Adding png scanline decoding to kIndex8

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 6f313a1c7058715c8e7c95a82ca1448b152b245e..523181a3d42f0e0fec8a46b42d3692168074cc88 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -70,6 +70,15 @@ public:
      *
      *  @param dstInfo Info of the destination. If the dimensions do not match
      *      those of getInfo, this implies a scale.
+     *  @param options Contains decoding options, including if memory is zero
+     *      initialized.
+     *  @param ctable A pointer to a color table.  This color table will be
+     *      populated if the png has a color palette and dstInfo.colorType() is
+     *      kIndex8 mode.
+     *  @param ctableCount A pointer to the size of the color table.  When
+     *      dstInfo.colorType() is kIndex8, this should be non-NULL and should
+     *      be a pointer to 256.  It will be modified to the actual size of

[scroggo, 2015/04/22 17:43:53]

It does not make a lot of sense to me to say it must be a pointer to 256 - it
seems like it doesn't even need to be initialized if we always expect it to be
256.

[msarett, 2015/04/22 18:05:38]

I agree with you.  I was trying to match the requirements of getPixels().  I
will change and remove the appropriate asserts.

[scroggo, 2015/04/22 18:41:36]

I *think* the idea was that a client could use a smaller number than 256, to
state that they had storage for a smaller number of colors.

In practice, I am not sure how to deal with that. Why would they use a smaller
storage? Do they think they know how many colors the image uses?

Upon re-reading the comments for getPixels, it wasn't clear to me that passing a
number less than 256 actually is supposed to behave that way, though. Hal, any
thoughts?

[msarett, 2015/04/22 20:03:32]

I think I misinterpreted the comments in SkImageGenerator.h as well - hopefully
I understand them better now.

It seems to me that we always require storage for at least 256 colors and a
non-NULL count_ptr, when decoding to kIndex8.  Then, we will set the value of
the count_ptr based on the actual number of colors in the image, ignoring the
previous value.

This seems reasonable enough to me, given that it may be tricky to deal with
situations when storage is less than 256.  I believe the latest patch set
follows this convention.

+     *      the color table (<= 256) after decoding the palette.
      *  @return New SkScanlineDecoder, or NULL on failure.
      *
      *  NOTE: If any rows were previously decoded, this requires rewinding the
@@ -78,6 +87,13 @@ public:
      *  NOTE: The scanline decoder is owned by the SkCodec and will delete it
      *  when the SkCodec is deleted.
      */
+    SkScanlineDecoder* getScanlineDecoder(const SkImageInfo& dstInfo, const Options* options,
+                                          SkPMColor ctable[], int* ctableCount);
+
+    /**
+     *  Simplified version of getScanlineDecoder() that asserts that info is NOT
+     *  kIndex8_SkColorType and uses the default Options.
+     */
     SkScanlineDecoder* getScanlineDecoder(const SkImageInfo& dstInfo);
 
     /**
@@ -126,10 +142,22 @@ protected:
      *
      *  @param dstInfo Info of the destination. If the dimensions do not match
      *      those of getInfo, this implies a scale.
+     *  @param options Contains decoding options, including if memory is zero
+     *      initialized.
+     *  @param ctable A pointer to a color table.  This color table will be
+     *      populated if the png has a color palette and dstInfo.colorType() is
+     *      kIndex8 mode.
+     *  @param ctableCount A pointer to the size of the color table.  When
+     *      dstInfo.colorType() is kIndex8, this should be non-NULL and should
+     *      be a pointer to 256.  It will be modified to the actual size of
+     *      the color table (<= 256) after decoding the palette.
      *  @return New SkScanlineDecoder on success, NULL otherwise. The SkCodec
      *      will take ownership of the returned scanline decoder.
      */
-    virtual SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo) {
+    virtual SkScanlineDecoder* onGetScanlineDecoder(const SkImageInfo& dstInfo,
+                                                    const Options& options,
+                                                    SkPMColor ctable[],
+                                                    int* ctableCount) {
         return NULL;
     }