Make SkPngCodec decode progressively.

src/codec/SkCodec.cpp

Index: src/codec/SkCodec.cpp
diff --git a/src/codec/SkCodec.cpp b/src/codec/SkCodec.cpp
index 8468f8a09996d1c7e79363970f9103e106049c7a..fd0cda0c66a69004e7461c6cd319612203b10b56 100644
--- a/src/codec/SkCodec.cpp
+++ b/src/codec/SkCodec.cpp
@@ -228,6 +228,66 @@ SkCodec::Result SkCodec::getPixels(const SkImageInfo& info, void* pixels, size_t
     return this->getPixels(info, pixels, rowBytes, nullptr, nullptr, nullptr);
 }
 
+SkCodec::Result SkCodec::startIncrementalDecode(const SkImageInfo& dstInfo,
+        const SkCodec::Options* options, SkPMColor* ctable, int* ctableCount) {
+    fStartedIncrementalDecode = false;
+
+    // FIXME: Share more code with getPixels/startScanlineDecode?

[msarett, 2016/05/20 15:04:40]

I think this would be a good chance to share the code that should be common with
getPixels().  Though I guess a lot of it is slightly different...

IMHO, I'm not worried about sharing with startScanlineDecode(), since we'll be
deleting that anyway.

[scroggo_chromium, 2016/05/20 16:51:59]

I moved the checks for index8, colortables, etc into a macro. That's the part I
really meant, that is shared across all methods. The rest is slightly different
or trivial.

+    // Ensure that valid color ptrs are passed in for kIndex8 color type
+    if (kIndex_8_SkColorType == dstInfo.colorType()) {
+        if (nullptr == ctable || nullptr == ctableCount) {
+            return kInvalidParameters;
+        }
+    } else {
+        if (ctableCount) {
+            *ctableCount = 0;
+        }
+        ctableCount = nullptr;
+        ctable = nullptr;
+    }
+
+    // FIXME: If the rows come after the rows of a previous incremental decode,
+    // we might be able to skip the rewind, but only the implementation knows
+    // that. (e.g. PNG will always need to rewind, since we called longjmp, but
+    // a bottom-up BMP could skip rewinding if the new rows are above the old
+    // rows.)
+    if (!this->rewindIfNeeded()) {
+        return kCouldNotRewind;
+    }
+
+    // Set options.
+    Options optsStorage;
+    if (nullptr == options) {
+        options = &optsStorage;
+    } else if (options->fSubset) {
+        SkIRect size = SkIRect::MakeSize(dstInfo.dimensions());
+        if (!size.contains(*options->fSubset)) {
+            return kInvalidParameters;
+        }
+
+        const int top = options->fSubset->top();
+        const int bottom = options->fSubset->bottom();
+        if (top < 0 || top >= dstInfo.height() || top >= bottom || bottom > dstInfo.height()) {
+            return kInvalidParameters;
+        }
+    }
+
+    if (!this->dimensionsSupported(dstInfo.dimensions())) {
+        return kInvalidScale;
+    }
+
+    fDstInfo = dstInfo;
+    fOptions = *options;
+
+    const Result result = this->onStartIncrementalDecode(dstInfo, fOptions, ctable,
+                                                         ctableCount);
+    if (kSuccess == result) {
+        fStartedIncrementalDecode = true;
+    }
+    return result;
+}
+
+
 SkCodec::Result SkCodec::startScanlineDecode(const SkImageInfo& dstInfo,
         const SkCodec::Options* options, SkPMColor ctable[], int* ctableCount) {
     // Reset fCurrScanline in case of failure.
@@ -331,7 +391,6 @@ int SkCodec::outputScanline(int inputScanline) const {
 int SkCodec::onOutputScanline(int inputScanline) const {
     switch (this->getScanlineOrder()) {
         case kTopDown_SkScanlineOrder:
-        case kNone_SkScanlineOrder:
             return inputScanline;
         case kBottomUp_SkScanlineOrder:
             return this->getInfo().height() - inputScanline - 1;
@@ -365,8 +424,7 @@ void SkCodec::fillIncompleteImage(const SkImageInfo& info, void* dst, size_t row
     }
 
     switch (this->getScanlineOrder()) {
-        case kTopDown_SkScanlineOrder:
-        case kNone_SkScanlineOrder: {
+        case kTopDown_SkScanlineOrder: {
             const SkImageInfo fillInfo = info.makeWH(fillWidth, linesRemaining);
             fillDst = SkTAddOffset<void>(dst, linesDecoded * rowBytes);
             fill_proc(fillInfo, fillDst, rowBytes, fillValue, zeroInit, sampler);