QuickTime Graphics is a

lossy In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data si ...

video compression and decompression algorithm (

codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or ...

) developed by Apple Inc. and first released as part of QuickTime 1.x in the early 1990s. The codec is also known by the name Apple Graphics and its FourCC SMC. The codec operates on 8-bit palettized

RGB The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...

data. The bit-stream format of QuickTime Graphics has been reverse-engineered and a decoder has been implemented in the projects XAnim and

libavcodec libavcodec is a free and open-source library of codecs for encoding and decoding video and audio data. libavcodec is an integral part of many open-source multimedia applications and frameworks. The popular MPV, xine and VLC media players u ...

Technical Details

The input video that the codec operates on is in an 8-bit palettized

colorspace. Compression is achieved by

conditional replenishment Conditional (if then) may refer to: * Causal conditional, if X then Y, where X is a cause of Y *Conditional probability, the probability of an event A given that another event B has occurred * Conditional proof, in logic: a proof that asserts a ...

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 The Moving Picture Experts Group (MPEG) is an alliance of working groups established jointly by ISO and IEC that sets standards for media coding, including compression coding of audio, video, graphics, and genomic data; and transmission and f ...

-style codecs which use

motion compensation Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...

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 QuickTime Animation format (also known as QuickTime RLE) is a video compression format and codec created by Apple Computer to enable playback of RGB video in real time without expensive hardware. It is generally found in the QuickTime container ...

codec. 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

. 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. Runs of skip blocks are coded in a

run-length encoding Run-length encoding (RLE) is a form of lossless data compression in which ''runs'' of data (sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original ...

scheme, enabling a high compression ratio in static areas of the picture.

Single color

In single color mode, the entire 4×4 block is painted with a single color. 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. 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. 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. Interpreted as

vector quantization Vector quantization (VQ) is a classical quantization technique from signal processing that allows the modeling of probability density functions by the distribution of prototype vectors. It was originally used for data compression. It works by di ...

, 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. In the single block repeat mode, the previous block is repeated a specified number of times. In the two block repeat mode, the previous two blocks are repeated a specified number of times.

PCM (16 color) mode

In 16-color mode, the color of each pixel in a block is explicitly written to the bit-stream. This mode is lossless and equivalent to raw

PCM Pulse-code modulation (PCM) is a method used to digitally represent sampled analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio applications. In a PCM stream, the am ...

without any compression.

References

External links

QuickTime Graphics decoder - FFmpeg
{{Compression formats

Graphics Graphics () are visual images or designs on some surface, such as a wall, canvas, screen, paper, or stone, to inform, illustrate, or entertain. In contemporary usage, it includes a pictorial representation of data, as in design and manufacture, ...

Lossy compression algorithms Video codecs