Add Alpha8 support to SkPNGImageEncoder

src/images/SkPNGImageEncoder.cpp

Index: src/images/SkPNGImageEncoder.cpp
diff --git a/src/images/SkPNGImageEncoder.cpp b/src/images/SkPNGImageEncoder.cpp
index a9c8b3d7857e2dd604fe9a531e3d676b04158c0e..445f8c73e8a41289c84e48b57828360925e5131e 100644
--- a/src/images/SkPNGImageEncoder.cpp
+++ b/src/images/SkPNGImageEncoder.cpp
@@ -77,6 +77,7 @@ static transform_scanline_proc choose_proc(SkColorType ct, bool hasAlpha) {
         { kARGB_4444_SkColorType,   true,   transform_scanline_4444 },
         { kIndex_8_SkColorType,     false,  transform_scanline_memcpy },
         { kGray_8_SkColorType,      false,  transform_scanline_memcpy },
+        { kAlpha_8_SkColorType,     true,   transform_scanline_memcpy },
     };
 
     for (int i = SK_ARRAY_COUNT(gMap) - 1; i >= 0; --i) {
@@ -180,13 +181,13 @@ bool SkPNGImageEncoder::onEncode(SkWStream* stream,
     switch (originalBitmap.colorType()) {
         case kIndex_8_SkColorType:
         case kGray_8_SkColorType:
+        case kAlpha_8_SkColorType:
         case kN32_SkColorType:
         case kARGB_4444_SkColorType:
         case kRGB_565_SkColorType:
             break;
         default:
             // TODO(scroggo): support 8888-but-not-N32 natively.
-            // TODO(scroggo): support Alpha_8 as Grayscale(black)+Alpha
             if (originalBitmap.copyTo(&copy, kN32_SkColorType)) {
                 bitmap = ©
             }
@@ -212,6 +213,15 @@ bool SkPNGImageEncoder::onEncode(SkWStream* stream,
             colorType = PNG_COLOR_TYPE_GRAY;
             SkASSERT(!hasAlpha);
             break;
+        case kAlpha_8_SkColorType:
+            // This is a bit of a trick.  PNG does not have an alpha-only format, so
+            // we tell libpng that we are encoding a gray image.  We'll also encode
+            // an extra custom chunk, so our decoder knows that the image is actually
+            // Alpha8.
+            sig_bit.gray = 8;
+            colorType = PNG_COLOR_TYPE_GRAY;
+            SkASSERT(hasAlpha);
+            break;
         case kN32_SkColorType:
             sig_bit.red = 8;
             sig_bit.green = 8;
@@ -313,6 +323,22 @@ bool SkPNGImageEncoder::doEncode(SkWStream* stream, const SkBitmap& bitmap,
     }
 
     png_set_sBIT(png_ptr, info_ptr, &sig_bit);
+
+    if (kAlpha_8_SkColorType == ct) {
+        static constexpr char key[] = "SkColorType";
+        static constexpr char value[] = "Alpha8";
+
+        png_text text;
+        text.compression = 0; // Use deflate for text compression.
+        text.key = const_cast<char*>(key);
+        text.text = const_cast<char*>(value);
+        text.text_length = sizeof(value) - 1;
+        text.itxt_length = 0;
+        text.lang = nullptr;
+        text.lang_key = nullptr;
+        png_set_text(png_ptr, info_ptr, &text, 1);
+    }
+
     png_write_info(png_ptr, info_ptr);
 
     const char* srcImage = (const char*)bitmap.getPixels();