Add subsetting to SkScaledCodec

include/codec/SkCodec.h

Index: include/codec/SkCodec.h
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index fecd3d44ea171aa0492a3e007dd5cd41d1f007aa..9d97e5d193d5131311dce3063bd8ca21e3bd98e0 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -55,20 +55,7 @@ public:
      *  The returned value is the codec's suggestion for the closest valid
      *  scale that it can natively support
      */
-    SkISize getScaledDimensions(float desiredScale) const {
-        // Negative and zero scales are errors.
-        SkASSERT(desiredScale > 0.0f);
-        if (desiredScale <= 0.0f) {
-            return SkISize::Make(0, 0);
-        }
-
-        // Upscaling is not supported. Return the original size if the client
-        // requests an upscale.
-        if (desiredScale >= 1.0f) {
-            return this->getInfo().dimensions();
-        }
-        return this->onGetScaledDimensions(desiredScale);
-    }
+    SkISize getScaledDimensions(float desiredScale) const;
 
     /**
      *  Return (via desiredSubset) a subset which can decoded from this codec,
@@ -85,9 +72,7 @@ public:
      *  @return true if this codec supports decoding desiredSubset (as
      *      returned, potentially modified)
      */
-    bool getValidSubset(SkIRect* desiredSubset) const {
-        return this->onGetValidSubset(desiredSubset);
-    }
+    bool getValidSubset(SkIRect* desiredSubset) const;
 
     /**
      *  Format of the encoded data.
@@ -163,21 +148,79 @@ public:
         Options()
             : fZeroInitialized(kNo_ZeroInitialized)
             , fSubset(NULL)
+            , fScaledDimensions(SkISize::Make(0, 0))
+            , fScaledSubset(SkIRect::MakeEmpty())
         {}
 
         ZeroInitialized fZeroInitialized;
+
         /**
-         *  If not NULL, represents a subset of the original image to decode.
+         *  fSubset represents a subset of the original image to decode.
+         *  It must be within the bounds returned by getInfo().
+         *  If the EncodedFormat is kWEBP_SkEncodedFormat, the top and left
+         *  values must be even.
+         *  If fSubset is NULL, we are not performing a subset decode.
+         *
+         *  fScaledDimensions and fScaledSubset are used together to help
+         *  specify a scaled subset decode.  Both should be specified for a
+         *  scaled subset decode, and both should be zeroed otherwise.
          *
-         *  Must be within the bounds returned by getInfo().
+         *  If fScaledDimensions and fScaledSubset are zeroed, we are not
+         *  performing a scaled subset decode:
+         *      (1) If fSubset is non-NULL, we are performing an unscaled
+         *          subset decode, and the subset dimensions must match the
+         *          dstInfo dimensions.
+         *      (2) If fSubset is NULL and the dstInfo dimensions do not
+         *          match the original dimensions, we are performing a
+         *          scaled decode.
+         *      (3) If fSubset is NULL and the dstInfo dimensions match the
+         *          original dimensions, we are performing an unscaled, full
+         *          image decode.
          *
-         *  If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
-         *  currently supports subsets), the top and left values must be even.
+         *  If both are non-NULL we are performing a scaled subset decode:
+         *      fSubset must be non-NULL.
+         *      fScaledSubset must be within the bounds of fScaledDimensions.
+         *      The dimensions of fScaledSubset must match the dimensions
+         *      specified by dstInfo.
+         *
+         *  Usage:
+         *      Call getScaledSubsetDimensions() with the desired fSubset of
+         *      the original image and the desired scale.  The function will
+         *      populate the options object with valid values of
+         *      fScaledDimensions and fScaledSubset that can be decoded
+         *      efficiently.
          */
         SkIRect*        fSubset;
+        SkISize         fScaledDimensions;
+        SkIRect         fScaledSubset;
     };
 
     /**
+     *  @param desiredScale The requested scale factor.
+     *  @param options      In/out parameter.
+     *                      options->fSubset (in/out) specifies the
+     *                      requested subset in terms of the original image
+     *                      dimensions.  This may be modified to a subset
+     *                      that can be decoded more efficiently.
+     *                      options->fScaledDimensions is an output and
+     *                      specifies the best scaled (full image) output
+     *                      dimensions that can be achieved for the
+     *                      desiredScale.
+     *                      options->fScaledSubset is an output and specifies
+     *                      the fSubset in terms of fScaledDimensions.
+     *
+     *  The codec may not be able to scale and subset efficiently to the exact
+     *  scale and subset requested, so fScaledDimensions and fScaledSubset may
+     *  be approximate.  These output values are the codec's suggestion for
+     *  the closest valid scaled subset that it can support.
+     *
+     *  @return true if fScaledDimensions and fScaledSubset have been
+     *               successfully set to values that the codec supports.
+     *          false otherwise.
+     */
+    bool getScaledSubsetDimensions(float desiredScale, Options* options) const;
+
+    /**
      *  Decode into the given pixels, a block of memory of size at
      *  least (info.fHeight - 1) * rowBytes + (info.fWidth *
      *  bytesPerPixel)
@@ -256,14 +299,25 @@ public:
      *      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.
+     *  @param subsetLeft The left offset at which to begin decoding each
+     *      scanline.  This enables partial scanline decodes.
+     *  @param subsetWidth The number of pixels to decode in each scanline row.
      *  @return Enum representing success or reason for failure.
      */
     Result startScanlineDecode(const SkImageInfo& dstInfo, const SkCodec::Options* options,
-                 SkPMColor ctable[], int* ctableCount);
+                 SkPMColor ctable[], int* ctableCount, int subsetLeft, int subsetWidth);
+
+    /**
+     *  Simplified version of startScanlineDecode() that does not enable partial
+     *  scanline decoding.
+     */
+    Result startScanlineDecode(const SkImageInfo& dstInfo, const SkCodec::Options* options,
+            SkPMColor ctable[], int* ctableCount);
 
     /**
      *  Simplified version of startScanlineDecode() that asserts that info is NOT
-     *  kIndex8_SkColorType and uses the default Options.
+     *  kIndex8_SkColorType, uses the default Options, and does not enable
+     *  partial scanline decoding.
      */
     Result startScanlineDecode(const SkImageInfo& dstInfo);
 
@@ -398,6 +452,16 @@ protected:
         return this->getInfo().dimensions();
     }
 
+    virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
+        // By default, subsets are not supported.
+        return false;
+    }
+
+    virtual bool onGetScaledSubsetDimensions(float desiredScale, Options* options) const {
+        // By default, scaled subsetting is not supported.
+        return false;
+    }
+
     // FIXME: What to do about subsets??
     /**
      *  Subclasses should override if they support dimensions other than the
@@ -420,11 +484,6 @@ protected:
                                SkPMColor ctable[], int* ctableCount,
                                int* rowsDecoded) = 0;
 
-    virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
-        // By default, subsets are not supported.
-        return false;
-    }
-
     virtual bool onReallyHasAlpha() const { return false; }
 
     /**
@@ -529,7 +588,8 @@ private:
 
     // Methods for scanline decoding.
     virtual SkCodec::Result onStartScanlineDecode(const SkImageInfo& dstInfo,
-            const SkCodec::Options& options, SkPMColor ctable[], int* ctableCount) {
+            const SkCodec::Options& options, SkPMColor ctable[], int* ctableCount, int subsetLeft,
+            int subsetWidth) {
         return kUnimplemented;
     }