Apple Video
Apple Video is a lossy video compression and decompression algorithm (codec) developed by Apple Inc. and first released as part of QuickTime 1.0 in 1991.[1] The codec is also known as QuickTime Video, by its FourCC RPZA and the name Road Pizza.[2][3] When used in the AVI container, the FourCC AZPR is also used.[3] The bit-stream format of Apple Video has been reverse-engineered and a decoder has been implemented in the projects XAnim and libavcodec.[2][4]
Contents
Technical Details
The codec operates on 4×4 blocks of pixels in the RGB colorspace. Each frame is segmented into 4×4 blocks in raster-scan order. Each block is coded in one of four coding modes: skip, single color, four color, or 16 color.[3] Colors are represented by 16 bits with a bit-depth of 5 bit for each of the three components red, green, and blue, a format known as RGB555.[3] 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 Video is lower.
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.[3] Runs of skip blocks are coded in a run-length encoding scheme, enabling a high compression ratio in static areas of the picture.[3]
Single color mode
In single color mode, all pixels in a block are decoded in the same color.[3] This can be interpreted as a palette with a single color.
Four color mode
In four color mode, each pixel in a block is decoded as one of four colors which are specified in a palette.[3] To select one of the four entries, 2 bits per pixel are written to the bit-stream. The same palette is used for a run of length between one and 32 blocks.[3] Of the four colors, two are explicitly written to the bit-stream, while the other two are calculated at the decoder by linear interpolation in the RGB colorspace using the following equations:
where color0 and color3 are the two colors which are written in the bit-stream.[3] The four colors can be interpreted as lying equidistantly spaced on a line segment in the three-dimensional vector space with the three components red, green, and blue. The end-points of this line are written in the bit-stream. A similar color-interpolation scheme is used in S3 Texture Compression.
Interpreted as vector quantization, a three-dimensional vector with the components red, green, and blue is quantized using a codebook with four entries.
16 color mode
In 16-color mode, the color of each pixel in a block is explicitly written in the bit-stream.[3] 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
- Microsoft Video 1, a codec based on a similar design
- QuickTime Graphics, 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
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