Make swizzler optional for SkPngCodec, refactor xforms

src/codec/SkPngCodec.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkColorSpaceXform.h"
 #include "SkColorTable.h"
@@ skipping 28 matching lines @@
         void* fPtr;
     };
 
     Result onGetPixels(const SkImageInfo&, void*, size_t, const Options&, SkPMColor*, int*, int*)
             override;
     SkEncodedFormat onGetEncodedFormat() const override { return kPNG_SkEncodedFormat; }
     bool onRewind() override;
     uint32_t onGetFillValue(SkColorType) const override;
 
     // Helper to set up swizzler, color xforms, and color table. Also calls png_read_update_info.
-    bool initializeXforms(const SkImageInfo& requestedInfo, const Options&, SkPMColor* colorPtr,
+    bool initializeXforms(const SkImageInfo& dstInfo, const Options&, SkPMColor* colorPtr,
                           int* colorCount);
-    SkSampler* getSampler(bool createIfNecessary) override {
-        SkASSERT(fSwizzler);
-        return fSwizzler;
-    }
-    void allocateStorage();
+    void initializeSwizzler(const SkImageInfo& dstInfo, const Options&);
+    SkSampler* getSampler(bool createIfNecessary) override;
+    void allocateStorage(const SkImageInfo& dstInfo);
+    void applyXformRow(void* dst, const void* src, SkColorType, SkAlphaType, int width);
 
     virtual int readRows(const SkImageInfo& dstInfo, void* dst, size_t rowBytes, int count,
                          int startRow) = 0;
 
     SkPngCodec(const SkEncodedInfo&, const SkImageInfo&, SkStream*, SkPngChunkReader*,
                void* png_ptr, void* info_ptr, int, int);
 
     SkAutoTUnref<SkPngChunkReader> fPngChunkReader;
     voidp                          fPng_ptr;
     voidp                          fInfo_ptr;
 
     // These are stored here so they can be used both by normal decoding and scanline decoding.
     SkAutoTUnref<SkColorTable>         fColorTable;    // May be unpremul.
     SkAutoTDelete<SkSwizzler>          fSwizzler;
     std::unique_ptr<SkColorSpaceXform> fColorXform;
     SkAutoTMalloc<uint8_t>             fStorage;
     uint8_t*                           fSwizzlerSrcRow;
     uint32_t*                          fColorXformSrcRow;
     size_t                             fSrcRowBytes;
 
     const int                          fNumberPasses;
     int                                fBitDepth;
 
 private:
+
+    enum XformMode {

[msarett, 2016/08/26 16:37:45]

There are two more cases where we are not quite optimal.
(1) When we could decode directly to dst and skip swizzling and color xforms.
(2) When we could perform the color xform in place (don't need an extra row
buffer).

I considered adding both of these modes, but ultimately decided the gain wasn't
worth the complexity.

+        // Requires only a swizzle pass.
+        kSwizzleOnly_XformMode,
+
+        // Requires only a color xform pass.
+        kColorOnly_XformMode,
+
+        // Requires a swizzle and a color xform.
+        kSwizzleColor_XformMode,
+    };
+
     bool createColorTable(const SkImageInfo& dstInfo, int* ctableCount);
     void destroyReadStruct();
 
+    XformMode                          fXformMode;
+
     typedef SkCodec INHERITED;
 };