, opsInv.size());
OCIO_CHECK_EQUAL("<MatrixOffsetOp>", opsInv[0]->getInfo());
OCIO_CHECK_EQUAL("<GammaOp>", opsInv[1]->getInfo());

for (const auto & op : opsInv)
{
op->apply(srcImage, 3);
op->apply(testImage.data(), 3);
}

// Values outside [0.0, 1.0] are clamped and won't round-trip.
const float bckImage[] = {
0.0f, 0.0f, 0.3f, 0.0f,
0.4f, 0.5f, 0.6f, 0.5f,
0.7f, 1.0f, 1.0f, 1.0f };

// Compare results
// For pure-gamma TRCs, values outside [0.0, 1.0] are NOT clamped
// thus those values should round-trip correctly.
const float error2 = 2e-4f;

for (unsigned int i = 0; i<12; ++i)
{
OCIO_CHECK_CLOSE(srcImage[i], bckImage[i], error2);
OCIO_CHECK_CLOSE(testImage[i], srcImage[i], error2);
}
}

Expand Down