Add subsetting to SkScanlineDecoder

src/codec/SkSwizzler.cpp

Index: src/codec/SkSwizzler.cpp
diff --git a/src/codec/SkSwizzler.cpp b/src/codec/SkSwizzler.cpp
index 8d13e56bd25a44e221c7f4bc290e51cf4922449f..8c7dd10126cd97cb40620b6f3736f32965b27bea 100644
--- a/src/codec/SkSwizzler.cpp
+++ b/src/codec/SkSwizzler.cpp
@@ -230,9 +230,6 @@ 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.
-        SkASSERT(0 == offset);
         memcpy(dst, src, dstWidth);

[scroggo, 2015/10/08 20:16:08]

Is the assumption here that dstWidth is smaller when we have an offset, so this
is safe?

[msarett, 2015/10/09 20:03:44]

Sorry I meant to respond to your original comment on this.

Yes we are relying on the subset being valid.  We check this in SkCodec.cpp.

It used to be the case that every time offset did not equal zero, it meant that
we were sampling.  Now offset can be nonzero because we are subsetting, but we
can still use a memcpy if we are not sampling.

         for (int x = 0; x < dstWidth; x++) {
             UPDATE_RESULT_ALPHA(ctable[src[x]] >> SK_A32_SHIFT);
@@ -514,8 +511,8 @@ static bool swizzle_rgba_to_n32_unpremul_skipZ(void* SK_RESTRICT dstRow,
 
 SkSwizzler* SkSwizzler::CreateSwizzler(SkSwizzler::SrcConfig sc,
                                        const SkPMColor* ctable,
-                                       const SkImageInfo& dstInfo, 
-                                       SkCodec::ZeroInitialized zeroInit) {
+                                       const SkImageInfo& dstInfo,
+                                       const SkCodec::Options& options) {
     if (dstInfo.colorType() == kUnknown_SkColorType || kUnknown == sc) {
         return nullptr;
     }
@@ -524,7 +521,7 @@ SkSwizzler* SkSwizzler::CreateSwizzler(SkSwizzler::SrcConfig sc,
         return nullptr;
     }
     RowProc proc = nullptr;
-
+    SkCodec::ZeroInitialized zeroInit = options.fZeroInitialized;
     switch (sc) {
         case kBit:
             switch (dstInfo.colorType()) {
@@ -683,37 +680,43 @@ 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);
+    
+    int srcOffset = 0;
+    int srcWidth = dstInfo.width();
+    if (options.fSubset) {
+        srcOffset = options.fSubset->left();
+        srcWidth = options.fSubset->width();
+    }
 
-    return new SkSwizzler(proc, ctable, deltaSrc, dstInfo.width());
+    return new SkSwizzler(proc, ctable, srcOffset, srcWidth, bpp);
 }
 
-SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable,
-                       int deltaSrc, int srcWidth)
+SkSwizzler::SkSwizzler(RowProc proc, const SkPMColor* ctable, int srcOffset, int srcWidth, int bpp)
     : fRowProc(proc)
     , fColorTable(ctable)
-    , fDeltaSrc(deltaSrc)
+    , fSrcOffset(srcOffset)
+    , fX0(srcOffset)
     , fSrcWidth(srcWidth)
     , fDstWidth(srcWidth)
+    , fBPP(bpp)
     , fSampleX(1)
-    , fX0(0)
 {}
 
+SkSwizzler::ResultAlpha SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {

[scroggo, 2015/10/08 20:16:08]

Why did this move?

[msarett, 2015/10/09 20:03:44]

I think it moved on a rebase, I'll put it back.

+    SkASSERT(nullptr != dst && nullptr != src);
+    return fRowProc(dst, src, fDstWidth, fBPP, fSampleX * fBPP, fX0 * fBPP, fColorTable);
+}
+
 int SkSwizzler::onSetSampleX(int sampleX) {
     SkASSERT(sampleX > 0); // Surely there is an upper limit? Should there be
                            // way to report failure?
     fSampleX = sampleX;
-    fX0 = get_start_coord(sampleX);
+    fX0 = fSrcOffset + get_start_coord(sampleX);

[scroggo, 2015/10/08 20:16:08]

Any reason this is the opposite order of mask swizzler?

[msarett, 2015/10/09 20:03:44]

No I'll reverse it.

     fDstWidth = get_scaled_dimension(fSrcWidth, sampleX);
 
     // check that fX0 is less than original width
     SkASSERT(fX0 >= 0 && fX0 < fSrcWidth);
     return fDstWidth;
 }
-
-SkSwizzler::ResultAlpha SkSwizzler::swizzle(void* dst, const uint8_t* SK_RESTRICT src) {
-    SkASSERT(nullptr != dst && nullptr != src);
-    return fRowProc(dst, src, fDstWidth, fDeltaSrc, fSampleX * fDeltaSrc,
-            fX0 * fDeltaSrc, fColorTable);
-}