Add support for multiple frames in SkCodec

src/codec/SkGifCodec.cpp

Index: src/codec/SkGifCodec.cpp
diff --git a/src/codec/SkGifCodec.cpp b/src/codec/SkGifCodec.cpp
index c35cd24ae498f5d12f62c39a687a95a57b9fcb08..233b0cf86fe2899dc79011d6ca1a32a2415f0ed2 100644
--- a/src/codec/SkGifCodec.cpp
+++ b/src/codec/SkGifCodec.cpp
@@ -5,6 +5,9 @@
  * found in the LICENSE file.
  */
 
+#include "SkBitmap.h"
+#include "SkCanvas.h"
+#include "SkCodecAnimation.h"
 #include "SkCodecPriv.h"
 #include "SkColorPriv.h"
 #include "SkColorTable.h"
@@ -59,9 +62,11 @@ static GifFileType* open_gif(SkStream* stream) {
 }
 
 /*
- * Check if a there is an index of the color table for a transparent pixel
+ * Read the graphics extension and report info from it. Returns false if there is none.
  */
-static uint32_t find_trans_index(const SavedImage& image) {
+static bool read_graphics_extension(const SavedImage& image, uint32_t* transIndex,
+                                    size_t* duration,
+                                    SkCodecAnimation::DisposalMethod* disposalMethod) {
     // If there is a transparent index specified, it will be contained in an
     // extension block.  We will loop through extension blocks in reverse order
     // to check the most recent extension blocks first.
@@ -78,44 +83,45 @@ static uint32_t find_trans_index(const SavedImage& image) {
             // Check the transparent color flag which indicates whether a
             // transparent index exists.  It is the least significant bit of
             // the first byte of the extension block.
-            if (1 == (extBlock.Bytes[0] & 1)) {
-                // Use uint32_t to prevent sign extending
-                return extBlock.Bytes[3];
+            if (transIndex) {
+                if (1 == (extBlock.Bytes[0] & 1)) {
+                    // Use uint32_t to prevent sign extending
+                    *transIndex = extBlock.Bytes[3];
+                } else {
+                    // Use maximum unsigned int (surely an invalid index) to indicate that a valid
+                    // index was not found.
+                    *transIndex = SK_MaxU32;
+                }
+            }
+
+            // The second and third bytes represent the duration.
+            if (duration) {
+                *duration = extBlock.Bytes[2] << 8 | extBlock.Bytes[1];
+            }
+
+            if (disposalMethod) {
+                int rawDisposalMethod = (extBlock.Bytes[0] >> 2) & 7;
+                switch (rawDisposalMethod) {
+                    case 1:
+                    case 2:
+                    case 3:
+                        *disposalMethod = (SkCodecAnimation::DisposalMethod) rawDisposalMethod;
+                        break;
+                    case 4:
+                        *disposalMethod = SkCodecAnimation::RestorePrevious_DisposalMethod;
+                        break;
+                    default:
+                        *disposalMethod = SkCodecAnimation::Keep_DisposalMethod;
+                        break;
+                }
             }
 
             // There should only be one graphics control extension for the image frame
-            break;
+            return true;
         }
     }
 
-    // Use maximum unsigned int (surely an invalid index) to indicate that a valid
-    // index was not found.
-    return SK_MaxU32;
-}
-
-inline uint32_t ceil_div(uint32_t a, uint32_t b) {
-    return (a + b - 1) / b;
-}
-
-/*
- * Gets the output row corresponding to the encoded row for interlaced gifs
- */
-inline uint32_t get_output_row_interlaced(uint32_t encodedRow, uint32_t height) {
-    SkASSERT(encodedRow < height);
-    // First pass
-    if (encodedRow * 8 < height) {
-        return encodedRow * 8;
-    }
-    // Second pass
-    if (encodedRow * 4 < height) {
-        return 4 + 8 * (encodedRow - ceil_div(height, 8));
-    }
-    // Third pass
-    if (encodedRow * 2 < height) {
-        return 2 + 4 * (encodedRow - ceil_div(height, 4));
-    }
-    // Fourth pass
-    return 1 + 2 * (encodedRow - ceil_div(height, 2));
+    return false;
 }
 
