attr/m49601: add YCgCoR colour transform support

This commit adds the lossless YCgCoR colour transform.
The CTC configuration is updated to use YCgCoR for lossless
attribute coding instead of inter-component prediction on
GBR (colourMatrix=0) data.

cfg/octree-predt-ctc-lossless-geom-lossless-attrs.yaml

@@ -21,7 +21,8 @@ categories:
      # attribute coding (common options -- relies on option ordering)
      #  - skip attribute coding if dist2 parameters are not defined
      #     -- this may seem odd, but is required for lossy-geom data
-     #  - avoid measuring loss of colourspace conversion
+     #  - use YCgCoR colour space to avoid conversion losses
+     #    NB: inter component prediction doesn't work well in non-RGB space
      #  - scale 16bit reflectance data to 8bit
      #  - use predicting transform for lossless conditions
      #  - automatically derive dist2 based on single initial value by the encoder:
@@ -29,7 +30,7 @@ categories:
      #    - generates dist2 per lod
      -
        - !conditional 'defined ${seq_lod} && defined ${seq_dist2}'
-       - convertPlyColourspace: 0
+       - convertPlyColourspace: 1
        -
          - !conditional '${reflectance8b16b_scale_factor}'
          - hack.reflectanceScale: ${reflectance8b16b_scale_factor}
@@ -43,7 +44,7 @@ categories:
        - positionQuantizationScaleAdjustsDist2: 1
        - dist2: ${seq_dist2}
        - intraLodPredictionEnabled: 1
-       - interComponentPredictionEnabled: 1
+       - interComponentPredictionEnabled: 0
 
        ##
        # attribute coding -- reflectance
@@ -62,12 +63,12 @@ categories:
          - qp: 4
          - qpChromaOffset: 0
          - bitdepth: 8
-         - colourMatrix: 0
+         - colourMatrix: 8
          - attribute: color
 
     decflags:
      - mode: 1
-     - convertPlyColourspace: 0
+     - convertPlyColourspace: 1
      -
        - !conditional '${reflectance8b16b_scale_factor}'
        - hack.reflectanceScale: ${reflectance8b16b_scale_factor}

doc/README.options.md

@@ -302,11 +302,14 @@ decoder if supported.
   | 5     | No            | YCbCr ITU-R BT.601                        |
   | 6     | No            | YCbCr SMPTE 170M                          |
   | 7     | No            | YCbCr SMPTE 240M                          |
-  | 8     | No            | YCgCo / YCgCoR                            |
+  | 8     | Yes (YCgCoR)  | YCgCo / YCgCoR                            |
   | 9     | No            | YCbCr ITU-R BT.2020                       |
   | 10    | No            | YCbCr ITU-R BT.2020 (constant luminance)  |
   | 11    | No            | YDzDx SMPTE ST 2085                       |
 
+NB: the use of YCgCoR and `bitdepth=N` implies that the bitdepth of the
+chroma component bitdepth is N + 1.
+
 ### `--bitdepth=INT-VALUE`
 The bitdepth of the attribute data.  NB, this is not necessarily the
 same as the bitdepth of the PLY property.  

tmc3/TMC3.cpp

@@ -561,7 +561,8 @@ ParseParameters(int argc, char* argv[], Parameters& params)
     params_attr.desc.cicp_matrix_coefficients_idx, ColourMatrix::kBt709,
     "Matrix used in colourspace conversion\n"
     "  0: none (identity)\n"
-    "  1: ITU-T BT.709")
+    "  1: ITU-T BT.709\n"
+    "  8: YCgCo")
 
   ("transformType",
     params_attr.aps.attr_encoding, AttributeEncoding::kPredictingTransform,
@@ -775,6 +776,10 @@ ParseParameters(int argc, char* argv[], Parameters& params)
     //  -- but there are a few edge cases to handle
     attr_sps.attr_bitdepth_secondary = attr_sps.attr_bitdepth;
 
