Support color xforms for kIndex8 pngs

src/codec/SkCodecPriv.h

 /*
  * Copyright 2015 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkCodecPriv_DEFINED
 #define SkCodecPriv_DEFINED
 
@@ skipping 292 matching lines @@
         }
     } else {
         if (isRGBA) {
             return &SkPackARGB_as_RGBA;
         } else {
             return &SkPackARGB_as_BGRA;
         }
     }
 }
 
+static inline bool needs_premul(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo) {
+    return kPremul_SkAlphaType == dstInfo.alphaType() &&
+           kUnpremul_SkAlphaType == srcInfo.alphaType();
+}
+
 static inline bool needs_color_xform(const SkImageInfo& dstInfo, const SkImageInfo& srcInfo) {
-    return (kRGBA_F16_SkColorType == dstInfo.colorType()) ||
-           (dstInfo.colorSpace() && !SkColorSpace::Equals(srcInfo.colorSpace(),
-                                                          dstInfo.colorSpace()));
+    // Color xform is necessary in order to correctly perform premultiply in linear space.
+    bool needsPremul = needs_premul(dstInfo, srcInfo);
+
+    // F16 is by definition a linear space, so we always must perform a color xform.
+    bool isF16 = kRGBA_F16_SkColorType == dstInfo.colorType();
+
+    // Need a color xform when dst space does not match the src.
+    bool srcDstNotEqual = !SkColorSpace::Equals(srcInfo.colorSpace(), dstInfo.colorSpace());
+
+    // We never perform a color xform in legacy mode.
+    bool isLegacy = nullptr == dstInfo.colorSpace();
+
+    return !isLegacy && (needsPremul || isF16 || srcDstNotEqual);
 }
 
 #endif // SkCodecPriv_DEFINED