#include <image_generator_apng.h>

Inheritance diagram for flutter::APNGImageGenerator:

Public Member Functions
	~APNGImageGenerator ()

const SkImageInfo &	GetInfo () override
	Returns basic information about the contents of the encoded image. This information can almost always be collected by just interpreting the header of a decoded image.

unsigned int	GetFrameCount () const override
	Get the number of frames that the encoded image stores. This method is always expected to be called before `GetFrameInfo`, as the underlying image decoder may interpret frame information that is then used when calling `GetFrameInfo`.

unsigned int	GetPlayCount () const override
	The number of times an animated image should play through before playback stops.

const ImageGenerator::FrameInfo	GetFrameInfo (unsigned int frame_index) override
	Get information about a single frame in the context of a multi-frame image, useful for animation and frame blending. This method should only ever be called after `GetFrameCount` has been called. This information is nonsensical for single-frame images.

SkISize	GetScaledDimensions (float desired_scale) override
	Given a scale value, find the closest image size that can be used for efficiently decoding the image. If subpixel image decoding is not supported by the decoder, this method should just return the original image size.

bool	GetPixels (const SkImageInfo &info, void *pixels, size_t row_bytes, unsigned int frame_index, std::optional< unsigned int > prior_frame) override
	Decode the image into a given buffer. This method is currently always used for sub-pixel image decoding. For full-sized still images, `GetImage` is always attempted first.

Public Member Functions inherited from flutter::ImageGenerator
virtual	~ImageGenerator ()

sk_sp< SkImage >	GetImage ()
	Creates an `SkImage` based on the current `ImageInfo` of this `ImageGenerator`.

Static Public Member Functions
static std::unique_ptr< ImageGenerator >	MakeFromData (sk_sp< SkData > data)

static uint32_t	ComputeCrc32 (const uint8_t *data, size_t length)
	Computes the CRC of the data in a PNG chunk.

Additional Inherited Members
Static Public Attributes inherited from flutter::ImageGenerator
static const unsigned int	kInfinitePlayCount
	Frame count value to denote infinite looping.

Detailed Description

Definition at line 27 of file image_generator_apng.h.

Constructor & Destructor Documentation

◆ ~APNGImageGenerator()

flutter::APNGImageGenerator::~APNGImageGenerator ( )

default

Member Function Documentation

◆ ComputeCrc32()

uint32_t flutter::APNGImageGenerator::ComputeCrc32	(	const uint8_t *	data,
		size_t	length
	)

static

Computes the CRC of the data in a PNG chunk.

Definition at line 665 of file image_generator_apng.cc.

                                                                            {
  uint32_t crc = 0;
  const uint8_t* data_p = data;
 
  // zlib's crc32 can only take 16 bits at a time for the length, but PNG
  // supports a 32 bit chunk length, so looping is necessary here.
  // Note that crc32 is always called at least once, even if the chunk has an
  // empty data section.
  do {
    uint16_t length16 = length;
    if (length16 == 0 && length > 0) {
      length16 = std::numeric_limits<uint16_t>::max();
    }
 
    crc = crc32(crc, data_p, length16);
    length -= length16;
    data_p += length16;
  } while (length > 0);
 
  return crc;
}

References flutter::data, and length.

◆ GetFrameCount()

unsigned int flutter::APNGImageGenerator::GetFrameCount ( ) const

overridevirtual

Get the number of frames that the encoded image stores. This method is always expected to be called before GetFrameInfo, as the underlying image decoder may interpret frame information that is then used when calling GetFrameInfo.

Returns: The number of frames that the encoded image stores. This will always be 1 for single-frame images.

Implements flutter::ImageGenerator.

Definition at line 44 of file image_generator_apng.cc.

                                                     {
  return frame_count_;
}

◆ GetFrameInfo()

const ImageGenerator::FrameInfo flutter::APNGImageGenerator::GetFrameInfo ( unsigned int frame_index )

overridevirtual

Get information about a single frame in the context of a multi-frame image, useful for animation and frame blending. This method should only ever be called after GetFrameCount has been called. This information is nonsensical for single-frame images.

Parameters

[in] frame_index The index of the frame to get information about.

Returns: Information about the given frame. If the image is single-frame, a default result is returned.

See also: GetFrameCount

Implements flutter::ImageGenerator.

Definition at line 52 of file image_generator_apng.cc.

