Make SkPngCodec decode progressively.

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index 50875e46c4a42cfbef1d204a0af3bfa01a74b38c..28239d2ad8e4ee103ff5e691b7e3f9a9c24702b0 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -23,6 +23,10 @@ class SkData;
 class SkPngChunkReader;
 class SkSampler;
 
+namespace DM {
+class CodecSrc;
+}
+
 /**
  *  Abstraction layer directly on top of an image codec.
  */
@@ -253,8 +257,8 @@ public:
          *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
          *  currently supports subsets), the top and left values must be even.
          *
-         *  In getPixels, we will attempt to decode the exact rectangular
-         *  subset specified by fSubset.
+         *  In getPixels and incremental decode, we will attempt to decode the
+         *  exact rectangular subset specified by fSubset.
          *
          *  In a scanline decode, it does not make sense to specify a subset
          *  top or subset height, since the client already controls which rows
@@ -349,6 +353,67 @@ public:
     }
 
     /**
+     *  Prepare for an incremental decode with the specified options.
+     *
+     *  This may require a rewind.
+     *
+     *  @param dstInfo Info of the destination. If the dimensions do not match
+     *      those of getInfo, this implies a scale.
+     *  @param dst Memory to write to. Needs to be large enough to hold the subset,
+     *      if present, or the full image as described in dstInfo.
+     *  @param options Contains decoding options, including if memory is zero
+     *      initialized and whether to decode a subset.
+     *  @param ctable A pointer to a color table.  When dstInfo.colorType() is
+     *      kIndex8, this should be non-NULL and have enough storage for 256
+     *      colors.  The color table will be populated after decoding the palette.
+     *  @param ctableCount A pointer to the size of the color table.  When
+     *      dstInfo.colorType() is kIndex8, this should be non-NULL.  It will
+     *      be modified to the true size of the color table (<= 256) after
+     *      decoding the palette.
+     *  @return Enum representing success or reason for failure.
+     */
+    Result startIncrementalDecode(const SkImageInfo& dstInfo, void* dst, size_t rowBytes,

[reed1, 2016/05/24 15:11:38]

The first 3 parameters could be SkPixmap ...

[scroggo, 2016/05/26 16:10:57]

+1. I've been thinking about that for getPixels as well. I'll follow up on that
in a separate CL.

+            const SkCodec::Options*, SkPMColor* ctable, int* ctableCount);
+
+    Result startIncrementalDecode(const SkImageInfo& dstInfo, void* dst, size_t rowBytes,
+            const SkCodec::Options* options) {
+        return this->startIncrementalDecode(dstInfo, dst, rowBytes, options, nullptr, nullptr);
+    }
+
+    Result startIncrementalDecode(const SkImageInfo& dstInfo, void* dst, size_t rowBytes) {
+        return this->startIncrementalDecode(dstInfo, dst, rowBytes, nullptr, nullptr, nullptr);
+    }
+
+    /**
+     *  Start/continue the incremental decode.
+     *
+     *  Not valid to call before calling startIncrementalDecode().
+     *
+     *  After the first call, should only be called again if more data has been
+     *  provided to the source SkStream.
+     *
+     *  Unlike getPixels and getScanlines, this does not do any filling. This is
+     *  left up to the caller, since they may be skipping lines or continuing the
+     *  decode later. In the latter case, they may choose to initialize all lines
+     *  first, or only initialize the remaining lines after the first call.
+     *
+     *  @param rowsDecoded Optional output variable returning the total number of
+     *      lines initialized. Only meaningful if this method returns kIncompleteInput.
+     *      Otherwise the implementation may not set it.
+     *      Note that some implementations may have initialized this many rows, but
+     *      not necessarily finished those rows (e.g. interlaced PNG). This may be
+     *      useful for determining what rows the client needs to initialize.
+     *  @return kSuccess if all lines requested in startIncrementalDecode have
+     *      been completely decoded. kIncompleteInput otherwise.
+     */
+    Result incrementalDecode(int* rowsDecoded = nullptr) {
+        if (!fStartedIncrementalDecode) {
+            return kInvalidParameters;
+        }
+        return this->onIncrementalDecode(rowsDecoded);
+    }
+
+    /**
      * The remaining functions revolve around decoding scanlines.
      */
 
@@ -468,17 +533,6 @@ public:
          * Interlaced gifs are an example.
          */
         kOutOfOrder_SkScanlineOrder,
-
-        /*
-         * Indicates that the entire image must be decoded in order to output
-         * any amount of scanlines.  In this case, it is a REALLY BAD IDEA to
-         * request scanlines 1-by-1 or in small chunks.  The client should
-         * determine which scanlines are needed and ask for all of them in
-         * a single call to getScanlines().
-         *
-         * Interlaced pngs are an example.
-         */
-        kNone_SkScanlineOrder,
     };
 
     /**
@@ -658,6 +712,7 @@ private:
     SkImageInfo                 fDstInfo;
     SkCodec::Options            fOptions;
     int                         fCurrScanline;
+    bool                        fStartedIncrementalDecode;
 
     /**
      *  Return whether these dimensions are supported as a scale.
@@ -678,6 +733,16 @@ private:
         return kUnimplemented;
     }
 
+    virtual Result onStartIncrementalDecode(const SkImageInfo& /*dstInfo*/, void*, size_t,
+            const SkCodec::Options&, SkPMColor*, int*) {
+        return kUnimplemented;
+    }
+
+    virtual Result onIncrementalDecode(int*) {
+        return kUnimplemented;
+    }
+
+
     virtual bool onSkipScanlines(int /*countLines*/) { return false; }
 
     virtual int onGetScanlines(void* /*dst*/, int /*countLines*/, size_t /*rowBytes*/) { return 0; }
@@ -709,6 +774,7 @@ private:
      */
     virtual SkSampler* getSampler(bool /*createIfNecessary*/) { return nullptr; }
 
+    friend class DM::CodecSrc;  // for fillIncompleteImage
     friend class SkSampledCodec;
     friend class SkIcoCodec;
 };