Common Intermediate Format
CIF (Common Intermediate Format or Common Interchange 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) frames like NTSC, with colour encoded using YCbCr 4:2:0.
QCIF means "Quarter CIF". To have one quarter of the area, as "quarter" implies, the height and width of the frame are halved.
Terms also used are SQCIF (Sub Quarter CIF, sometimes subQCIF), 4CIF (4× CIF) and 16CIF (16× CIF). The resolutions for all of these formats are summarized in the table below.
|SQCIF||128 × 96|
|QCIF||176 × 144|
|SCIF||256 x 192|
|SIF(525)||352 x 240|
|CIF/SIF(625)||352 × 288|
|4SIF(525)||704 x 480|
|4CIF/4SIF(625)||704 × 576|
|16CIF||1408 × 1152|
|DCIF||528 × 384|
xCIF pixels are not square, instead having 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 × 288 square pixels.
The CIF "image sizes" were specifically chosen to be multiples of macroblocks (i.e. 16 × 16 pixels) because of the way that discrete cosine transform based video compression/decompression is handled. So, by example, a CIF-size image (352 × 288) corresponds to 22 × 18 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 × 240, and on 625-line ("PAL") based systems, it is identical to CIF (352 × 288). SIF and 4SIF are commonly used in certain video conferencing systems.
DCIF means Double CIF, proposed as a compromise resolution between CIF and 4CIF that is more balanced (in terms of horizontal vs vertical resolution) and suited to common CCTV equipment (with 480+ scanlines but a maximum of about 560 TVL) than 2CIF (704x288). The pixel and therefore data rate is exactly double that of CIF , but the 1:1.375 image aspect ratio is a lot closer to standard 4:3, with essentially square pixels.