QuickTime Graphics

QuickTime Graphics is a lossy video compression and decompression algorithm (codec) developed by Apple Inc. and first released as part of QuickTime 1.x in the early 1990s.^[1] The codec is also known by the name Apple Graphics and its FourCC SMC.^[2][1] The codec operates on 8-bit palettized RGB data.^[1] The bit-stream format of QuickTime Graphics has been reverse-engineered and a decoder has been implemented in the projects XAnim and libavcodec.^[3][2]

Technical Details

The input video that the codec operates on is in an 8-bit palettized RGB colorspace. Compression is achieved by conditional replenishment and by reducing the palette from 256 colors to a per-4×4 block adaptive palette of 1-16 colors. Because Apple Video operates in the image domain without motion compensation, decoding is much faster than MPEG-style codecs which use motion compensation and perform coding in a transform domain. As a tradeoff, the compression performance of Apple Graphics is lower. The decoding complexity is approximately 50% that of the QuickTime Animation codec.^[4]

Each frame is segmented into 4×4 blocks in raster-scan order. Each block can be coded in one of the following coding modes: skip mode, single color, 2-, 4-, and 8 color palette modes, two repeat modes, and PCM.

Skip mode

The skip mode realizes conditional replenishment. If a block is coded in skip mode, the content of the block at same location in the previous frame is copied to the current frame.^[1] Runs of skip blocks are coded in a run-length encoding scheme, enabling a high compression ratio in static areas of the picture.^[1]

Single color

In single color mode, the entire 4×4 block is painted with a single color.^[1] This mode can also be considered as a 1-color palette mode.

Palette (2, 4, or 8-color) modes

In the palette modes, each 4×4 block is coded with a 2, 4, or 8-color palette.^[1] To select one of the colors from the palette, 1, 2, or 3 bits per pixel are used, respectively. The palette can be written to the bitstream either explicitly or as a reference to an entry in the palette cache.^[1] The palette cache is a set of three circular buffers which store the 256 most recently used palettes, one each for of the 2, 4, and 8-color modes.^[1]

Interpreted as vector quantization, three-dimensional vectors with components red, green, and blue are quantized using a forward adaptive codebook with between 1 and 8 entries.

Repeat modes

There are two different repeat modes.^[1] In the single block repeat mode, the previous block is repeated a specified number of times.^[1] In the two block repeat mode, the previous two blocks are repeated a specified number of times.^[1]

PCM (16 color) mode

In 16-color mode, the color of each pixel in a block is explicitly written to the bit-stream.^[1] This mode is lossless and equivalent to raw PCM without any compression.

See also

• Indexed color
• Color quantization
• Block truncation coding, a similar coding technique for grayscale content
• Color Cell Compression, a similar coding technique for color content, based on block truncation coding
• Apple Video, a codec based on a similar design
• Microsoft Video 1, a codec based on a similar design
• Smacker video, a codec based on a similar design
• S3 Texture Compression, a texture compression format based on a similar design

References

External links

• QuickTime Graphics decoder - FFmpeg