Add the ability to decode a subset to SkCodec
This allows codecs that support subsets natively (i.e. WEBP) to do so.
Add a field on SkCodec::Options representing the subset.
Add a method on SkCodec to find a valid subset which approximately
matches a desired subset.
Implement subset decodes in SkWebpCodec.
Add a test in DM for decoding subsets.
Notice that we only start on even boundaries. This is due to the
way libwebp's API works. SkWEBPImageDecoder does not take this into
account, which results in visual artifacts.
FIXME: Subsets with scaling are not pixel identical, but close. (This
may be fine, though - they are not perceptually different. We'll just
need to mark another set of images in gold as valid, once
https://skbug.com/4038 is fixed, so we can tests scaled webp without
generating new images on each run.)
Review URL: https://codereview.chromium.org/1240143002
diff --git a/include/codec/SkCodec.h b/include/codec/SkCodec.h
index cc635e0..1cdc88d 100644
--- a/include/codec/SkCodec.h
+++ b/include/codec/SkCodec.h
@@ -58,6 +58,25 @@
}
/**
+ * Return (via desiredSubset) a subset which can decoded from this codec,
+ * or false if this codec cannot decode subsets or anything similar to
+ * desiredSubset.
+ *
+ * @param desiredSubset In/out parameter. As input, a desired subset of
+ * the original bounds (as specified by getInfo). If true is returned,
+ * desiredSubset may have been modified to a subset which is
+ * supported. Although a particular change may have been made to
+ * desiredSubset to create something supported, it is possible other
+ * changes could result in a valid subset.
+ * If false is returned, desiredSubset's value is undefined.
+ * @return true if this codec supports decoding desiredSubset (as
+ * returned, potentially modified)
+ */
+ bool getValidSubset(SkIRect* desiredSubset) const {
+ return this->onGetValidSubset(desiredSubset);
+ }
+
+ /**
* Format of the encoded data.
*/
SkEncodedFormat getEncodedFormat() const { return this->onGetEncodedFormat(); }
@@ -128,9 +147,20 @@
*/
struct Options {
Options()
- : fZeroInitialized(kNo_ZeroInitialized) {}
+ : fZeroInitialized(kNo_ZeroInitialized)
+ , fSubset(NULL)
+ {}
ZeroInitialized fZeroInitialized;
+ /**
+ * If not NULL, represents a subset of the original image to decode.
+ *
+ * Must be within the bounds returned by getInfo().
+ *
+ * If the EncodedFormat is kWEBP_SkEncodedFormat (the only one which
+ * currently supports subsets), the top and left values must be even.
+ */
+ SkIRect* fSubset;
};
/**
@@ -228,6 +258,11 @@
void* pixels, size_t rowBytes, const Options&,
SkPMColor ctable[], int* ctableCount) = 0;
+ virtual bool onGetValidSubset(SkIRect* /* desiredSubset */) const {
+ // By default, subsets are not supported.
+ return false;
+ }
+
/**
* Override if your codec supports scanline decoding.
*