Fixes for decoding to A8.

src/images/SkImageDecoder_libpng.cpp

Index: src/images/SkImageDecoder_libpng.cpp
diff --git a/src/images/SkImageDecoder_libpng.cpp b/src/images/SkImageDecoder_libpng.cpp
index fc220eae1a098c73892ef130a62b4c6ba6568ab7..d3b45c3847488f013d3746fdd12ecac6e9500b25 100644
--- a/src/images/SkImageDecoder_libpng.cpp
+++ b/src/images/SkImageDecoder_libpng.cpp
@@ -435,6 +435,9 @@ bool SkPNGImageDecoder::onDecode(SkStream* sk_stream, SkBitmap* decodedBitmap,
         // return false, since the result will have premultiplied colors.
         return false;
     }
+    if (SkBitmap::kA8_Config == decodedBitmap->config()) {

[reed1, 2013/10/08 16:25:57]

Shouldn't this block be done before line 431?

+        reallyHasAlpha = true;

[reed1, 2013/10/04 08:56:27]

I thought this variable was set when we actually saw a non-opaque value. If that
is true, why do we need to force it to true here? What if the alpha-only PNG was
in fact 100% opaque?

[scroggo, 2013/10/04 19:45:53]

Setting the image to be opaque is an optimization for drawing fast. An alpha
only PNG that is 100% opaque is not very interesting, and I suspect not used
much. Right now we don't know whether the image has alpha for certain, and we'd
have to look at each pixel to determine the answer (we could update the sampler
to check, but in the fast case of skipping the sampler, we don't look at the
pixels directly). I don't think it makes sense to add the opaque optimization
for the rare case while penalizing the common case.

[reed1, 2013/10/07 13:31:39]

Ah, I think I see. You're saying we don't always perform the check for
reallyHasAlpha (I thought we did). But now I see around line 350 we seem to not
inspect the values. If that is the case, I suggest we a the line

reallyHasAlpha = true;

... in that spot, with a comment to the effect of // likely we do have non-0xFF,
but faster not to check...
rather than down here.

[scroggo, 2013/10/07 22:10:29]

Sounds good to me. Uploaded https://codereview.chromium.org/26210007/, which
includes fixes for other bugs I found while making these changes.

+    }
     decodedBitmap->setIsOpaque(!reallyHasAlpha);
     return true;
 }
@@ -888,6 +891,9 @@ bool SkPNGImageDecoder::onDecodeSubset(SkBitmap* bm, const SkIRect& region) {
     if (0 != theTranspColor) {
         reallyHasAlpha |= substituteTranspColor(&decodedBitmap, theTranspColor);
     }
+    if (SkBitmap::kA8_Config == decodedBitmap.config()) {
+        reallyHasAlpha = true;
+    }
     decodedBitmap.setIsOpaque(!reallyHasAlpha);
 
     if (swapOnly) {