Creating a new wrapper for gif decoder

src/codec/SkCodec_libgif.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_libgif.h"
+#include "SkCodecPriv.h"
+#include "SkColorPriv.h"
+#include "SkColorTable.h"
+#include "SkGifInterlaceIter.h"
+#include "SkStream.h"
+#include "SkSwizzler.h"
+#include "SkUtils.h"
+
+/*
+ * Checks the start of the stream to see if the image is a gif
+ */
+bool SkGifCodec::IsGif(SkStream* stream) {
+    char buf[GIF_STAMP_LEN];
+    if (stream->read(buf, GIF_STAMP_LEN) == GIF_STAMP_LEN) {
+        if (memcmp(GIF_STAMP,   buf, GIF_STAMP_LEN) == 0 ||
+                memcmp(GIF87_STAMP, buf, GIF_STAMP_LEN) == 0 ||
+                memcmp(GIF89_STAMP, buf, GIF_STAMP_LEN) == 0) {
+            return true;
+        }
+    }
+    return false;
+}
+
+/*
+ * Warning reporting function
+ */
+static void gif_warning(const char* msg) {
+    SkCodecPrintf("Gif Warning: %s\n", msg);
+}
+
+/*
+ * Error function
+ */
+static SkCodec::Result gif_error(const char* msg,
+        SkCodec::Result result = SkCodec::kInvalidInput) {
+    SkCodecPrintf("Gif Error: %s\n", msg);
+    return result;
+}
+
+
+/*
+ * Read function that will be passed to gif_lib
+ */
+static int32_t read_bytes_callback(GifFileType* fileType, GifByteType* out,
+        int32_t size) {
+    SkStream* stream = (SkStream*) fileType->UserData;
+    return (int32_t) stream->read(out, size);
+}
+
+/*
+ * Open the gif file
+ */
+static GifFileType* open_gif(SkStream* stream) {
+#if GIFLIB_MAJOR < 5
+    return DGifOpen(stream, read_bytes_callback);
+#else
+    return DGifOpen(stream, read_bytes_callback, NULL);
+#endif
+}
+
+ /*
+ * This function cleans up the gif object after the decode completes
+ * It is used in a SkAutoTCallIProc template
+ */
+int32_t SkGifCodec::CloseGif(GifFileType* gif) {
+#if GIFLIB_MAJOR < 5 || (GIFLIB_MAJOR == 5 && GIFLIB_MINOR == 0)
+    return DGifCloseFile(gif);
+#else
+    return DGifCloseFile(gif, NULL);
+#endif
+}
+
+/*
+ * This function free extension data that has been saved to assist the image
+ * decoder
+ */
+void SkGifCodec::FreeExtension(SavedImage* image) {
+    if (NULL != image->ExtensionBlocks) {
+#if GIFLIB_MAJOR < 5
+        FreeExtension(image);
+#else
+        GifFreeExtensions(&image->ExtensionBlockCount, &image->ExtensionBlocks);
+#endif
+    }
+}
+
+/*
+ * Check if a there is an index of the color table for a transparent pixel
+ */
+static uint32_t find_trans_index(const SavedImage& image) {
+    // 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.
+    for (int32_t i = image.ExtensionBlockCount - 1; i >= 0; i--) {
+        // Get an extension block
+        const ExtensionBlock& extBlock = image.ExtensionBlocks[i];
+
+        // Specifically, we need to check for a graphics control extension,
+        // which may contain transparency information.  Also, note that a valid
+        // graphics control extension is always four bytes.  The fourth byte
+        // is the transparent index (if it exists), so we need at least four
+        // bytes.
+        if (GRAPHICS_EXT_FUNC_CODE == extBlock.Function &&
+                extBlock.ByteCount >= 4) {
+
+            // 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];
+            }
+
+            // There should only be one graphics control extension for the image frame
+            break;
+        }
+    }
+
+    // Flag indicating that a valid index was not found
+    return (uint32_t) -1;

[scroggo, 2015/03/31 20:39:11]

Why not return a signed type?

[msarett, 2015/03/31 21:10:56]

It's my personal preference for indices to be unsigned when possible.  Hopefully
using max int as the invalid flag is better than this ridiculous cast.

