词条 | Apple Video |
释义 |
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 DetailsThe 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 modeThe 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 modeIn 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 modeIn 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 modeIn 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
References1. ^{{cite web |url=https://developer.apple.com/legacy/mac/library/technotes/tn2140/tn2140_QuickTimeDevelop7Article.pdf |title=QuickTime 1.0: "You oughta be in pictures" |publisher=Apple Computer |author=Guillermo A. Ortiz |format=PDF |date=Summer 1991 |accessdate=14 April 2013}} 2. ^1 {{cite web |url=http://ffmpeg.org/general.html |title=FFmpeg Documentation |author= |work=FFmpeg |accessdate=4 April 2013}} 3. ^1 2 3 4 5 6 7 8 9 10 {{cite web |url=http://wiki.multimedia.cx/index.php?title=Apple_RPZA |title=Apple RPZA |author= |date=11 December 2008 |work=MultimediaWiki |accessdate=4 April 2013}} 4. ^{{cite web|title=CHM Live: The Origins of QuickTime|url=http://www.computerhistory.org/events/video/264/|publisher=Computer History Museum|accessdate=27 March 2018}} 5. ^{{cite web |url=http://www.math.unl.edu/~rdieter1/OpenStep/X11/xanim.2.70.6.4.2.README |title=xanim.2.70.6.4.2 README |author=Mark Podlipec |date=10 December 1997 |work=XAnim |accessdate=4 April 2013}} External links
3 : Video codecs|Lossy compression algorithms|QuickTime |
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