 /*
@@ -174,31 +180,21 @@ bool SkGifCodec::ReadHeader(SkStream* stream, SkCodec** codecOut, GifFileType**
         return false;
     }
 
-    // Read through gif extensions to get to the image data.  Set the
-    // transparent index based on the extension data.
-    uint32_t transIndex;
-    SkCodec::Result result = ReadUpToFirstImage(gif, &transIndex);
-    if (kSuccess != result){
+    // Read the whole file.
+    if (GIF_ERROR == DGifSlurp(gif) || gif->ImageCount == 0) {
         return false;
     }
 
-    // Read the image descriptor
-    if (GIF_ERROR == DGifGetImageDesc(gif)) {
+    SkISize size;
+    if (!GetDimensions(gif, &size)) {
+        gif_error("Invalid gif size.\n");
         return false;
     }
-    // If reading the image descriptor is successful, the image count will be
-    // incremented.
-    SkASSERT(gif->ImageCount >= 1);
 
-    if (nullptr != codecOut) {
-        SkISize size;
-        SkIRect frameRect;
-        if (!GetDimensions(gif, &size, &frameRect)) {
-            gif_error("Invalid gif size.\n");
-            return false;
-        }
-        bool frameIsSubset = (size != frameRect.size());
+    uint32_t transIndex = SK_MaxU32;
+    read_graphics_extension(gif->SavedImages[0], &transIndex, nullptr, nullptr);
 
+    if (nullptr != codecOut) {
         // Determine the encoded alpha type.  The transIndex might be valid if it less
         // than 256.  We are not certain that the index is valid until we process the color
         // table, since some gifs have color tables with less than 256 colors.  If
@@ -215,7 +211,7 @@ bool SkGifCodec::ReadHeader(SkStream* stream, SkCodec** codecOut, GifFileType**
         // FIXME: Gifs can actually be encoded with 4-bits per pixel.  Can we support this?
         SkEncodedInfo info = SkEncodedInfo::Make(SkEncodedInfo::kPalette_Color, alpha, 8);
         *codecOut = new SkGifCodec(size.width(), size.height(), info, streamDeleter.release(),
-                gif.release(), transIndex, frameRect, frameIsSubset);
+                gif.release(), transIndex);
     } else {
         SkASSERT(nullptr != gifOut);
         streamDeleter.release();
@@ -232,134 +228,105 @@ bool SkGifCodec::ReadHeader(SkStream* stream, SkCodec** codecOut, GifFileType**
 SkCodec* SkGifCodec::NewFromStream(SkStream* stream) {
     SkCodec* codec = nullptr;
     if (ReadHeader(stream, &codec, nullptr)) {
+        SkASSERT(codec);
         return codec;
     }
     return nullptr;
 }
 
+static SkColorTable* create_color_table(const ColorMapObject& colorMap, uint32_t transIndex,
+                                        uint32_t backgroundIndex);
 SkGifCodec::SkGifCodec(int width, int height, const SkEncodedInfo& info, SkStream* stream,
-        GifFileType* gif, uint32_t transIndex, const SkIRect& frameRect, bool frameIsSubset)
+        GifFileType* gif, uint32_t transIndex)
     : INHERITED(width, height, info, stream)
     , fGif(gif)
     , fSrcBuffer(new uint8_t[this->getInfo().width()])
-    , fFrameRect(frameRect)
-    // If it is valid, fTransIndex will be used to set fFillIndex.  We don't know if
-    // fTransIndex is valid until we process the color table, since fTransIndex may
-    // be greater than the size of the color table.
-    , fTransIndex(transIndex)
-    // Default fFillIndex is 0.  We will overwrite this if fTransIndex is valid, or if
-    // there is a valid background color.
     , fFillIndex(0)
-    , fFrameIsSubset(frameIsSubset)
     , fSwizzler(NULL)
     , fColorTable(NULL)
-{}
-
-bool SkGifCodec::onRewind() {
-    GifFileType* gifOut = nullptr;
-    if (!ReadHeader(this->stream(), nullptr, &gifOut)) {
-        return false;
+    , fFrameInfos(gif->ImageCount)
+{
+    if (gif->SColorMap) {
+        // FIXME: Note - this is the global color table.
+        fColorTable.reset(create_color_table(*gif->SColorMap, transIndex, gif->SBackGroundColor));
+    } else {
+        // Read the first image's local color table, for now.
+        const SavedImage* image = &gif->SavedImages[0];
+        const GifImageDesc& desc = image->ImageDesc;
+
+        if (desc.ColorMap) {
+            // I'm guessing the background index only makes sense for the global color table.
+            // A lot of users (including Chromium!) ignore the background color, but we (currently)
+            // use it to fill in the color table if it's less than 256 colors.
+            fColorTable.reset(create_color_table(*desc.ColorMap, transIndex, SK_MaxU32));
+        }
     }
 
