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] (The codename "Road Pizza" is a reference to the idea that "when you run over an animal, you're basically compressing it on the freeway".^[4]) 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][5]

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:

${\displaystyle \mathrm {color1} ={\frac {21}{32}}*\mathrm {color0} +{\frac {11}{32}}*\mathrm {color3} \approx {\frac {2}{3}}*\mathrm {color0} +{\frac {1}{3}}*\mathrm {color3} }$

${\displaystyle \mathrm {color2} ={\frac {11}{32}}*\mathrm {color0} +{\frac {21}{32}}*\mathrm {color3} \approx {\frac {1}{3}}*\mathrm {color0} +{\frac {2}{3}}*\mathrm {color3} }$

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

External links

• Apple Video decoder - FFmpeg

This page was last edited on 12 February 2022, at 11:48