                              {
  unsigned int image_index = first_frame_index_ + frame_index;
  if (!DemuxToImageIndex(image_index)) {
    return {};
  }
 
  auto frame_info = images_[image_index].frame_info;
  if (frame_info.has_value()) {
    return frame_info.value();
  }
  return {};
}

◆ GetInfo()

const SkImageInfo & flutter::APNGImageGenerator::GetInfo ( )

overridevirtual

Returns basic information about the contents of the encoded image. This information can almost always be collected by just interpreting the header of a decoded image.

Returns: Size and color information describing the image.

Note: This method is executed on the UI thread and used for layout purposes by the framework, and so this method should not perform long synchronous tasks.

Implements flutter::ImageGenerator.

Definition at line 40 of file image_generator_apng.cc.

                                               {
  return image_info_;
}

◆ GetPixels()

bool flutter::APNGImageGenerator::GetPixels	(	const SkImageInfo &	info,
		void *	pixels,
		size_t	row_bytes,
		unsigned int	frame_index,
		std::optional< unsigned int >	prior_frame
	)

overridevirtual

Decode the image into a given buffer. This method is currently always used for sub-pixel image decoding. For full-sized still images, GetImage is always attempted first.

Parameters

[in]	info	The desired size and color info of the decoded image to be returned. The implementation of `GetScaledDimensions` determines which sizes are supported by the image decoder.
[in]	pixels	The location where the raw decoded image data should be written.
[in]	row_bytes	The total number of bytes that should make up a single row of decoded image data (i.e. width * bytes_per_pixel).
[in]	frame_index	Which frame to decode. This is only useful for multi-frame images.
[in]	prior_frame	Optional frame index parameter for multi-frame images which specifies the previous frame that should be use for blending. This hints to the decoder that it should use a previously cached frame instead of decoding dependency frame(s). If an empty value is supplied, the decoder should decode any necessary frames first.

Returns: True if the image was successfully decoded.

Note: This method performs potentially long synchronous work, and so it should never be executed on the UI thread. Image decoders do not require GPU acceleration, and so threads without a GPU context may also be used.

See also: GetScaledDimensions

Demux the frame from the APNG stream.

Decode the frame.

Composite the frame onto the canvas.

Implements flutter::ImageGenerator.

Definition at line 70 of file image_generator_apng.cc.

                                                                          {
  FML_DCHECK(images_.size() > 0);
  unsigned int image_index = first_frame_index_ + frame_index;
 
  //----------------------------------------------------------------------------
  /// 1. Demux the frame from the APNG stream.
  ///
 
  if (!DemuxToImageIndex(image_index)) {
    FML_DLOG(ERROR) << "Couldn't demux image at index " << image_index
                    << " (frame index: " << frame_index
                    << ") from APNG stream.";
    return RenderDefaultImage(info, pixels, row_bytes);
  }
 
  //----------------------------------------------------------------------------
  /// 2. Decode the frame.
  ///
 
  APNGImage& frame = images_[image_index];
  SkImageInfo frame_info = frame.codec->getInfo();
 
  fml::SafeMath safe;
  size_t frame_row_bytes =
      safe.mul(frame_info.bytesPerPixel(), frame_info.width());
  if (safe.overflow_detected()) {
    FML_DLOG(ERROR) << "Failed to decode image at index " << image_index
                    << " (frame index: " << frame_index
                    << ") of APNG due to frame row bytes overflow.";
    return false;
  }
 
  if (frame.pixels.empty()) {
    size_t pixels_bytes = safe.mul(frame_row_bytes, frame_info.height());
    if (safe.overflow_detected()) {
      FML_DLOG(ERROR) << "Failed to decode image at index " << image_index
                      << " (frame index: " << frame_index
                      << ") of APNG due to pixel buffer size overflow.";
      return false;
    }
    frame.pixels.resize(pixels_bytes);
    SkCodec::Result result = frame.codec->getPixels(
        frame.codec->getInfo(), frame.pixels.data(), frame_row_bytes);
    if (result != SkCodec::kSuccess) {
      FML_DLOG(ERROR) << "Failed to decode image at index " << image_index
                      << " (frame index: " << frame_index
                      << ") of APNG. SkCodec::Result: " << result;
      return RenderDefaultImage(info, pixels, row_bytes);
    }
  }
  if (!frame.frame_info.has_value()) {
    FML_DLOG(ERROR) << "Failed to decode image at index " << image_index
                    << " (frame index: " << frame_index
                    << ") of APNG due to the frame missing data (frame_info).";
    return false;
  }
  if (
      // Check for unsigned integer wrapping for
      // frame.{x|y}_offset + frame_info.{width|height}().
      frame.x_offset >
          std::numeric_limits<uint32_t>::max() - frame_info.width() ||
      frame.y_offset >
          std::numeric_limits<uint32_t>::max() - frame_info.height() ||
 
      frame.x_offset + frame_info.width() >
          static_cast<unsigned int>(info.width()) ||
      frame.y_offset + frame_info.height() >
          static_cast<unsigned int>(info.height())) {
    FML_DLOG(ERROR)
        << "Decoded image at index " << image_index
        << " (frame index: " << frame_index
        << ") rejected because the destination region (x: " << frame.x_offset
        << ", y: " << frame.y_offset << ", width: " << frame_info.width()
        << ", height: " << frame_info.height()
        << ") is not entirely within the destination surface (width: "
        << info.width() << ", height: " << info.height() << ").";
    return false;
  }
 
