Create a scanline decoder without creating a codec

src/codec/SkJpegCodec.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCodec.h"
 #include "SkJpegCodec.h"
 #include "SkJpegDecoderMgr.h"
@@ skipping 371 matching lines @@
     turbo_jpeg_finish_decompress(dinfo);
 
     return kSuccess;
 }
 
 /*
  * Enable scanline decoding for jpegs
  */
 class SkJpegScanlineDecoder : public SkScanlineDecoder {
 public:
-    SkJpegScanlineDecoder(const SkImageInfo& dstInfo, SkJpegCodec* codec,
-            const SkCodec::Options& opts)
-        : INHERITED(dstInfo)
+    SkJpegScanlineDecoder(const SkImageInfo& srcInfo, SkJpegCodec* codec)
+        : INHERITED(srcInfo)
         , fCodec(codec)
-        , fOpts(opts)
+        , fOpts()

[scroggo_chromium, 2015/08/03 18:35:58]

The latest patch is a rebase, but I had to make some changes around
SkJpegScanlineDecoder::fOpts.

Previously, it was a const reference. (This is a bad idea anyway, since we do
not know how long the input object will last.)

It is no longer const, and it is no longer a reference. It cannot be const,
since start() (which calls this class's onStart()) may change the options. We
now default construct it in our constructor, and update it in onStart.

     {}
 
+    SkCodec::Result onStart(const SkImageInfo& dstInfo, const SkCodec::Options& options,
+                            SkPMColor ctable[], int* ctableCount) override {
+
+        // Rewind the stream if needed
+        if (!fCodec->handleRewind()) {
+            return SkCodec::kCouldNotRewind;
+        }
+
+        // Set the jump location for libjpeg errors
+        if (setjmp(fCodec->fDecoderMgr->getJmpBuf())) {
+            SkCodecPrintf("setjmp: Error from libjpeg\n");
+            return SkCodec::kInvalidInput;
+        }
+
+        // Check if we can decode to the requested destination and set the output color space
+        if (!fCodec->setOutputColorSpace(dstInfo)) {
+            return SkCodec::kInvalidConversion;
+        }
+
+        // Perform the necessary scaling
+        if (!fCodec->scaleToDimensions(dstInfo.width(), dstInfo.height())) {
+            return SkCodec::kInvalidScale;
+        }
+
+        // Now, given valid output dimensions, we can start the decompress
+        if (!turbo_jpeg_start_decompress(fCodec->fDecoderMgr->dinfo())) {
+            SkCodecPrintf("start decompress failed\n");
+            return SkCodec::kInvalidInput;
+        }
+
+        fOpts = options;
+
+        return SkCodec::kSuccess;
+    }
+
     virtual ~SkJpegScanlineDecoder() {
         if (setjmp(fCodec->fDecoderMgr->getJmpBuf())) {
             SkCodecPrintf("setjmp: Error in libjpeg finish_decompress\n");
             return;
         }
 
         // We may not have decoded the entire image.  Prevent libjpeg-turbo from failing on a
         // partial decode.
         fCodec->fDecoderMgr->dinfo()->output_scanline = fCodec->getInfo().height();
         turbo_jpeg_finish_decompress(fCodec->fDecoderMgr->dinfo());
@@ skipping 48 matching lines @@
             return fCodec->fDecoderMgr->returnFailure("setjmp", SkCodec::kInvalidInput);
         }
 
         turbo_jpeg_skip_scanlines(fCodec->fDecoderMgr->dinfo(), count);
 
         return SkCodec::kSuccess;
     }
 
 private:
     SkAutoTDelete<SkJpegCodec> fCodec;
-    const SkCodec::Options&    fOpts;
+    SkCodec::Options           fOpts;
 
     typedef SkScanlineDecoder INHERITED;
 };
 
-SkScanlineDecoder* SkJpegCodec::onGetScanlineDecoder(const SkImageInfo& dstInfo,
-        const Options& options, SkPMColor ctable[], int* ctableCount) {
-
-    // Rewind the stream if needed
-    if (!this->handleRewind()) {
-        SkCodecPrintf("Could not rewind\n");
-        return NULL;
-    }
-
-    // Set the jump location for libjpeg errors
-    if (setjmp(fDecoderMgr->getJmpBuf())) {
-        SkCodecPrintf("setjmp: Error from libjpeg\n");
-        return NULL;
-    }
-
-    SkStream* stream = this->stream()->duplicate();
-    if (!stream) {
-        return NULL;
-    }
+SkScanlineDecoder* SkJpegCodec::NewSDFromStream(SkStream* stream) {
     SkAutoTDelete<SkJpegCodec> codec(static_cast<SkJpegCodec*>(SkJpegCodec::NewFromStream(stream)));
     if (!codec) {
         return NULL;
     }
 
-    // Check if we can decode to the requested destination and set the output color space
-    if (!codec->setOutputColorSpace(dstInfo)) {
-        SkCodecPrintf("Cannot convert to output type\n");
-        return NULL;
-    }
-
-    // Perform the necessary scaling
-    if (!codec->scaleToDimensions(dstInfo.width(), dstInfo.height())) {
-        SkCodecPrintf("Cannot scale to output dimensions\n");
-        return NULL;
-    }
-
-    // Now, given valid output dimensions, we can start the decompress
-    if (!turbo_jpeg_start_decompress(codec->fDecoderMgr->dinfo())) {
-        SkCodecPrintf("start decompress failed\n");
-        return NULL;
-    }
-
+    const SkImageInfo& srcInfo = codec->getInfo();
     // Return the new scanline decoder
-    return SkNEW_ARGS(SkJpegScanlineDecoder, (dstInfo, codec.detach(), options));
+    return SkNEW_ARGS(SkJpegScanlineDecoder, (srcInfo, codec.detach()));
 }