Add scaled subset API to SkCodec

src/codec/SkWebpCodec.cpp

Index: src/codec/SkWebpCodec.cpp
diff --git a/src/codec/SkWebpCodec.cpp b/src/codec/SkWebpCodec.cpp
index 3c61b937622097d5e1c5be483770a6fb2336543a..5d3cb78039dc5a7a968d6e17f923398cf44eac4e 100644
--- a/src/codec/SkWebpCodec.cpp
+++ b/src/codec/SkWebpCodec.cpp
@@ -120,6 +120,13 @@ bool SkWebpCodec::onDimensionsSupported(const SkISize& dim) {
             && dim.height() >= 1 && dim.height() <= info.height();
 }
 
+bool SkWebpCodec::onSubsetSupported(const SkIRect& subset, bool isScanlineDecode) {
+    return !((subset.left() & 1) || (subset.top() & 1));

[scroggo, 2015/10/12 20:47:07]

Could you add a comment explaining this? (Left and top must be even.)

I suppose we don't need to worry about containment, because that was checked in
in subsetSupported?

+}
+
+bool SkWebpCodec::onScaledSubsetSupported(const Options& options, bool isScanlineDecode) {
+    return this->onSubsetSupported(*options.fSubset, isScanlineDecode);
+}
 
 static WEBP_CSP_MODE webp_decode_mode(SkColorType ct, bool premultiply) {
     switch (ct) {
@@ -139,21 +146,26 @@ static WEBP_CSP_MODE webp_decode_mode(SkColorType ct, bool premultiply) {
 // is arbitrary.
 static const size_t BUFFER_SIZE = 4096;
 
-bool SkWebpCodec::onGetValidSubset(SkIRect* desiredSubset) const {
-    if (!desiredSubset) {
-        return false;
-    }
-
-    SkIRect dimensions  = SkIRect::MakeSize(this->getInfo().dimensions());
-    if (!dimensions.contains(*desiredSubset)) {
-        return false;
-    }
+bool SkWebpCodec::onGetScaledSubsetDimensions(float desiredScale, const Options& options) const {
+    options.fSubset->fLeft = (options.fSubset->fLeft >> 1) << 1;
+    options.fSubset->fTop = (options.fSubset->fTop >> 1) << 1;
+
+    // Notice that we may round the size of the subset up to 1.  This means that we must
+    // round up the scaled output dimensions to avoid suggesting a subset that is off the
+    // edge of the image.  This rounding behvaior is consistent with SkJpegCodec.
+    *(options.fScaledDimensions) = SkISize::Make(
+            sk_float_ceil2int(desiredScale * this->getInfo().width()),
+            sk_float_ceil2int(desiredScale * this->getInfo().height()));
+
+    // Notice that we may round the size of the subset up to 1.  This means that we must
+    // floor the left and top offsets to ensure that we do not suggest a subset that is
+    // off the edge of the image.
+    *(options.fScaledSubset) = SkIRect::MakeXYWH(
+            int (desiredScale * options.fSubset->left()),
+            int (desiredScale * options.fSubset->top()),
+            SkTMax(1, SkScalarRoundToInt(desiredScale * options.fSubset->width())),
+            SkTMax(1, SkScalarRoundToInt(desiredScale * options.fSubset->height())));
 
-    // As stated below, libwebp snaps to even left and top. Make sure top and left are even, so we
-    // decode this exact subset.
-    // Leave right and bottom unmodified, so we suggest a slightly larger subset than requested.
-    desiredSubset->fLeft = (desiredSubset->fLeft >> 1) << 1;
-    desiredSubset->fTop = (desiredSubset->fTop >> 1) << 1;
     return true;
 }
 
@@ -193,16 +205,6 @@ SkCodec::Result SkWebpCodec::onGetPixels(const SkImageInfo& dstInfo, void* dst,
             return kInvalidParameters;
         }
 
-#ifdef SK_DEBUG

[msarett, 2015/10/12 18:33:29]

I tried to keep a debug check here, but it is too tricky now that we use
getScaledSubsetDimensions() instead of getValidSubset().

We would need to somehow calculate the float scale that we are using in order to
call the getScaledSubsetDimensions() API.

-        {
-            // Make a copy, since getValidSubset can change its input.
-            SkIRect subset(bounds);
-            // That said, getValidSubset should *not* change its input, in this case; otherwise
-            // getValidSubset does not match the actual subsets we can do.
-            SkASSERT(this->getValidSubset(&subset) && subset == bounds);
-        }
-#endif
-
         config.options.use_cropping = 1;
         config.options.crop_left = bounds.fLeft;
         config.options.crop_top = bounds.fTop;