  //----------------------------------------------------------------------------
  /// 3. Composite the frame onto the canvas.
  ///
 
  if (info.colorType() != kN32_SkColorType) {
    FML_DLOG(ERROR) << "Failed to composite image at index " << image_index
                    << " (frame index: " << frame_index
                    << ") of APNG due to the destination surface having an "
                       "unsupported color type.";
    return false;
  }
  if (frame_info.colorType() != kN32_SkColorType) {
    FML_DLOG(ERROR)
        << "Failed to composite image at index " << image_index
        << " (frame index: " << frame_index
        << ") of APNG due to the frame having an unsupported color type.";
    return false;
  }
 
  // Regardless of the byte order (RGBA vs BGRA), the blending operations are
  // the same.
  struct Pixel {
    uint8_t channel[4];
 
    uint8_t GetAlpha() { return channel[3]; }
 
    void Premultiply() {
      for (int i = 0; i < 3; i++) {
        channel[i] = channel[i] * GetAlpha() / 0xFF;
      }
    }
 
    void Unpremultiply() {
      if (GetAlpha() == 0) {
        channel[0] = channel[1] = channel[2] = 0;
        return;
      }
      for (int i = 0; i < 3; i++) {
        channel[i] = channel[i] * 0xFF / GetAlpha();
      }
    }
  };
 
  FML_DCHECK(frame_info.bytesPerPixel() == sizeof(Pixel));
 
  bool result = true;
 
  if (frame.frame_info->blend_mode == SkCodecAnimation::Blend::kSrc) {
    SkPixmap src_pixmap(frame_info, frame.pixels.data(), frame_row_bytes);
    uint8_t* dst_pixels = static_cast<uint8_t*>(pixels) +
                          frame.y_offset * row_bytes +
                          frame.x_offset * frame_info.bytesPerPixel();
    result = src_pixmap.readPixels(info, dst_pixels, row_bytes);
    if (!result) {
      FML_DLOG(ERROR) << "Failed to copy pixels at index " << image_index
                      << " (frame index: " << frame_index << ") of APNG.";
    }
  } else if (frame.frame_info->blend_mode ==
             SkCodecAnimation::Blend::kSrcOver) {
    for (int y = 0; y < frame_info.height(); y++) {
      auto src_row = frame.pixels.data() + y * frame_row_bytes;
      auto dst_row = static_cast<uint8_t*>(pixels) +
                     (y + frame.y_offset) * row_bytes +
                     frame.x_offset * frame_info.bytesPerPixel();
 
      for (int x = 0; x < frame_info.width(); x++) {
        auto x_offset_bytes = x * frame_info.bytesPerPixel();
 
        Pixel src = *reinterpret_cast<Pixel*>(src_row + x_offset_bytes);
        Pixel* dst_p = reinterpret_cast<Pixel*>(dst_row + x_offset_bytes);
        Pixel dst = *dst_p;
 
        // Ensure both colors are premultiplied for the blending operation.
        if (info.alphaType() == kUnpremul_SkAlphaType) {
          dst.Premultiply();
        }
        if (frame_info.alphaType() == kUnpremul_SkAlphaType) {
          src.Premultiply();
        }
 
        for (int i = 0; i < 4; i++) {
          dst.channel[i] =
              src.channel[i] + dst.channel[i] * (0xFF - src.GetAlpha()) / 0xFF;
        }
 
        // The final color is premultiplied. Unpremultiply to match the
        // backdrop surface if necessary.
        if (info.alphaType() == kUnpremul_SkAlphaType) {
          dst.Unpremultiply();
        }
 
        *dst_p = dst;
      }
    }
  }
 
  return result;
}

References channel, FML_DCHECK, FML_DLOG, i, fml::SafeMath::mul(), fml::SafeMath::overflow_detected(), x, and y.

◆ GetPlayCount()

unsigned int flutter::APNGImageGenerator::GetPlayCount ( ) const

overridevirtual

The number of times an animated image should play through before playback stops.

