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

* Addressing issue #1915, remove [0, 1] clamping from ICC profiles.

Currently the clamping is removed ONLY when the the type 0 (simple gamma) TRC is used. In this case the the mirroring around origin is used for the negative values.

Other parametric curves are currently implemented as 1DLuts thus will not handle out of bound values properly. In a later wave we can either convert LUTs to half-domain ones or try to implement them with more complex ops (such as exponent with linear).

Signed-off-by: cuneyt.ozdas <cuneyt.ozdas@autodesk.com>

* - Fixing the op tests in the inverse direction
- More detailed explanation for the clamp removal in pure-gamma TRC cases.

Signed-off-by: cuneyt.ozdas <cuneyt.ozdas@autodesk.com>

---------

Signed-off-by: cuneyt.ozdas <cuneyt.ozdas@autodesk.com>
Co-authored-by: Doug Walker <doug.walker@autodesk.com>

Author: cozdas

src/OpenColorIO/fileformats/FileFormatICC.cpp

@@ -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"
 
@@ -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)
     {
@@ -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);
         }
 
@@ -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;
     }

tests/cpu/fileformats/FileFormatICC_tests.cpp

@@ -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));
@@ -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,
@@ -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.
@@ -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);
         }
     }