Convert XYZ values from PNGs to D50

src/codec/SkPngCodec.cpp

Index: src/codec/SkPngCodec.cpp
diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 32308b263095676990884503a51376efb75ceabd..ee72af58ad46c517e3ec63da18746d4fe69257ef 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -13,6 +13,7 @@
 #include "SkMath.h"
 #include "SkOpts.h"
 #include "SkPngCodec.h"
+#include "SkPoint3.h"
 #include "SkSize.h"
 #include "SkStream.h"
 #include "SkSwizzler.h"
@@ -178,6 +179,62 @@ static constexpr float gSRGB_toXYZD50[] {
     0.1430f, 0.0606f, 0.7139f,    // * B
 };
 
+static bool convert_to_D50(SkMatrix44* toXYZD50, float toXYZ[9], float whitePoint[2]) {
+    float wX = whitePoint[0];
+    float wY = whitePoint[1];
+    if (wX < 0.0f || wY < 0.0f || (wX + wY > 1.0f)) {
+        return false;
+    }
+
+    // Calculate the XYZ illuminant.  Call this the src illuminant.
+    float wZ = 1.0f - wX - wY;
+    float scale = 1.0f / wY;
+    // TODO (msarett):
+    // What are common src illuminants?  I'm guessing we will almost always see D65.  Should
+    // we go ahead and save a precomputed D65->D50 Bradford matrix?  Should we exit early if
+    // if the src illuminant is D50?
+    SkVector3 srcXYZ = SkVector3::Make(wX * scale, 1.0f, wZ * scale);
+
+    // The D50 illuminant.
+    SkVector3 dstXYZ = SkVector3::Make(0.96422f, 1.0f, 0.82521f);
+
+    // Calculate the chromatic adaptation matrix.  We will use the Bradford method, thus
+    // the matrices below.  The Bradford method is used by Adobe and is widely considered
+    // to be the best.
+    // http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html
+    SkMatrix mA, mAInv;
+    mA.setAll(0.8951f, 0.2664f, -0.1614f, -0.7502f, 1.7135f, 0.0367f, 0.0389f, -0.0685f, 1.0296f);
+    mAInv.setAll(0.9869929f, -0.1470543f, 0.1599627f, 0.4323053f, 0.5183603f, 0.0492912f,
+                 -0.0085287f, 0.0400428f, 0.9684867f);
+
+    // Map illuminant into cone response domain.
+    SkVector3 srcCone;
+    srcCone.fX = mA[0] * srcXYZ.fX + mA[1] * srcXYZ.fY + mA[2] * srcXYZ.fZ;
+    srcCone.fY = mA[3] * srcXYZ.fX + mA[4] * srcXYZ.fY + mA[5] * srcXYZ.fZ;
+    srcCone.fZ = mA[6] * srcXYZ.fX + mA[7] * srcXYZ.fY + mA[8] * srcXYZ.fZ;
+    SkVector3 dstCone;
+    dstCone.fX = mA[0] * dstXYZ.fX + mA[1] * dstXYZ.fY + mA[2] * dstXYZ.fZ;
+    dstCone.fY = mA[3] * dstXYZ.fX + mA[4] * dstXYZ.fY + mA[5] * dstXYZ.fZ;
+    dstCone.fZ = mA[6] * dstXYZ.fX + mA[7] * dstXYZ.fY + mA[8] * dstXYZ.fZ;
+
+    SkMatrix DXToD50;
+    DXToD50.setIdentity();
+    DXToD50[0] = dstCone.fX / srcCone.fX;
+    DXToD50[4] = dstCone.fY / srcCone.fY;
+    DXToD50[8] = dstCone.fZ / srcCone.fZ;
+    DXToD50.postConcat(mAInv);
+    DXToD50.preConcat(mA);
+
+    SkMatrix toXYZ3x3;
+    toXYZ3x3.setAll(toXYZ[0], toXYZ[3], toXYZ[6], toXYZ[1], toXYZ[4], toXYZ[7], toXYZ[2], toXYZ[5],
+                    toXYZ[8]);
+    toXYZ3x3.postConcat(DXToD50);
+
+    toXYZD50->set3x3(toXYZ3x3[0], toXYZ3x3[1], toXYZ3x3[2], toXYZ3x3[3], toXYZ3x3[4], toXYZ3x3[5],
+                     toXYZ3x3[6], toXYZ3x3[7], toXYZ3x3[8]);
+    return true;
+}
+
 // Returns a colorSpace object that represents any color space information in
 // the encoded data.  If the encoded data contains no color space, this will
 // return NULL.
@@ -213,24 +270,28 @@ sk_sp<SkColorSpace> read_color_space(png_structp png_ptr, png_infop info_ptr) {
     }
 
