Editing Common Intermediate Format (section)

==Technical details==
The simple way to convert NTSC video to CIF is to capture every other [[field (video)|field]] (e.g., the top fields) of [[interlaced video]], [[Sample-rate conversion|downsample]] it by 2:1 horizontally to convert 704 samples per line to 352 samples per line, and [[Sample-rate conversion|upsample]] it vertically by a ratio of 6:5 vertically to convert 240 lines to 288 lines. The simple way to convert PAL video to CIF is to similarly capture every other field, downsample it horizontally by 2:1, and introduce some [[jitter]] in the frame rate by skipping or repeating frames as necessary. Since H.261 systems typically operated at low [[bit rate]]s, they also typically operated at low frame rates by skipping many of the camera source frames, so introducing some jitter in the frame rate tended not to be noticeable. More sophisticated conversion schemes (e.g., using [[deinterlacing]] to improve the vertical resolution from an NTSC camera) could also be used in higher quality systems.

In contrast to the CIF compromise that originated with the H.261 standard, there are two variants of the SIF (''[[Source Input Format]]'') that was first defined in the [[MPEG-1]] standard. SIF is otherwise very similar to CIF. SIF on 525-line ("NTSC") based systems is 352 × 240 with a frame rate of 30000/1001 frames per second, and on 625-line ("PAL") based systems, it has the same picture size as CIF (352 × 288) but with a frame rate of 25 frames per second.

Some references to CIF are intended to refer only to its ''resolution'' (352 × 288), without intending to refer to its frame rate.

The YCbCr color representation had been previously defined in the first standard digital video source format, [[CCIR 601]], in 1982. However, CCIR 601 uses [[4:2:2]] color sampling, which subsamples the Cb and Cr components only horizontally. H.261 additionally used vertical color subsampling, resulting in what is known as 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 '''Sub-QCIF'''<!-- Avidemux 2.5.1 (r5429) -->), '''SCIF''' (sometimes '''Sub-CIF''' <!-- Very uncommon, but a number of papers and sources include this resolution and term -->), '''4CIF''' (4 × CIF), '''9CIF''' (9 × CIF) and '''16CIF''' (16 × CIF). The resolutions for all of these formats are summarized in the table below.

{| class="wikitable"
|-
!Format
!Video Resolution
![[Aspect ratio (image)|Storage aspect ratio (SAR)]]
|-
| SQCIF || 128 × 96 || 4:3
|-
| QCIF  || 176 × 144 || 11:9
|-
| SCIF || 256 x 192 || 4:3
|-
| [[Source Input Format|SIF]](525)|| 352 x 160 || 11:5 (≈13:6)
|-
| CIF/SIF(625)|| 352 × 288 || 11:9
|-
| 4SIF(525) || 704 x 320 || 11:5 (≈13:6)
|-
| 4CIF/4SIF(625)|| 704 × 576 || 11:9
|-
| 9CIF || 1056 × 864 || 11:9
|-
| 16CIF ||1408 × 1152 || 11:9
|}

xCIF [[pixel]]s are not square, instead having a ″native″ aspect ratio ([[pixel aspect ratio#Pixel aspect ratios of common video formats|pixel aspect ratio (PAR)]]) of 12:11 (PAR&nbsp;= [[Display aspect ratio|DAR]]&nbsp;:&nbsp;[[Storage aspect ratio|SAR]]&nbsp;= {{sfrac|4|3}}&nbsp;:&nbsp;{{sfrac|11|9}}&nbsp;=&nbsp;{{sfrac|12|11}}), as with the standard for 625-line systems (see [[CCIR 601]]). On square-pixel displays (e.g., computer screens and many modern televisions) xCIF rasters should be rescaled so that the picture covers a 4:3 area, in order to avoid a "stretched" look: CIF content expanded horizontally by 12:11 results in a 4:3 raster of 384 × 288 square pixels (384 = 352 * 12/11). (This can happen on larger graphics displays of any aspect ratio in a [[Window (computing)|window]] of {{resx|384|288}} square pixels or enlarged to full screen on any larger 4:3 graphic display.){{cn|reason=A citation is needed that there are graphics displays with a resolution of 384 × 288 pixels|date=May 2023}}

The CIF and QCIF picture dimensions were specifically chosen to be multiples of 16 because of the way that [[discrete cosine transform]] based video compression/decompression was handled in H.261, using 16 × 16 [[macroblock]]s and 8 × 8 transform blocks. So a CIF-size image (352 × 288) contains 22 × 18 macroblocks and a QCIF image (176 × 144) contains 11 × 9 macroblocks. The 16 × 16 macroblock concept was later also used in other compression standards such as [[MPEG-1]], [[H.262/MPEG-2 Part 2|MPEG-2]], [[MPEG-4 Part 2]], [[H.263]], and [[H.264/MPEG-4 AVC]].