-    SkASSERT(nullptr != gifOut);
-    fGif.reset(gifOut);
-    return true;
-}
-
-SkCodec::Result SkGifCodec::ReadUpToFirstImage(GifFileType* gif, uint32_t* transIndex) {
-    // Use this as a container to hold information about any gif extension
-    // blocks.  This generally stores transparency and animation instructions.
-    SavedImage saveExt;
-    SkAutoTCallVProc<SavedImage, FreeExtension> autoFreeExt(&saveExt);
-    saveExt.ExtensionBlocks = nullptr;
-    saveExt.ExtensionBlockCount = 0;
-    GifByteType* extData;
-    int32_t extFunction;
-
-    // We will loop over components of gif images until we find an image.  Once
-    // we find an image, we will decode and return it.  While many gif files
-    // contain more than one image, we will simply decode the first image.
-    GifRecordType recordType;
-    do {
-        // Get the current record type
-        if (GIF_ERROR == DGifGetRecordType(gif, &recordType)) {
-            return gif_error("DGifGetRecordType failed.\n", kInvalidInput);
+    for (int i = 0; i < gif->ImageCount; i++) {
+        const SavedImage* image = &gif->SavedImages[i];
+        const GifImageDesc& desc = image->ImageDesc;
+
+        auto& frame = fFrameInfos.push_back();
+        // FIXME: Probably want to intersect this with the bounds, just in case.
+        // But then we'll need to make sure we draw the right thing...
+        // i.e. if the top is cut off (desc.Top < 0), we want to start off drawing line -desc.Top
+        // (probably?)
+        // Or if we're cut off width-wise, we need to ensure we use the original row bytes
+        // (desc.Width) but the corrected width.
+        frame.fFrameRect = SkIRect::MakeXYWH(desc.Left, desc.Top, desc.Width, desc.Height);
+        if (!read_graphics_extension(*image, &frame.fTransIndex, &frame.fDuration,
+                                     &frame.fDisposalMethod)) {
+            frame.fDisposalMethod = SkCodecAnimation::Keep_DisposalMethod;
+            frame.fDuration = 0;
+            frame.fTransIndex = SK_MaxU32;
         }
-        switch (recordType) {
-            case IMAGE_DESC_RECORD_TYPE: {
-                *transIndex = find_trans_index(saveExt);
-
-                // FIXME: Gif files may have multiple images stored in a single
-                //        file.  This is most commonly used to enable
-                //        animations.  Since we are leaving animated gifs as a
-                //        TODO, we will return kSuccess after decoding the
-                //        first image in the file.  This is the same behavior
-                //        as SkImageDecoder_libgif.
-                //
-                //        Most times this works pretty well, but sometimes it
-                //        doesn't.  For example, I have an animated test image
-                //        where the first image in the file is 1x1, but the
-                //        subsequent images are meaningful.  This currently
-                //        displays the 1x1 image, which is not ideal.  Right
-                //        now I am leaving this as an issue that will be
-                //        addressed when we implement animated gifs.
-                //
-                //        It is also possible (not explicitly disallowed in the
-                //        specification) that gif files provide multiple
-                //        images in a single file that are all meant to be
-                //        displayed in the same frame together.  I will
-                //        currently leave this unimplemented until I find a
-                //        test case that expects this behavior.
-                return kSuccess;
-            }
-            // Extensions are used to specify special properties of the image
-            // such as transparency or animation.
-            case EXTENSION_RECORD_TYPE:
-                // Read extension data
-                if (GIF_ERROR == DGifGetExtension(gif, &extFunction, &extData)) {
-                    return gif_error("Could not get extension.\n", kIncompleteInput);
-                }
-
-                // Create an extension block with our data
-                while (nullptr != extData) {
-                    // Add a single block
 