+}
+
+/*
+ * Assumes IsGif was called and returned true
+ * Creates a gif decoder
+ * Reads enough of the stream to determine the image format
+ */
+SkCodec* SkGifCodec::NewFromStream(SkStream* stream) {
+    // Read gif header, logical screen descriptor, and global color table
+    SkAutoTCallIProc<GifFileType, CloseGif> gif(open_gif(stream));
+
+    if (NULL == gif) {
+        gif_error("DGifOpen failed.\n");
+        return NULL;
+    }
+
+    // Get fields from header
+    const int32_t width = gif->SWidth;
+    const int32_t height = gif->SHeight;
+    if (width <= 0 || height <= 0) {
+        gif_error("Invalid dimensions.\n");
+        return NULL;
+    }
+
+    // Return the codec
+    // kIndex is the most natural color type for gifs, so we set this as
+    // the default.
+    // Many gifs specify a color table index for transparent pixels.  Every
+    // other pixel is guaranteed to be opaque.  Despite this, because of the
+    // possiblity of transparent pixels, we cannot assume that the image is
+    // opaque.  We have the option to set the alpha type as kPremul or
+    // kUnpremul.  Both are valid since the alpha component will always be
+    // 0xFF or the entire 32-bit pixel will be set to zero.  We prefer
+    // kPremul because we support kPremul, and it is more efficient to
+    // use kPremul directly even when kUnpremul is supported.
+    const SkImageInfo& imageInfo = SkImageInfo::Make(width, height,
+            kIndex_8_SkColorType, kPremul_SkAlphaType);
+    return SkNEW_ARGS(SkGifCodec, (imageInfo, stream, gif.detach()));
+}
+
+SkGifCodec::SkGifCodec(const SkImageInfo& srcInfo, SkStream* stream,
+                       GifFileType* gif)
+    : INHERITED(srcInfo, stream)
+    , fGif(gif)
+{}
+
+/*
+ * Checks if the conversion between the input image and the requested output
+ * image has been implemented
+ */
+static bool conversion_possible(const SkImageInfo& dst,
+                                const SkImageInfo& src) {
+    // Ensure that the profile type is unchanged
+    if (dst.profileType() != src.profileType()) {
+        return false;
+    }
+
+    // Check for supported color and alpha types
+    switch (dst.colorType()) {
+        case kN32_SkColorType:
+            return kPremul_SkAlphaType == dst.alphaType() ||
+                    kUnpremul_SkAlphaType == dst.alphaType();
+        default:
+            return false;
+    }
+}
+
+/*
+ * Initiates the gif decode
+ */
+SkCodec::Result SkGifCodec::onGetPixels(const SkImageInfo& dstInfo,
+                                        void* dst, size_t dstRowBytes,
+                                        const Options& opts, SkPMColor*, int*) {
+    // Check for valid input parameters
+    if (!this->rewindIfNeeded()) {
+        return kCouldNotRewind;
+    }
+    if (dstInfo.dimensions() != this->getInfo().dimensions()) {
+        return gif_error("Scaling not supported.\n", kInvalidScale);
+    }
+    if (!conversion_possible(dstInfo, this->getInfo())) {
+        return gif_error("Cannot convert input type to output type.\n",
+                kInvalidConversion);
+    }
+
+    // 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 = NULL;
+    saveExt.ExtensionBlockCount = 0;
+    GifByteType* extData;
+#if GIFLIB_MAJOR >= 5
+    int32_t extFunction;
+#endif
+
+    // 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.
+    const int32_t width = dstInfo.width();
+    const int32_t height = dstInfo.height();
+    GifRecordType recordType = UNDEFINED_RECORD_TYPE;
+    while (TERMINATE_RECORD_TYPE != recordType) {

[scroggo, 2015/03/31 20:39:11]

nit: It seems like this could be a do while loop. Then you don't need to
initialize recordType (it will be set by DGifGetRecordType before we ever read
it).

[msarett, 2015/03/31 21:10:56]

Done.

