Common Intermediate Format

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CIF (Common Intermediate Format), also known as FCIF (Full Common Intermediate Format), is a format used to standardize the horizontal and vertical resolutions in pixels of YCbCr sequences in video signals, commonly used in video teleconferencing systems. It was first proposed in the H.261 standard.

CIF was designed to be easy to convert to PAL or NTSC standards. CIF defines a video sequence with a resolution of 352×288 like PAL Source Input Format, a framerate of 30000/1001 (roughly 29.97) frame/s like NTSC, with colour encoded using YCbCr 4:2:0.

QCIF means "Quarter CIF". To have one fourth of the area as "quarter" implies the height and width of the frame are halved.

Terms also used are SQCIF (Sub Quarter CIF), 4CIF (4× CIF) and 16CIF (16× CIF). The resolutions for all of these formats are summarized in the table below.

Format Video Resolution
SQCIF 128 × 96
QCIF 176 × 144
CIF 352 × 288
4CIF 704 × 576
16CIF 1408 × 1152

xCIF pixels are not square: xCIF formats have a native aspect ratio of ~1.222:1. (On older television systems, a pixel aspect ratio of 1.2:1 was the standard for 525-line systems- see CCIR_601). On square-pixel displays (computer screens, many modern televisions) xCIF rasters should be rescaled horizontally by ~109% to 4:3 in order to avoid a "stretched" look: CIF content expanded horizontally by ~109% results in a 4:3 raster of 384 x 288 square pixels.

The CIF "image sizes" were specifically chosen to be multiples of macroblocks (i.e. 16x16 pixels) due to the way that Discrete cosine transform based video compression/decompression is handled. So, by example, a CIF-size image (352x288) corresponds to 22x18 macroblocks.

SIF (Source Input Format) is practically identical to CIF, but taken from MPEG-1 rather than ITU standards. SIF on 525-Line ("NTSC") based systems is 352 x 240, and on 625-line ("PAL") based systems, it is identical to CIF (352 x 288)[1]. SIF and 4SIF are commonly used in certain video conferencing systems ( [2], p48.)


[edit] References

  • ITU-T H.261 standard [3]

[edit] See also

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