Add subsetting to SkScaledCodec

src/codec/SkSwizzler.cpp

Index: src/codec/SkSwizzler.cpp
diff --git a/src/codec/SkSwizzler.cpp b/src/codec/SkSwizzler.cpp
index 8fcd18e8be499a0dd07e06acd4f860091f7e6f60..648f021ef7c31a18f340942730eed9cb68c92199 100644
--- a/src/codec/SkSwizzler.cpp
+++ b/src/codec/SkSwizzler.cpp
@@ -231,9 +231,6 @@ static SkSwizzler::ResultAlpha swizzle_index_to_index(
     //                 SkScaledBitmap sampler just guesses that it is opaque.  This is dangerous
     //                 and probably wrong since gif and bmp (rarely) may have alpha.
     if (1 == deltaSrc) {
-        // A non-zero offset is only used when sampling, meaning that deltaSrc will be
-        // greater than 1. The below loop relies on the fact that src remains unchanged.

[scroggo, 2015/10/02 18:27:03]

Does the below loop not need to be updated to take this into account?

[msarett, 2015/10/06 23:01:27]

Don't think so...

[scroggo, 2015/10/07 17:49:39]

Can you help me understand why that is? I thought we previously ran into a
problem where the loop did not match, causing problems...

See Emmalee's comment https://codereview.chromium.org/1260673002#msg92. It looks
like we had to revert patch set 32, and the fix included this assert.

Put another way, did we update dstWidth to take the offset into account? Because
if not, it seems like the memcpy will copy too much, and then the loop will read
too much.

-        SkASSERT(0 == offset);
         memcpy(dst, src, dstWidth);
         for (int x = 0; x < dstWidth; x++) {
             UPDATE_RESULT_ALPHA(ctable[src[x]] >> SK_A32_SHIFT);
@@ -517,7 +514,8 @@ SkSwizzler* SkSwizzler::CreateSwizzler(SkSwizzler::SrcConfig sc,
                                        const SkPMColor* ctable,
                                        const SkImageInfo& dstInfo, 
                                        SkCodec::ZeroInitialized zeroInit, 
-                                       const SkImageInfo& srcInfo) {
+                                       const SkImageInfo& srcInfo,
+                                       int subsetLeft, int subsetWidth) {
     if (dstInfo.colorType() == kUnknown_SkColorType || kUnknown == sc) {
         return nullptr;
     }
@@ -685,33 +683,30 @@ SkSwizzler* SkSwizzler::CreateSwizzler(SkSwizzler::SrcConfig sc,
         return nullptr;
     }
 
-    // Store deltaSrc in bytes if it is an even multiple, otherwise use bits
-    int deltaSrc = SkIsAlign8(BitsPerPixel(sc)) ? BytesPerPixel(sc) : BitsPerPixel(sc);
+    // Store bpp in bytes if it is an even multiple, otherwise use bits
+    int bpp = SkIsAlign8(BitsPerPixel(sc)) ? BytesPerPixel(sc) : BitsPerPixel(sc);
 
     // get sampleX based on srcInfo and dstInfo dimensions
     int sampleX;
-    SkScaledCodec::ComputeSampleSize(dstInfo, srcInfo, &sampleX, nullptr);
-
-    return new SkSwizzler(proc, ctable, deltaSrc, dstInfo, sampleX);
+    SkScaledCodec::ComputeSampleSize(dstInfo.dimensions(), srcInfo.dimensions(), &sampleX, nullptr);
+    int x0 = get_start_coord(sampleX) + subsetLeft;
+    
+    return new SkSwizzler(proc, ctable, x0 * bpp, subsetWidth, bpp, sampleX * bpp);
 }
 
 SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable,
-                       int deltaSrc, const SkImageInfo& info, int sampleX)
+                       int left, int width, int bpp, int deltaSrc)
     : fRowProc(proc)
     , fColorTable(ctable)
+    , fLeft(left)
+    , fWidth(width)
+    , fBPP(bpp)
     , fDeltaSrc(deltaSrc)
-    , fDstInfo(info)
-    , fSampleX(sampleX)
-    , fX0(get_start_coord(sampleX))
-{
-    // check that fX0 is less than original width
-    SkASSERT(fX0 >= 0 && fX0 < fDstInfo.width() * fSampleX);
-}
+{}
 
 SkSwizzler::ResultAlpha SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
     SkASSERT(nullptr != dst && nullptr != src);
-    return fRowProc(dst, src, fDstInfo.width(), fDeltaSrc, fSampleX * fDeltaSrc,
-            fX0 * fDeltaSrc, fColorTable);
+    return fRowProc(dst, src, fWidth, fBPP, fDeltaSrc, fLeft, fColorTable);
 }
 
 void SkSwizzler::Fill(void* fillDst, const SkImageInfo& fillInfo, size_t rowBytes,