Handle gray alpha conversions in SkSwizzler

Nothing fancy here.  By doing the entire conversion ourselves, we
only need to make one conversion pass over each row.  Additionally,
we optimize the premultiply since we know each color component of
the pixel is identical.

This will also enable us to follow up with platform specific
optimizations.

PNG Decode Time Nexus 6P (for a test set of GrayAlpha encoded PNGs)
Regular   Unpremul 0.95x
Zero Init Unpremul 0.94x
Regular   Premul   0.91x
Zero Init Premul   0.90x

BUG=skia:4767
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1665583002

Review URL: https://codereview.chromium.org/1665583002

1 files changed, 24 insertions, 9 deletions

diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 710630d472..232373c044 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -297,9 +297,10 @@ static bool read_header(SkStream* stream, SkPngChunkReader* chunkReader,
             }
 
             if (png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS)) {
-                // Convert to RGBA if there is a transparency chunk.
                 png_set_tRNS_to_alpha(png_ptr);
-                png_set_gray_to_rgb(png_ptr);
+
+                // We will recommend kN32 here since we do not support kGray
+                // with alpha.
                 colorType = kN32_SkColorType;
                 alphaType = kUnpremul_SkAlphaType;
             } else {
@@ -308,8 +309,8 @@ static bool read_header(SkStream* stream, SkPngChunkReader* chunkReader,
             }
             break;
         case PNG_COLOR_TYPE_GRAY_ALPHA:
-            // Convert to RGBA if the image has alpha.
-            png_set_gray_to_rgb(png_ptr);
+            // We will recommend kN32 here since we do not support anything
+            // similar to GRAY_ALPHA.
             colorType = kN32_SkColorType;
             alphaType = kUnpremul_SkAlphaType;
             break;
@@ -384,10 +385,10 @@ SkCodec::Result SkPngCodec::initializeSwizzler(const SkImageInfo& requestedInfo,
     }
     png_read_update_info(fPng_ptr, fInfo_ptr);
 
-    // srcColorType was determined in read_header() which determined png color type
-    const SkColorType srcColorType = this->getInfo().colorType();
+    // suggestedColorType was determined in read_header() based on the encodedColorType
+    const SkColorType suggestedColorType = this->getInfo().colorType();
 
-    switch (srcColorType) {
+    switch (suggestedColorType) {
         case kIndex_8_SkColorType:
             //decode palette to Skia format
             fSrcConfig = SkSwizzler::kIndex;
@@ -399,13 +400,27 @@ SkCodec::Result SkPngCodec::initializeSwizzler(const SkImageInfo& requestedInfo,
         case kGray_8_SkColorType:
             fSrcConfig = SkSwizzler::kGray;
             break;
-        case kN32_SkColorType:
-            if (this->getInfo().alphaType() == kOpaque_SkAlphaType) {
+        case kN32_SkColorType: {
+            const uint8_t encodedColorType = png_get_color_type(fPng_ptr, fInfo_ptr);
+            if (PNG_COLOR_TYPE_GRAY_ALPHA == encodedColorType ||
+                    PNG_COLOR_TYPE_GRAY == encodedColorType) {
+                // If encodedColorType is GRAY, there must be a transparent chunk.
+                // Otherwise, suggestedColorType would be kGray.  We have already
+                // instructed libpng to convert the transparent chunk to alpha,
+                // so we can treat both GRAY and GRAY_ALPHA as kGrayAlpha.
+                SkASSERT(encodedColorType == PNG_COLOR_TYPE_GRAY_ALPHA ||
+                        png_get_valid(fPng_ptr, fInfo_ptr, PNG_INFO_tRNS));
+
+                fSrcConfig = SkSwizzler::kGrayAlpha;
+            } else {
+                if (this->getInfo().alphaType() == kOpaque_SkAlphaType) {
                     fSrcConfig = SkSwizzler::kRGB;
                 } else {
                     fSrcConfig = SkSwizzler::kRGBA;
+                }
             }
             break;
+        }
         default:
             // We will always recommend one of the above colorTypes.
             SkASSERT(false);