Add Alpha8 support to SkPNGImageEncoder

src/codec/SkPngCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
@@ skipping 746 matching lines @@
         if (SkEncodedInfo::kOpaque_Alpha == alpha) {
             png_color_8p sigBits;
             if (png_get_sBIT(png_ptr, info_ptr, &sigBits)) {
                 if (5 == sigBits->red && 6 == sigBits->green && 5 == sigBits->blue) {
                     // Recommend a decode to 565 if the sBIT indicates 565.
                     imageInfo = imageInfo.makeColorType(kRGB_565_SkColorType);
                 }
             }
         }
 
+        if (PNG_COLOR_TYPE_GRAY == encodedColorType) {
+            png_textp text;
+            if (png_get_text(png_ptr, info_ptr, &text, nullptr)) {
+                if (0 == strcmp("SkColorType", text->key) && 0 == strcmp("Alpha8", text->text)) {

[msarett, 2016/08/30 14:30:38]

Ideally, we store these strings somewhere that is shared by the decoder and
encoder... Any ideas?

+                    // Recommend a decode to Alpha8 if our custom encoder has marked
+                    // the image as Alpha8.
+                    imageInfo = imageInfo.makeColorType(kAlpha_8_SkColorType)
+                                         .makeAlphaType(kPremul_SkAlphaType);
+                }
+            }
+        }
+
         if (1 == numberPasses) {
             *outCodec = new SkPngNormalCodec(encodedInfo, imageInfo, stream,
                     chunkReader, png_ptr, info_ptr, bitDepth);
         } else {
             *outCodec = new SkPngInterlacedCodec(encodedInfo, imageInfo, stream,
                     chunkReader, png_ptr, info_ptr, bitDepth, numberPasses);
         }
     }
 
     return true;
@@ skipping 149 matching lines @@
     SkCodec* outCodec;
     if (read_header(stream, chunkReader, &outCodec, nullptr, nullptr)) {
         // Codec has taken ownership of the stream.
         SkASSERT(outCodec);
         streamDeleter.release();
         return outCodec;
     }
 
     return nullptr;
 }