Add subsetting to SkScanlineDecoder

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index bafca852554db21a1e15fdda6b5b3c2a0dc2de45..20032e984d4d2b4e940f0f916f37ddf8bf6eddd7 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -111,6 +111,34 @@ bool SkCodec::rewindIfNeeded() {
     return this->onRewind();
 }
 
+SkISize SkCodec::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);
+}
+
+bool SkCodec::getValidSubset(SkIRect* desiredSubset) const {
+    if (!desiredSubset) {
+        return false;
+    }
+
+    SkIRect dimensions  = SkIRect::MakeSize(this->getInfo().dimensions());
+    if (!dimensions.contains(*desiredSubset)) {
+        return false;
+    }
+
+    return this->onGetValidSubset(desiredSubset);
+}
+
 SkCodec::Result SkCodec::getPixels(const SkImageInfo& info, void* pixels, size_t rowBytes,
                                    const Options* options, SkPMColor ctable[], int* ctableCount) {
     if (kUnknown_SkColorType == info.colorType()) {
@@ -198,6 +226,7 @@ SkCodec::Result SkCodec::startScanlineDecode(const SkImageInfo& dstInfo,
         const SkCodec::Options* options, SkPMColor ctable[], int* ctableCount) {
     // Reset fCurrScanline in case of failure.
     fCurrScanline = -1;
+
     // Ensure that valid color ptrs are passed in for kIndex8 color type
     if (kIndex_8_SkColorType == dstInfo.colorType()) {
         if (nullptr == ctable || nullptr == ctableCount) {
@@ -220,10 +249,14 @@ SkCodec::Result SkCodec::startScanlineDecode(const SkImageInfo& dstInfo,
     if (nullptr == options) {
         options = &optsStorage;
     } else if (options->fSubset) {
-        SkIRect subset(*options->fSubset);
-        if (!this->onGetValidSubset(&subset) || subset != *options->fSubset) {
-            // FIXME: How to differentiate between not supporting subset at all
-            // and not supporting this particular subset?
+        SkIRect size = SkIRect::MakeSize(dstInfo.dimensions());
+        if (!size.contains(*options->fSubset)) {
+            return kInvalidInput;
+        }
+
+        // We only support subsetting in the x-dimension for scanline decoder.
+        // Subsetting in the y-dimension can be accomplished using skipScanlines().
+        if (options->fSubset->top() != 0 || options->fSubset->height() != dstInfo.height()) {

[msarett, 2015/10/07 22:15:14]

Alternatively we can just ignore the y-coordinates of the subset.  But I think
it might be better to be sure that the client knows that they are responsible
for subsetting in the y-dimension.

             return kUnimplemented;
         }
     }
@@ -233,14 +266,14 @@ SkCodec::Result SkCodec::startScanlineDecode(const SkImageInfo& dstInfo,
         return kInvalidScale;
     }
 
+    fDstInfo = dstInfo;
+    fOptions = *options;

[msarett, 2015/10/07 22:15:14]

SkJpegCodec might need to call getSampler() in startScanlineDecode().
We cannot call getSampler() until fDstInfo and fOptions are set.

I know you had a reason for not setting these fields until the end of this
function, but I can't remember what it was...

[scroggo, 2015/10/08 20:16:07]

The reason is because I did not want to update them until we knew that
onStartScanlineDecode succeeded. This way we do not have some dstInfo and
options that perhaps did not work. It does not go all the way though - we could
be even more defensive about this and reset them at the beginning (like we do
with fCurrScanline).
Do you mean in onStartScanlineDecode? That function receives dstInfo and options
as parameters, so it seems like you can just read those.

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

Acknowledged.

     const Result result = this->onStartScanlineDecode(dstInfo, *options, ctable, ctableCount);
     if (result != SkCodec::kSuccess) {
         return result;
     }
 
     fCurrScanline = 0;
-    fDstInfo = dstInfo;
-    fOptions = *options;
     return kSuccess;
 }