+        // Get the current record type
+        if (GIF_ERROR == DGifGetRecordType(fGif, &recordType)) {
+            return gif_error("DGifGetRecordType failed.\n", kInvalidInput);
+        }
+
+        switch (recordType) {
+            case IMAGE_DESC_RECORD_TYPE: {
+                // Read the image descriptor
+                if (GIF_ERROR == DGifGetImageDesc(fGif)) {
+                    return gif_error("DGifGetImageDesc failed.\n",
+                            kInvalidInput);
+                }
+
+                // If reading the image descriptor is successful, the image
+                // count will be incremented
+                SkASSERT(fGif->ImageCount >= 1);
+                SavedImage* image = &fGif->SavedImages[fGif->ImageCount - 1];
+
+                // Process the descriptor
+                const GifImageDesc& desc = image->ImageDesc;
+                int32_t imageLeft = desc.Left;
+                int32_t imageTop = desc.Top;
+                int32_t innerWidth = desc.Width;
+                int32_t innerHeight = desc.Height;
+                // Fail on non-positive dimensions
+                if (innerWidth <= 0 || innerHeight <= 0) {
+                    return gif_error("Invalid dimensions for inner image.\n",
+                            kInvalidInput);
+                }
+                // Treat the following cases as warnings and try to fix
+                if (innerWidth > width) {
+                    gif_warning("Inner image too wide, shrinking.\n");
+                    innerWidth = width;
+                    imageLeft = 0;
+                } else if (imageLeft + innerWidth > width) {
+                    gif_warning("Shifting inner image to left to fit.\n");
+                    imageLeft = width - innerWidth;
+                } else if (imageLeft < 0) {
+                    gif_warning("Shifting image to right to fit\n");
+                    imageLeft = 0;
+                }
+                if (innerHeight > height) {
+                    gif_warning("Inner image too tall, shrinking.\n");
+                    innerHeight = height;
+                    imageTop = 0;
+                } else if (imageTop + innerHeight > height) {
+                    gif_warning("Shifting inner image up to fit.\n");
+                    imageTop = height - innerHeight;
+                } else if (imageTop < 0) {
+                    gif_warning("Shifting image down to fit\n");
+                    imageTop = 0;
+                }
+
+                // Set up the color table
+                uint32_t colorCount = 0;
+                // Allocate maximum storage to deal with invalid indices safely
+                const uint32_t maxColors = 256;
+                SkPMColor colorTable[maxColors];
+                ColorMapObject* colorMap = fGif->Image.ColorMap;
+                // If there is no local color table, use the global color table
+                if (NULL == colorMap) {
+                    colorMap = fGif->SColorMap;
+                }
+                if (NULL != colorMap) {
+                    colorCount = colorMap->ColorCount;
+                    SkASSERT(colorCount ==
+                            (unsigned) (1 << (colorMap->BitsPerPixel)));
+                    SkASSERT(colorCount <= 256);
+                    for (uint32_t i = 0; i < colorCount; i++) {
+                        colorTable[i] = SkPackARGB32(0xFF,
+                                                     colorMap->Colors[i].Red,
+                                                     colorMap->Colors[i].Green,
+                                                     colorMap->Colors[i].Blue);
+                    }
+                }
+
+                // This is used to fill unspecified pixels in the image data.
+                uint32_t fillIndex = fGif->SBackGroundColor;
+                bool fillBackground = true;
+                ZeroInitialized zeroInit = opts.fZeroInitialized;
+
+                // Gifs have the option to specify the color at a single
+                // index of the color table as transparent.
+                {
+                    uint32_t transIndex = find_trans_index(saveExt);
+
+                    // If the background is already zeroed and we have a valid
+                    // transparent index, we do not need to fill the background.
+                    if (transIndex < colorCount) {

[scroggo, 2015/03/31 20:39:11]

This seems a little sneaky:

It seems that find_trans_index returns max uint32_t on failure, so we can use
one if statement to take care of both failure cases (i.e. not found and out of
range). I think this could use a comment (plus one by find_trans_index).

[msarett, 2015/03/31 21:10:56]

Done.

