Add SkPngChunkReader.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index d90fea8cedcf6d92ecdc8535e71962faaa376689..dffab6b3779207aeba99084810afb27d8416a097 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -17,6 +17,7 @@
 #include "SkTypes.h"
 
 class SkData;
+class SkPngChunkReader;
 class SkSampler;
 
 /**
@@ -28,18 +29,24 @@ public:
      *  If this stream represents an encoded image that we know how to decode,
      *  return an SkCodec that can decode it. Otherwise return NULL.
      *
+     *  If SkPngChunkReader is not NULL, take a ref and pass it to libpng if
+     *  the image is a png.
+     *
      *  If NULL is returned, the stream is deleted immediately. Otherwise, the
      *  SkCodec takes ownership of it, and will delete it when done with it.
      */
-    static SkCodec* NewFromStream(SkStream*);
+    static SkCodec* NewFromStream(SkStream*, SkPngChunkReader* = NULL);

[msarett, 2015/11/20 20:21:23]

This is in contrast to SkImageDecoder's setPeeker() API?

Any thoughts on the tradeoffs?  I'm just curious, I don't really have a
preference.

[scroggo, 2015/11/20 21:14:58]

Mike and I talked about this a while ago (this is from my earlier comments,
which you can view in the discussion at the bottom):
I dug into it a bit more, and I learned that libpng's chunks come in the
following order:
IHDR
PLTE
tRNS (if present)
IDAT

Android always uses PNG_HAVE_PLTE when inserting custom chunks, which is
documented to be after PLTE and before IDAT, but I'm not sure whether it comes
before tRNS or after. We read tRNS in the factory, so if it comes before, that
would be a problem. If we want to install it elsewhere (Mike doesn't like the
setPeeker API - he prefers something that is not stateful, but we could attach
it to Options), we need to verify that tRNS comes before, but if libpng does not
promise that order (I haven't found that promise), I'm nervous about relying on
it.

If we were to supply it later, we also wouldn't be able to support PNG_HAVE_IHDR
(not used by Android, but it is an option - see
http://www.ludism.org/~rwhe/LSB/png/libpng-png-set-unknown-chunks-1.html), so we
would need to document that as well.

[msarett, 2015/11/20 21:27:26]

I think I would prefer it in Options, but, if that may not be early enough, then
I'm in favor of this.

 
     /**
      *  If this data represents an encoded image that we know how to decode,
      *  return an SkCodec that can decode it. Otherwise return NULL.
      *
+     *  If SkPngChunkReader is not NULL, take a ref and pass it to libpng if
+     *  the image is a png.
+     *
      *  Will take a ref if it returns a codec, else will not affect the data.
      */
-    static SkCodec* NewFromData(SkData*);
+    static SkCodec* NewFromData(SkData*, SkPngChunkReader* = NULL);
 
     virtual ~SkCodec();