Returns: If this image is animated, the number of times the animation should play through is returned, otherwise it'll just return 1. If the animation should loop forever, kInfinitePlayCount is returned.

Implements flutter::ImageGenerator.

Definition at line 48 of file image_generator_apng.cc.

                                                    {
  return frame_count_ > 1 ? play_count_ : 1;
}

◆ GetScaledDimensions()

SkISize flutter::APNGImageGenerator::GetScaledDimensions ( float scale )

overridevirtual

Given a scale value, find the closest image size that can be used for efficiently decoding the image. If subpixel image decoding is not supported by the decoder, this method should just return the original image size.

Parameters

[in] scale The desired scale factor of the image for decoding.

Returns: The closest image size that can be used for efficiently decoding the image.

Note: This method is called prior to GetPixels in order to query for supported sizes.

See also: GetPixels

Implements flutter::ImageGenerator.

Definition at line 66 of file image_generator_apng.cc.

                                                                   {
  return image_info_.dimensions();
}

◆ MakeFromData()

std::unique_ptr< ImageGenerator > flutter::APNGImageGenerator::MakeFromData ( sk_sp< SkData > data )

static

Definition at line 252 of file image_generator_apng.cc.

                        {
  // Ensure the buffer is large enough to at least contain the PNG signature
  // and a chunk header.
  if (data->size() < sizeof(kPngSignature) + sizeof(ChunkHeader)) {
    return nullptr;
  }
  // Validate the full PNG signature.
  const uint8_t* data_p = static_cast<const uint8_t*>(data.get()->data());
  if (memcmp(data_p, kPngSignature, sizeof(kPngSignature))) {
    return nullptr;
  }
 
  // Validate the header chunk.
  const ChunkHeader* chunk = reinterpret_cast<const ChunkHeader*>(data_p + 8);
  if (!IsValidChunkHeader(data_p, data->size(), chunk) ||
      chunk->get_data_length() != sizeof(ImageHeaderChunkData) ||
      chunk->get_type() != kImageHeaderChunkType) {
    return nullptr;
  }
 
  // Walk the chunks to find the "animation control" chunk. If an "image data"
  // chunk is found first, this PNG is not animated.
  while (true) {
    chunk = GetNextChunk(data_p, data->size(), chunk);
 
    if (chunk == nullptr) {
      return nullptr;
    }
    if (chunk->get_type() == kImageDataChunkType) {
      return nullptr;
    }
    if (chunk->get_type() == kAnimationControlChunkType) {
      break;
    }
  }
 
  if (chunk->get_data_length() < sizeof(AnimationControlChunkData)) {
    return nullptr;
  }
  const AnimationControlChunkData* animation_data =
      CastChunkData<AnimationControlChunkData>(chunk);
 
  // Extract the header signature and chunks to prepend when demuxing images.
  std::optional<std::vector<uint8_t>> header;
  const void* first_chunk_p;
  std::tie(header, first_chunk_p) = ExtractHeader(data_p, data->size());
  if (!header.has_value()) {
    return nullptr;
  }
 
  // Demux the first image in the APNG chunk stream in order to interpret
  // extent and blending info immediately.
  std::optional<APNGImage> default_image;
  const void* next_chunk_p;
  std::tie(default_image, next_chunk_p) =
      DemuxNextImage(data_p, data->size(), header.value(), first_chunk_p);
  if (!default_image.has_value()) {
    return nullptr;
  }
 
  unsigned int play_count = animation_data->get_num_plays();
  if (play_count == 0) {
    play_count = kInfinitePlayCount;
  }
 
  SkImageInfo image_info = default_image.value().codec->getInfo();
  return std::unique_ptr<APNGImageGenerator>(
      new APNGImageGenerator(data, image_info, std::move(default_image.value()),
                             animation_data->get_num_frames(), play_count,
                             next_chunk_p, header.value()));
}

References flutter::data.

Referenced by flutter::ImageGeneratorRegistry::ImageGeneratorRegistry(), and flutter::testing::TEST().

The documentation for this class was generated from the following files:

lib/ui/painting/image_generator_apng.h
lib/ui/painting/image_generator_apng.cc

Public Member Functions

Static Public Member Functions

Additional Inherited Members

Detailed Description

Constructor & Destructor Documentation

◆ ~APNGImageGenerator()

Member Function Documentation

◆ ComputeCrc32()

◆ GetFrameCount()

◆ GetFrameInfo()

◆ GetInfo()

◆ GetPixels()

◆ GetPlayCount()

◆ GetScaledDimensions()

◆ MakeFromData()