-#if GIFLIB_MAJOR < 5
-                    if (AddExtensionBlock(&saveExt, extData[0],
-                                          &extData[1]) == GIF_ERROR) {
-#else
-                    if (GIF_ERROR == GifAddExtensionBlock(&saveExt.ExtensionBlockCount,
-                                                          &saveExt.ExtensionBlocks,
-                                                          extFunction, extData[0], &extData[1])) {
-#endif
-                        return gif_error("Could not add extension block.\n", kIncompleteInput);
+        if (0 == i) {
+            frame.fRequiredFrame = kIndependentFrame;
+        } else {
+            // FIXME: We could correct these after decoding (i.e. some frames may turn out to be
+            // independent although we did not determine that here).
+            const SkCodecAnimation::FrameInfo& prevFrame = fFrameInfos[i-1];
+            switch (prevFrame.fDisposalMethod) {
+                case SkCodecAnimation::Keep_DisposalMethod:
+                    frame.fRequiredFrame = i - 1;
+                    break;
+                case SkCodecAnimation::RestorePrevious_DisposalMethod:
+                    frame.fRequiredFrame = prevFrame.fRequiredFrame;
+                    break;
+                case SkCodecAnimation::RestoreBGColor_DisposalMethod:
+                    if (prevFrame.fFrameRect == SkIRect::MakeWH(width, height)
+                            || prevFrame.fRequiredFrame == kIndependentFrame) {
+                        frame.fRequiredFrame = kIndependentFrame;
+                    } else {
+                        frame.fRequiredFrame = i - 1;
                     }
-                    // Move to the next block
-                    if (GIF_ERROR == DGifGetExtensionNext(gif, &extData)) {
-                        return gif_error("Could not get next extension.\n", kIncompleteInput);
-                    }
-                }
-                break;
-
-            // Signals the end of the gif file
-            case TERMINATE_RECORD_TYPE:
-                break;
-
-            default:
-                // DGifGetRecordType returns an error if the record type does
-                // not match one of the above cases.  This should not be
-                // reached.
-                SkASSERT(false);
-                break;
+                    break;
+            }
         }
-    } while (TERMINATE_RECORD_TYPE != recordType);
+    }
+}
 
-    return gif_error("Could not find any images to decode in gif file.\n", kInvalidInput);
+size_t SkGifCodec::onGetRequiredFrame(size_t index) {
+    if ((int) index >= fFrameInfos.count()) {
+        return kIndependentFrame;
+    }
+
+    return fFrameInfos[index].fRequiredFrame;
+}
+
+size_t SkGifCodec::onGetFrameDuration(size_t index) {
+    if ((int) index >= fFrameInfos.count()) {
+        return 0;
+    }
+
+    return fFrameInfos[index].fDuration;
 }
 
-bool SkGifCodec::GetDimensions(GifFileType* gif, SkISize* size, SkIRect* frameRect) {
-    // Get the encoded dimension values
-    SavedImage* image = &gif->SavedImages[gif->ImageCount - 1];
+bool SkGifCodec::GetDimensions(GifFileType* gif, SkISize* size) {
+    // Get the encoded dimension values for the first frame. Some GIFs have frames that are bigger
+    // than the screen width and height. For the first frame only, we will expand in that case.
+    SavedImage* image = &gif->SavedImages[0];
     const GifImageDesc& desc = image->ImageDesc;
     int frameLeft = desc.Left;
     int frameTop = desc.Top;
@@ -380,41 +347,36 @@ bool SkGifCodec::GetDimensions(GifFileType* gif, SkISize* size, SkIRect* frameRe
         return false;
     }
 
-    frameRect->setXYWH(frameLeft, frameTop, frameWidth, frameHeight);
     size->set(width, height);
     return true;
 }
 