+    // Assume that YCgCo is actually YCgCoR for now
+    if (attr_sps.cicp_matrix_coefficients_idx == ColourMatrix::kYCgCo)
+      attr_sps.attr_bitdepth_secondary++;
+
     bool isLifting =
       attr_aps.attr_encoding == AttributeEncoding::kPredictingTransform
       || attr_aps.attr_encoding == AttributeEncoding::kLiftingTransform;
@@ -1219,6 +1224,11 @@ convertToGbr(const SequenceParameterSet& sps, PCCPointSet3& cloud)
   switch (attrDesc->cicp_matrix_coefficients_idx) {
   case ColourMatrix::kBt709: convertYCbCrBt709ToGbr(cloud); break;
 
+  case ColourMatrix::kYCgCo:
+    // todo(df): select YCgCoR vs YCgCo
+    convertYCgCoRToGbr(attrDesc->attr_bitdepth, cloud);
+    break;
+
   default: break;
   }
 }
@@ -1235,6 +1245,11 @@ convertFromGbr(const SequenceParameterSet& sps, PCCPointSet3& cloud)
   switch (attrDesc->cicp_matrix_coefficients_idx) {
   case ColourMatrix::kBt709: convertGbrToYCbCrBt709(cloud); break;
 
+  case ColourMatrix::kYCgCo:
+    // todo(df): select YCgCoR vs YCgCo
+    convertGbrToYCgCoR(attrDesc->attr_bitdepth, cloud);
+    break;
+
   default: break;
   }
 }

tmc3/colourspace.h

@@ -79,4 +79,50 @@ transformYCbCrBt709ToGbr(T<Tv>& ycbcr)
 
 //============================================================================
 
+template<template<typename> class T, typename Tv>
+T<Tv>
+transformGbrToYCgCoR(int bitDepth, T<Tv>& gbr)
+{
+  int g = gbr[0];
+  int b = gbr[1];
+  int r = gbr[2];
+
+  int co = r - b;
+  int t = b + (co >> 1);
+  int cg = g - t;
+  int y = t + (cg >> 1);
+
+  int offset = 1 << bitDepth;
+
+  // NB: YCgCoR needs extra 1-bit for chroma
+  return {Tv(y), Tv(cg + offset), Tv(co + offset)};
+}
+
+//============================================================================
+
+template<template<typename> class T, typename Tv>
+T<Tv>
+transformYCgCoRToGbr(int bitDepth, T<Tv>& ycgco)
+{
+  int offset = 1 << bitDepth;
+  int y0 = ycgco[0];
+  int cg = ycgco[1] - offset;
+  int co = ycgco[2] - offset;
+
+  int t = y0 - (cg >> 1);
+
+  int g = cg + t;
+  int b = t - (co >> 1);
+  int r = co + b;
+
+  int maxVal = (1 << bitDepth) - 1;
+  g = PCCClip(g, 0, maxVal);
+  b = PCCClip(b, 0, maxVal);
+  r = PCCClip(r, 0, maxVal);
+
+  return {Tv(g), Tv(b), Tv(r)};
+}
+
+//============================================================================
+
 }  // namespace pcc

tmc3/pointset_processing.cpp

@@ -912,6 +912,28 @@ recolour(
 
 //============================================================================
 
+void
+convertGbrToYCgCoR(int bitDepth, PCCPointSet3& cloud)
+{
+  for (int i = 0; i < cloud.getPointCount(); i++) {
+    auto& val = cloud.getColor(i);
+    val = transformGbrToYCgCoR(bitDepth, val);
+  }
+}
+
+//============================================================================
+
+void
+convertYCgCoRToGbr(int bitDepth, PCCPointSet3& cloud)
+{
+  for (int i = 0; i < cloud.getPointCount(); i++) {
+    auto& val = cloud.getColor(i);
+    val = transformYCgCoRToGbr(bitDepth, val);
+  }
+}
+
+//============================================================================
+
 void
 convertGbrToYCbCrBt709(PCCPointSet3& cloud)
 {

tmc3/pointset_processing.h

@@ -180,6 +180,9 @@ int recolour(
 void convertGbrToYCbCrBt709(PCCPointSet3&);
 void convertYCbCrBt709ToGbr(PCCPointSet3&);
 
+void convertGbrToYCgCoR(int bitDepth, PCCPointSet3&);
+void convertYCgCoRToGbr(int bitDepth, PCCPointSet3&);
+
 //============================================================================
 
 }  // namespace pcc