QuickTime Graphics is alossy video compression and decompression algorithm (codec) developed byApple Inc. and first released as part ofQuickTime 1.x in the early 1990s.[1] The codec is also known by the nameApple Graphics and itsFourCCSMC.[2][1] The codec operates on 8-bit palettizedRGB data.[1] The bit-stream format of QuickTime Graphics has been reverse-engineered and a decoder has been implemented in the projectsXAnim andlibavcodec.[3][2]
The input video that the codec operates on is in an 8-bitpalettizedRGB colorspace. Compression is achieved byconditional 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 thanMPEG-style codecs which usemotion compensation and perform coding in atransform domain. As a tradeoff, the compression performance of Apple Graphics is lower. The decoding complexity is approximately 50% that of theQuickTime 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.
The skip mode realizesconditional 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 arun-length encoding scheme, enabling a high compression ratio in static areas of the picture.[1]
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.
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 asvector quantization, three-dimensional vectors with components red, green, and blue arequantized using a forward adaptive codebook with between 1 and 8 entries.
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]
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 rawPCM without any compression.