+constexpr uint32_t kMaxColors = 256;
+
 void SkGifCodec::initializeColorTable(const SkImageInfo& dstInfo, SkPMColor* inputColorPtr,
         int* inputColorCount) {
-    // Set up our own color table
-    const uint32_t maxColors = 256;
-    SkPMColor colorPtr[256];
-    if (NULL != inputColorCount) {
-        // We set the number of colors to maxColors in order to ensure
-        // safe memory accesses.  Otherwise, an invalid pixel could
-        // access memory outside of our color table array.
-        *inputColorCount = maxColors;
+    // FIXME: Use the correct one for the frame.
+    SkASSERT(fColorTable);
+    if (inputColorCount) {
+        *inputColorCount = fColorTable->count();
     }
 
-    // Get local color table
-    ColorMapObject* colorMap = fGif->Image.ColorMap;
-    // If there is no local color table, use the global color table
-    if (NULL == colorMap) {
-        colorMap = fGif->SColorMap;
-    }
+    copy_color_table(dstInfo, fColorTable, inputColorPtr, inputColorCount);
+}
 
-    uint32_t colorCount = 0;
-    if (NULL != colorMap) {
-        colorCount = colorMap->ColorCount;
-        // giflib guarantees these properties
-        SkASSERT(colorCount == (unsigned) (1 << (colorMap->BitsPerPixel)));
-        SkASSERT(colorCount <= 256);
-        PackColorProc proc = choose_pack_color_proc(false, dstInfo.colorType());
-        for (uint32_t i = 0; i < colorCount; i++) {
-            colorPtr[i] = proc(0xFF, colorMap->Colors[i].Red,
-                    colorMap->Colors[i].Green, colorMap->Colors[i].Blue);
-        }
+static SkColorTable* create_color_table(const ColorMapObject& colorMap, uint32_t transIndex,
+                                        uint32_t backgroundIndex) {
+    SkPMColor colorPtr[kMaxColors];
+    const uint32_t colorCount = colorMap.ColorCount;
+    // giflib guarantees these properties
+    SkASSERT(colorCount == (unsigned) (1 << (colorMap.BitsPerPixel)));
+    SkASSERT(colorCount <= 256);
+
+    // FIXME: We may be creating this at the wrong time - we don't know the dst color type.
+    PackColorProc proc = choose_pack_color_proc(false, kIndex_8_SkColorType);
+    for (uint32_t i = 0; i < colorCount; i++) {
+        const GifColorType& color = colorMap.Colors[i];
+        colorPtr[i] = proc(0xFF, color.Red, color.Green, color.Blue);
     }
 
     // Fill in the color table for indices greater than color count.
@@ -428,23 +390,24 @@ void SkGifCodec::initializeColorTable(const SkImageInfo& dstInfo, SkPMColor* inp
         // valid, we will let fFillIndex default to 0 (it is set to zero in the
         // constructor).  This behavior is not specified but matches
         // SkImageDecoder_libgif.
-        uint32_t backgroundIndex = fGif->SBackGroundColor;
-        if (fTransIndex < colorCount) {
-            colorPtr[fTransIndex] = SK_ColorTRANSPARENT;
-            fFillIndex = fTransIndex;
+        uint32_t fillIndex;
+        if (transIndex < colorCount) {
+            colorPtr[transIndex] = SK_ColorTRANSPARENT;
+            fillIndex = transIndex;
         } else if (backgroundIndex < colorCount) {
-            fFillIndex = backgroundIndex;
+            fillIndex = backgroundIndex;
+        } else {
+            fillIndex = 0;
         }
 
-        for (uint32_t i = colorCount; i < maxColors; i++) {
-            colorPtr[i] = colorPtr[fFillIndex];
+        for (uint32_t i = colorCount; i < kMaxColors; i++) {
+            colorPtr[i] = colorPtr[fillIndex];
         }
     } else {
-        sk_memset32(colorPtr, 0xFF000000, maxColors);
+        sk_memset32(colorPtr, 0xFF000000, kMaxColors);
     }
 
-    fColorTable.reset(new SkColorTable(colorPtr, maxColors));
-    copy_color_table(dstInfo, this->fColorTable, inputColorPtr, inputColorCount);
+    return new SkColorTable(colorPtr, kMaxColors);
 }
 
