Add scanline decoding to SkCodec.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index beb9cb97b32bdb51bed7cc4d76d24b968fe0858d..5b92285059c023b276247a3a4670aea13642cdf5 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -15,6 +15,7 @@
 #include "SkTypes.h"
 
 class SkData;
+class SkScanlineDecoder;
 class SkStream;
 
 /**
@@ -48,6 +49,39 @@ public:
      */
     SkISize getScaledDimensions(float desiredScale) const;
 
+    /**
+     *  Create a new scanline decoder.
+     *
+     *  Create a new object which can be used to decode individual scanlines.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo (or origSubset, if provided), this implies a scale.
+     *  @param origSubset If non null, represents a subset of the original
+     *      unscaled image for the scanline decoder to decode.
+     *  @return New SkScanlineDecoder, or NULL on failure.
+     *
+     *  NOTE: If any rows were previously decoded, this requires rewinding the
+     *  SkStream.
+     *
+     *  NOTE: The lifetime of the scanline decoder is tied to the lifetime of
+     *  the SkCodec. Do not attempt to use after the SkCodec has been deleted.
+     *  Further, the SkScanlineDecoder should be deleted *before* the SkCodec,
+     *  in case the implementation depends on the ordering.
+     */
+    SkScanlineDecoder* getScanlineDecoder(const SkImageInfo& dstInfo,
+                                          const SkIRect* origSubset = NULL);

[scroggo, 2015/03/18 20:19:06]

I haven't yet handled the subset case, but I wanted to bounce my thoughts of you
all before I start to do it.

We discussed passing the subset in here, but I'm wondering if it's necessary. If
I was going to implement a subset decode, I would probably do something like the
following:

scanlineDecoder->skipLines(origSubset->top());
void* tmpDest = malloc(getInfo()->minRowBytes());
for (int y = origSubset->top(); y < origSubset->bottom(); y++) {
  scanlineDecoder->getNextScanline(tmpDest);
  memcpy(dest, tmpDest + (origSubset->left() * bpp),
         origSubset->width() * bpp);
}
scanlineDecoder->skipLines(getInfo()->height() - origSubset->bottom());

(The scanline implementation looks a bit different, since we don't do it in a
loop, but this is the general idea.)

This reads some extra pixels (maybe a lot, if the caller wanted a narrow
subset), but at least for PNG/JPEG, I think we can only read a row at a time
anyway. I'm guessing BMP could do something smarter, but I imagine it would
greatly complicate the code (WDYT, Matt?).

Should we just leave this to the caller? The only obvious downside is that it
makes things a little bit trickier, but not impossible, to use getScanlines(int
count).

+
+    /**
+     *  Some images may initially report that they have alpha due to the format
+     *  of the encoded data, but then never use any colors which have alpha
+     *  less than 100%. This function can be called *after* decoding to
+     *  determine if such an image truly had alpha. Calling it before decoding
+     *  is undefined.
+     */
+    bool reallyHasAlpha() const {

[scroggo, 2015/03/18 20:19:06]

This maybe should have been a separate change, but it came up as I was
rearranging the PngCodec

+        return this->onReallyHasAlpha();
+    }
+
 protected:
     SkCodec(const SkImageInfo&, SkStream*);
 
@@ -70,6 +104,27 @@ protected:
     }
 
     /**
+     *  Override if your codec supports scanline decoding.
+     *
+     *  No need to call rewindIfNeeded(), which will have already been called
+     *  by the base class.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo (or origSubset, if different), this implies a
+     *      scale.
+     *  @param origSubset Subset of the original info to decode. The base class
+     *      will have already intersected with getInfo (or set to match if the
+     *      caller supplied NULL).
+     *  @return New SkScanlineDecoder on success, NULL otherwise.
+     */
+    virtual SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo,
+                                                    const SkIRect& origSubset) {
+        return NULL;
+    }
+
+    virtual bool onReallyHasAlpha() const { return false; }
+
+    /**
      *  If the stream was previously read, attempt to rewind.
      *  @returns:
      *      true