+                        colorTable[transIndex] = SK_ColorTRANSPARENT;
+                        // If there is a transparent index, we also use this as
+                        // the fill index.
+                        fillIndex = transIndex;
+                        fillBackground = (kYes_ZeroInitialized != zeroInit);
+                    } else if (fillIndex >= colorCount) {
+                        // If the fill index is invalid, we default to 0.  This
+                        // behavior is unspecified but matches SkImageDecoder.
+                        fillIndex = 0;
+                    }
+                }
+
+                // Fill in the color table for indices greater than color count.
+                // This allows for predictable, safe behavior.
+                for (uint32_t i = colorCount; i < maxColors; i++) {
+                    colorTable[i] = colorTable[fillIndex];
+                } 
+
+                // Check if image is only a subset of the image frame
+                SkAutoTDelete<SkSwizzler> swizzler(NULL);
+                if (innerWidth < width || innerHeight < height) {
+
+                    // Modify the destination info
+                    const SkImageInfo subsetDstInfo =
+                            dstInfo.makeWH(innerWidth, innerHeight);
+
+                    // Fill the destination with the fill color
+                    // FIXME: This may not be the behavior that we want for
+                    //        animated gifs where we draw on top of the
+                    //        previous frame.
+                    SkColorType dstColorType = dstInfo.colorType();
+                    if (fillBackground) {
+                        switch (dstColorType) {
+                            case kN32_SkColorType:
+                                sk_memset32((SkPMColor*) dst,
+                                    colorTable[fillIndex],
+                                    ((int) dstRowBytes) * height
+                                    / sizeof(SkPMColor));
+                                break;
+                            default:
+                                SkASSERT(false);
+                                break;
+                        }
+                    }
+
+                    // Modify the dst pointer
+                    const int32_t dstBytesPerPixel =
+                            SkColorTypeBytesPerPixel(dstColorType);
+                    void* subsetDst = SkTAddOffset<void*>(dst,
+                            dstRowBytes * imageTop +
+                            dstBytesPerPixel * imageLeft);
+
+                    // Create the subset swizzler
+                    swizzler.reset(SkSwizzler::CreateSwizzler(
+                            SkSwizzler::kIndex, colorTable, subsetDstInfo,
+                            subsetDst, dstRowBytes, zeroInit));
+                } else {
+                    // Create the fully dimensional swizzler
+                    swizzler.reset(SkSwizzler::CreateSwizzler(
+                            SkSwizzler::kIndex, colorTable, dstInfo, dst,
+                            dstRowBytes, zeroInit));
+                }
+
+                // Stores output from dgiflib and input to the swizzler
+                SkAutoTDeleteArray<uint8_t>
+                        buffer(SkNEW_ARRAY(uint8_t, innerWidth));
+
+                // Check the interlace flag and iterate over rows of the input
+                if (fGif->Image.Interlace) {
+                    // In interlace mode, the rows of input are rearranged in
+                    // the output image.  We use an iterator to take care of
+                    // the rearranging.
+                    SkGifInterlaceIter iter(innerHeight);
+                    for (int32_t y = 0; y < innerHeight; y++) {
+                        if (GIF_ERROR == DGifGetLine(fGif, buffer.get(),
+                                innerWidth)) {
+                            // Recover from error by filling remainder of image
+                            if (fillBackground) {
+                                memset(buffer.get(), fillIndex, innerWidth);
+                                for (; y < innerHeight; y++) {
+                                    swizzler->next(buffer.get(), iter.nextY());
+                                }
+                            }
+                            return gif_error(SkStringPrintf(
+                                    "Could not decode line %d of %d.\n",
+                                    y, height - 1).c_str(), kIncompleteInput);
+                        }
+                        swizzler->next(buffer.get(), iter.nextY());
+                    }
+                } else {
+                    // Standard mode
+                    for (int32_t y = 0; y < innerHeight; y++) {
+                        if (GIF_ERROR == DGifGetLine(fGif, buffer.get(),
+                                innerWidth)) {
+                            if (fillBackground) {
+                                SkPMColor* dstPtr = (SkPMColor*) SkTAddOffset
+                                        <void*>(dst, y * dstRowBytes);
+                                sk_memset32(dstPtr, colorTable[fillIndex],
+                                        (height - y) * ((int) dstRowBytes)
+                                        / sizeof(SkPMColor));
+                            }
+                            return gif_error(SkStringPrintf(
+                                    "Could not decode line %d of %d.\n",
+                                    y, height - 1).c_str(), kIncompleteInput);
+                        }
+                        swizzler->next(buffer.get());
+                    }
+                }
+
+                // 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 GIFLIB_MAJOR < 5
+                if (GIF_ERROR ==
+                        DGifGetExtension(fGif, &saveExt.Function, &extData)) {
+#else
+                if (GIF_ERROR ==
+                        DGifGetExtension(fGif, &extFunction, &extData)) {
+#endif
+                    return gif_error("Could not get extension.\n",
+                            kIncompleteInput);
+                }
+
+                // Create an extension block with our data
+                while (NULL != extData) {
+                    // Add a single block
+#if GIFLIB_MAJOR < 5
+                    if (GIF_ERROR == AddExtensionBlock(&saveExt, extData[0],
+                            &extData[1])) {
+#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);
+                    }
+                    // Move to the next block
+                    if (GIF_ERROR == DGifGetExtensionNext(fGif, &extData)) {
+                        return gif_error("Could not get next extension.\n",
+                                kIncompleteInput);
+                    }
+#if GIFLIB_MAJOR < 5
+                    saveExt.Function = 0;
+#endif
+                }
+                break;
+
+            // Signals the end of the gif file
+            case TERMINATE_RECORD_TYPE:
+                break;
+
+            default:
+                // giflib returns an error code if the record type is not known.
+                // We should catch this error immediately.
+                SkASSERT(false);
+                break;
+        }
+    }
+
+    return gif_error("Could not find any images to decode in gif file.\n",
+            kInvalidInput);
+}