 SkCodec::Result SkGifCodec::prepareToDecode(const SkImageInfo& dstInfo, SkPMColor* inputColorPtr,
@@ -454,30 +417,30 @@ SkCodec::Result SkGifCodec::prepareToDecode(const SkImageInfo& dstInfo, SkPMColo
         return gif_error("Cannot convert input type to output type.\n", kInvalidConversion);
     }
 
+    if (opts.fFrameIndex > 0 && dstInfo.colorType() == kIndex_8_SkColorType) {
+        // FIXME: It is possible that a later frame can be decoded to index8, if it does one of the
+        // following:
+        // - Covers the entire previous frame
+        // - Shares a color table (and transparent index) with any prior frames that are showing.
+        // We must support index8 for the first frame to be backwards compatible on Android.
+        return gif_error("Cannot decode multiframe gif (except frame 0) as index 8.\n",
+                         kInvalidConversion);
+    }
+
+    if ((int) opts.fFrameIndex > fFrameInfos.count()) {
+        return gif_error("frame index out of range!\n", kInvalidParameters);
+    }
+
     // Initialize color table and copy to the client if necessary
     this->initializeColorTable(dstInfo, inputColorPtr, inputColorCount);
-
-    this->initializeSwizzler(dstInfo, opts);
     return kSuccess;
 }
 
-void SkGifCodec::initializeSwizzler(const SkImageInfo& dstInfo, const Options& opts) {
-    const SkPMColor* colorPtr = get_color_ptr(fColorTable.get());
-    const SkIRect* frameRect = fFrameIsSubset ? &fFrameRect : nullptr;
-    fSwizzler.reset(SkSwizzler::CreateSwizzler(this->getEncodedInfo(), colorPtr, dstInfo, opts,
-            frameRect));
-    SkASSERT(fSwizzler);
-}
-
-bool SkGifCodec::readRow() {
-    return GIF_ERROR != DGifGetLine(fGif, fSrcBuffer.get(), fFrameRect.width());
-}
-
 /*
  * Initiates the gif decode
  */
 SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
-                                        void* dst, size_t dstRowBytes,
+                                        void* pixels, size_t dstRowBytes,
                                         const Options& opts,
                                         SkPMColor* inputColorPtr,
                                         int* inputColorCount,
@@ -491,117 +454,101 @@ SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
         return gif_error("Scaling not supported.\n", kInvalidScale);
     }
 
-    // Initialize the swizzler
-    if (fFrameIsSubset) {
-        // Fill the background
-        SkSampler::Fill(dstInfo, dst, dstRowBytes, this->getFillValue(dstInfo),
-                opts.fZeroInitialized);
-    }
-
-    // Iterate over rows of the input
-    for (int y = fFrameRect.top(); y < fFrameRect.bottom(); y++) {
-        if (!this->readRow()) {
-            *rowsDecoded = y;
-            return gif_error("Could not decode line.\n", kIncompleteInput);
-        }
-        void* dstRow = SkTAddOffset<void>(dst, dstRowBytes * this->outputScanline(y));
-        fSwizzler->swizzle(dstRow, fSrcBuffer.get());
-    }
+    this->decodeFrame(dstInfo, pixels, dstRowBytes, opts);
     return kSuccess;
 }
 