     // Next, check for chromaticities.
-    png_fixed_point XYZ[9];
-    float toXYZD50[9];
+    png_fixed_point toXYZFixed[9];
+    float toXYZ[9];
+    png_fixed_point whitePointFixed[2];
+    float whitePoint[2];
     png_fixed_point gamma;
     float gammas[3];
-    if (png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &XYZ[0], &XYZ[1], &XYZ[2], &XYZ[3], &XYZ[4],
-            &XYZ[5], &XYZ[6], &XYZ[7], &XYZ[8])) {
-
-        // FIXME (msarett): Here we are treating XYZ values as D50 even though the color
-        //                  temperature is unspecified.  I suspect that this assumption
-        //                  is most often ok, but we could also calculate the color
-        //                  temperature (D value) and then convert the XYZ to D50.  Maybe
-        //                  we should add a new constructor to SkColorSpace that accepts
-        //                  XYZ with D-Unkown?
+    if (png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &toXYZFixed[0], &toXYZFixed[1], &toXYZFixed[2],
+                               &toXYZFixed[3], &toXYZFixed[4], &toXYZFixed[5], &toXYZFixed[6],
+                               &toXYZFixed[7], &toXYZFixed[8]) &&
+        png_get_cHRM_fixed(png_ptr, info_ptr, &whitePointFixed[0], &whitePointFixed[1], nullptr,
+                           nullptr, nullptr, nullptr, nullptr, nullptr))
+    {
         for (int i = 0; i < 9; i++) {
-            toXYZD50[i] = png_fixed_point_to_float(XYZ[i]);
+            toXYZ[i] = png_fixed_point_to_float(toXYZFixed[i]);
+        }
+        whitePoint[0] = png_fixed_point_to_float(whitePointFixed[0]);
+        whitePoint[1] = png_fixed_point_to_float(whitePointFixed[1]);
+
+        SkMatrix44 toXYZD50(SkMatrix44::kUninitialized_Constructor);
+        if (!convert_to_D50(&toXYZD50, toXYZ, whitePoint)) {
+            toXYZD50.set3x3RowMajorf(gSRGB_toXYZD50);
         }
-        SkMatrix44 mat(SkMatrix44::kUninitialized_Constructor);
-        mat.set3x3RowMajorf(toXYZD50);
 
         if (PNG_INFO_gAMA == png_get_gAMA_fixed(png_ptr, info_ptr, &gamma)) {
             float value = png_inverted_fixed_point_to_float(gamma);
@@ -238,12 +299,12 @@ sk_sp<SkColorSpace> read_color_space(png_structp png_ptr, png_infop info_ptr) {
             gammas[1] = value;
             gammas[2] = value;
 
-            return SkColorSpace_Base::NewRGB(gammas, mat);
+            return SkColorSpace_Base::NewRGB(gammas, toXYZD50);
         }
 
         // Default to sRGB gamma if the image has color space information,
         // but does not specify gamma.
-        return SkColorSpace::NewRGB(SkColorSpace::kSRGB_GammaNamed, mat);
+        return SkColorSpace::NewRGB(SkColorSpace::kSRGB_GammaNamed, toXYZD50);
     }
 
     // Last, check for gamma.
@@ -256,10 +317,10 @@ sk_sp<SkColorSpace> read_color_space(png_structp png_ptr, png_infop info_ptr) {
         gammas[2] = value;
 
         // Since there is no cHRM, we will guess sRGB gamut.
-        SkMatrix44 mat(SkMatrix44::kUninitialized_Constructor);
-        mat.set3x3RowMajorf(gSRGB_toXYZD50);
+        SkMatrix44 toXYZD50(SkMatrix44::kUninitialized_Constructor);
+        toXYZD50.set3x3RowMajorf(gSRGB_toXYZD50);
 
-        return SkColorSpace_Base::NewRGB(gammas, mat);
+        return SkColorSpace_Base::NewRGB(gammas, toXYZD50);
     }
 
 #endif // LIBPNG >= 1.6