From 1c58e179b59408ed813a767f4d02096a3f4aca6b Mon Sep 17 00:00:00 2001
From: Kelsey Gilbert <kelsey.gilbert@mozilla.com>
Date: Mon, 6 Feb 2023 19:58:54 +0000
Subject: [PATCH] Bug 1799258 - No-op equal tfs rather than inverting.
 r=bradwerth

Differential Revision: https://phabricator.services.mozilla.com/D167471
---
 gfx/gl/Colorspaces.cpp           | 31 ++++++++++++++++++++-----------
 gfx/gl/Colorspaces.h             | 13 ++++++++-----
 gfx/gl/gtest/TestColorspaces.cpp |  2 +-
 3 files changed, 29 insertions(+), 17 deletions(-)

diff --git a/gfx/gl/Colorspaces.cpp b/gfx/gl/Colorspaces.cpp
index 44b96b53a596..c7c40b2abaa8 100644
--- a/gfx/gl/Colorspaces.cpp
+++ b/gfx/gl/Colorspaces.cpp
@@ -409,17 +409,26 @@ ColorProfileConversionDesc ColorProfileConversionDesc::From(
           dstLinearRgbFromXyzd65 * desc.src.xyzd65FromLinearRgb,
       .dstTfFromDstLinear = {},
   };
-  const auto Invert = [](const std::vector<float>& linearFromTf,
-                         std::vector<float>* const tfFromLinear) {
-    const auto size = linearFromTf.size();
-    MOZ_ASSERT(size != 1);  // Less than two is uninvertable.
-    if (size < 2) return;
-    (*tfFromLinear).resize(size);
-    InvertLut(linearFromTf, &*tfFromLinear);
-  };
-  Invert(desc.dst.linearFromTf.r, &ret.dstTfFromDstLinear.r);
-  Invert(desc.dst.linearFromTf.g, &ret.dstTfFromDstLinear.g);
-  Invert(desc.dst.linearFromTf.b, &ret.dstTfFromDstLinear.b);
+  bool sameTF = true;
+  sameTF &= desc.src.linearFromTf.r == desc.dst.linearFromTf.r;
+  sameTF &= desc.src.linearFromTf.g == desc.dst.linearFromTf.g;
+  sameTF &= desc.src.linearFromTf.b == desc.dst.linearFromTf.b;
+  if (sameTF) {
+    ret.srcLinearFromSrcTf = {};
+    ret.dstTfFromDstLinear = {};
+  } else {
+    const auto Invert = [](const std::vector<float>& linearFromTf,
+                           std::vector<float>* const tfFromLinear) {
+      const auto size = linearFromTf.size();
+      MOZ_ASSERT(size != 1);  // Less than two is uninvertable.
+      if (size < 2) return;
+      (*tfFromLinear).resize(size);
+      InvertLut(linearFromTf, &*tfFromLinear);
+    };
+    Invert(desc.dst.linearFromTf.r, &ret.dstTfFromDstLinear.r);
+    Invert(desc.dst.linearFromTf.g, &ret.dstTfFromDstLinear.g);
+    Invert(desc.dst.linearFromTf.b, &ret.dstTfFromDstLinear.b);
+  }
   return ret;
 }
 
diff --git a/gfx/gl/Colorspaces.h b/gfx/gl/Colorspaces.h
index fb7f346ebe40..1b4f6f9ba48b 100644
--- a/gfx/gl/Colorspaces.h
+++ b/gfx/gl/Colorspaces.h
@@ -81,7 +81,7 @@ struct PiecewiseGammaDesc final {
         4.5,
     };
   }
-  // static constexpr auto Rec2020_10bit() { return Rec709(); }
+  // FYI: static constexpr auto Rec2020_10bit() { return Rec709(); }
   static constexpr auto Rec2020_12bit() {
     return PiecewiseGammaDesc{
         1.0993,
@@ -704,7 +704,7 @@ mat3 XyzAFromXyzB_BradfordLinear(const vec2 xyA, const vec2 xyB);
 struct ColorProfileDesc {
   // ICC profiles are phrased as PCS-from-encoded (PCS is CIEXYZ-D50)
   // However, all of our colorspaces are D65, so let's normalize to that,
-  // even though it's a reverseable transform.
+  // even though it's a reversible transform.
   color::mat4 rgbFromYcbcr = color::mat4::Identity();
   RgbTransferTables linearFromTf;
   color::mat3 xyzd65FromLinearRgb = color::mat3::Identity();
@@ -743,7 +743,7 @@ inline float SampleInByOut(const C& outByIn, const float out) {
   const auto d_in = float(1) / (outByIn.size() - 1);
   const auto d_out = *(out0_itr + 1) - *out0_itr;
 
-  // printf("%f - (%f / %f) * (%f - %f)\n", in0, d_in, d_out, out, out0);
+  // printf("%f + (%f / %f) * (%f - %f)\n", in0, d_in, d_out, out, out0);
   const auto in = in0 + (d_in / d_out) * (out - out0);
   // printf("SampleInByOut(%f)->%f\n", out, in);
   return in;
@@ -786,10 +786,13 @@ struct TwoPoints {
   } p1;
 
   T y(const T x) const {
-    return p0.y + (x - p0.x) / (p1.x - p0.x) * (p1.y - p0.y);
+    const auto dx = p1.x - p0.x;
+    const auto dy = p1.y - p0.y;
+    return p0.y + dy / dx * (x - p0.x);
   }
 };
 
+/// Fills `vals` with `x:[0..vals.size()-1] => line.y(x)`.
 template <class T>
 static void LinearFill(T& vals, const TwoPoints<float>& line) {
   float x = -1;
@@ -837,7 +840,7 @@ inline void DequantizeMonotonic(const Span<float> vals) {
   // E.g. [0,1,1,2]
   //         ^^^ body
   // => f(0.5)->0.5, f(2.5)->1.5
-  // => f(x) = f(x0) + (x-x0) * (f(x1) - f(x0)) / (x1-x1)
+  // => f(x) = f(x0) + (x-x0) * (f(x1) - f(x0)) / (x1-x0)
   // => f(x) = f(x0) + (x-x0) * dfdx
 
   const auto head_end = *body_first;
diff --git a/gfx/gl/gtest/TestColorspaces.cpp b/gfx/gl/gtest/TestColorspaces.cpp
index 19886814e61c..c437e204af02 100644
--- a/gfx/gl/gtest/TestColorspaces.cpp
+++ b/gfx/gl/gtest/TestColorspaces.cpp
@@ -528,7 +528,7 @@ TEST(Colorspaces, GuessGamma)
 {
   EXPECT_NEAR(GuessGamma(MakeGamma(1, 11)), 1.0, 0);
   EXPECT_NEAR(GuessGamma(MakeGamma(2.2, 11)), 2.2, 4.8e-8);
-  EXPECT_NEAR(GuessGamma(MakeGamma(1 / 2.2, 11)), 1 / 2.2, 1.1e-7);
+  EXPECT_NEAR(GuessGamma(MakeGamma(1 / 2.2, 11)), 1 / 2.2, 1.7e-7);
 }
 
 // -
-- 
2.11.4.GIT