-// FIXME: This is similar to the implementation for bmp and png.  Can we share more code or
-//        possibly make this non-virtual?
-uint64_t SkGifCodec::onGetFillValue(const SkImageInfo& dstInfo) const {
-    const SkPMColor* colorPtr = get_color_ptr(fColorTable.get());
-    return get_color_table_fill_value(dstInfo.colorType(), dstInfo.alphaType(), colorPtr,
-                                      fFillIndex, nullptr);
-}
-
-SkCodec::Result SkGifCodec::onStartScanlineDecode(const SkImageInfo& dstInfo,
-        const SkCodec::Options& opts, SkPMColor inputColorPtr[], int* inputColorCount) {
-    return this->prepareToDecode(dstInfo, inputColorPtr, inputColorCount, opts);
-}
-
-void SkGifCodec::handleScanlineFrame(int count, int* rowsBeforeFrame, int* rowsInFrame) {
-    if (fFrameIsSubset) {
-        const int currRow = this->currScanline();
-
-        // The number of rows that remain to be skipped before reaching rows that we
-        // actually must decode into.
-        // This must be at least zero.  We also make sure that it is less than or
-        // equal to count, since we will skip at most count rows.
-        *rowsBeforeFrame = SkTMin(count, SkTMax(0, fFrameRect.top() - currRow));
-
-        // Rows left to decode once we reach the start of the frame.
-        const int rowsLeft = count - *rowsBeforeFrame;
-
-        // Count the number of that extend beyond the bottom of the frame.  We do not
-        // need to decode into these rows.
-        const int rowsAfterFrame = SkTMax(0, currRow + rowsLeft - fFrameRect.bottom());
+// FIXME: This should be SkCodec::Result, probably?
+void SkGifCodec::decodeFrame(const SkImageInfo& dstInfo, void* pixels, size_t dstRowBytes,
+                             const Options& opts) {
+    SkBitmap tmpBm;
+    void* dst = pixels;
+
+    SkASSERT((int) opts.fFrameIndex < fFrameInfos.count());
+    const auto& frameInfo = fFrameInfos[opts.fFrameIndex];
+    const SkIRect& frameRect = frameInfo.fFrameRect;
+    const bool independent = frameInfo.fRequiredFrame == kIndependentFrame;
+    if (!independent) {
+        if (!opts.fHasPriorFrame) {
+            // Decode that frame into pixels.
+            Options prevFrameOpts(opts);
+            prevFrameOpts.fFrameIndex = frameInfo.fRequiredFrame;
+            this->decodeFrame(dstInfo, pixels, dstRowBytes, prevFrameOpts);
+        }
+        const auto& prevFrame = fFrameInfos[frameInfo.fRequiredFrame];
+        if (prevFrame.fDisposalMethod == SkCodecAnimation::RestoreBGColor_DisposalMethod) {
+            const SkIRect& prevRect = prevFrame.fFrameRect;
+            void* const eraseDst = SkTAddOffset<void>(pixels, prevRect.fTop * dstRowBytes
+                    + prevRect.fLeft * SkColorTypeBytesPerPixel(dstInfo.colorType()));
+            // FIXME: Is this the right color?
+            SkSampler::Fill(dstInfo.makeWH(prevRect.width(), prevRect.height()), eraseDst,
+                            dstRowBytes, SK_ColorTRANSPARENT, SkCodec::kNo_ZeroInitialized);
+        }
 
-        // Set the actual number of source rows that we need to decode.
-        *rowsInFrame = rowsLeft - rowsAfterFrame;
+        // Now we need to swizzle to a temporary bitmap. That way we do not overwrite pixels
+        // with transparent. Later we'll draw to pixels
+        const int frameWidth = frameRect.width();
+        const int frameHeight = frameRect.height();
+        SkImageInfo tmpInfo = dstInfo.makeWH(frameWidth, frameHeight);
+        // FIXME: Could be more agressive than this (i.e. it could be out of the actual range of
+        // the color map).
+        if (frameInfo.fTransIndex < 256) {
+            // Need alpha to blend with prior frame.
+            tmpInfo = tmpInfo.makeAlphaType(kUnpremul_SkAlphaType);
+        }
+        tmpBm.allocPixels(tmpInfo);
+        dst = tmpBm.getPixels();
     } else {
-        *rowsBeforeFrame = 0;
-        *rowsInFrame = count;
-    }
-}
-
-int SkGifCodec::onGetScanlines(void* dst, int count, size_t rowBytes) {
-    int rowsBeforeFrame;
-    int rowsInFrame;
-    this->handleScanlineFrame(count, &rowsBeforeFrame, &rowsInFrame);
-
-    if (fFrameIsSubset) {
-        // Fill the requested rows
-        SkImageInfo fillInfo = this->dstInfo().makeWH(this->dstInfo().width(), count);
-        uint64_t fillValue = this->onGetFillValue(this->dstInfo());
-        fSwizzler->fill(fillInfo, dst, rowBytes, fillValue, this->options().fZeroInitialized);
-
-        // Start to write pixels at the start of the image frame
-        dst = SkTAddOffset<void>(dst, rowBytes * rowsBeforeFrame);
-    }
-
-    for (int i = 0; i < rowsInFrame; i++) {
-        if (!this->readRow()) {
-            return i + rowsBeforeFrame;
+        // This frame is independent
+        if (frameRect != dstInfo.bounds()) {
+            // Fill the background
+            // FIXME: Android may want the BG color, but Chromium wants transparent!
+            SkSampler::Fill(dstInfo, pixels, dstRowBytes, SK_ColorTRANSPARENT,
+                    opts.fZeroInitialized);
+
+            // Now offset dst to draw the frame:
+            dst = SkTAddOffset<void>(dst, frameRect.fTop * dstRowBytes
+                    + frameRect.fLeft * SkColorTypeBytesPerPixel(dstInfo.colorType()));
         }
-        fSwizzler->swizzle(dst, fSrcBuffer.get());
-        dst = SkTAddOffset<void>(dst, rowBytes);
     }
 
-    return count;
-}
+    SkASSERT((int) opts.fFrameIndex < fGif->ImageCount);
+    const SavedImage* image = &fGif->SavedImages[opts.fFrameIndex];
+    const GifImageDesc& desc = image->ImageDesc;
 
-bool SkGifCodec::onSkipScanlines(int count) {
-    int rowsBeforeFrame;
-    int rowsInFrame;
-    this->handleScanlineFrame(count, &rowsBeforeFrame, &rowsInFrame);
+    ColorMapObject* cmap = desc.ColorMap ? desc.ColorMap : fGif->SColorMap;
+    sk_sp<SkColorTable> ct(create_color_table(*cmap, frameInfo.fTransIndex, SK_MaxU32));
 
-    for (int i = 0; i < rowsInFrame; i++) {
-        if (!this->readRow()) {
-            return false;
-        }
-    }
+    std::unique_ptr<SkSwizzler> swizzler(SkSwizzler::CreateSwizzler(this->getEncodedInfo(),
+            ct->readColors(), dstInfo, opts));
+    SkASSERT(swizzler);
 
-    return true;
-}
+    // Iterate over rows of the input
+    void* dstRow = dst;
+    GifByteType* src = image->RasterBits;
+    for (int y = 0; y < desc.Height; y++) {
+        swizzler->swizzle(dstRow, src);
+        dstRow = SkTAddOffset<void>(dstRow, dstRowBytes);
+        src = SkTAddOffset<GifByteType>(src, desc.Width);
+    }
 
-SkCodec::SkScanlineOrder SkGifCodec::onGetScanlineOrder() const {
-    if (fGif->Image.Interlace) {
-        return kOutOfOrder_SkScanlineOrder;
+    if (!independent) {
+        // We drew to a temporary bitmap. Now we need to draw it to pixels.
+        SkASSERT(tmpBm.getPixels() == dst);
+        SkBitmap dstBm;
+        // FIXME: What to do if this fails?
+        dstBm.installPixels(dstInfo, pixels, dstRowBytes, fColorTable, nullptr, nullptr);
+        SkCanvas canvas(dstBm);
+        const SkScalar left = SkIntToScalar(frameRect.left());
+        const SkScalar top = SkIntToScalar(frameRect.top());
+        canvas.drawBitmap(tmpBm, left, top);
     }
-    return kTopDown_SkScanlineOrder;
 }
 
-int SkGifCodec::onOutputScanline(int inputScanline) const {
-    if (fGif->Image.Interlace) {
-        if (inputScanline < fFrameRect.top() || inputScanline >= fFrameRect.bottom()) {
-            return inputScanline;
-        }
-        return get_output_row_interlaced(inputScanline - fFrameRect.top(), fFrameRect.height()) +
-                fFrameRect.top();
-    }
-    return inputScanline;
+// FIXME: This is similar to the implementation for bmp and png.  Can we share more code or
+//        possibly make this non-virtual?
+uint64_t SkGifCodec::onGetFillValue(const SkImageInfo& dstInfo) const {
+    const SkPMColor* colorPtr = get_color_ptr(fColorTable.get());
+    return get_color_table_fill_value(dstInfo.colorType(), dstInfo.alphaType(), colorPtr,
+                                      fFillIndex, nullptr);
 }