Editing Chroma subsampling (section)

==Sampling systems and ratios==
The subsampling scheme is commonly expressed as a three-part ratio ''J'':''a'':''b'' (e.g. 4:2:2) or four parts, if alpha channel is present (e.g. 4:2:2:4), that describe the number of luminance and chrominance samples in a conceptual region that is ''J'' pixels wide and 2 pixels high. The parts are (in their respective order):
* ''J'': horizontal sampling reference (width of the conceptual region). Usually, 4.
<!-- This used to be in the article but seems plain wrong to me: J is a number of pixels, and expressing it as MHz implies a very particular use-case, still images do not necessarily have a notion of frequency ...

  (originally, as a multiple of 3.579 MHz in the NTSC television system)

if one wants to re-add this to the article, it should be better explained --~~~~ -->
* ''a'': number of chrominance samples ('''Cr''', '''Cb''') in the first row of ''J'' pixels.
* ''b'': number of changes of chrominance samples ('''Cr''', '''Cb''') between first and second row of ''J'' pixels. ''b'' is usually either zero or equal to ''a'' (except in rare irregular cases like 4:4:1 and 4:2:1, which do not follow this convention).
* ''[[Alpha compositing|Alpha]]'': horizontal factor (relative to first digit). May be omitted if alpha component is not present, and is equal to ''J'' when present.

This notation is not valid for all combinations and has exceptions, e.g. 4:1:0 (where the height of the region is not 2 pixels, but 4 pixels, so if 8 bits per component are used, the media would be 9 bits per pixel) and 4:2:1.<!-- the height of the squares are set in the first column and the width are set in the table's first row -->
{| style="border-collapse:collapse; text-align:center;"
| 
| rowspan="11" style="width:1ex;" | || colspan="4" style="font-size:larger; width:100px;" | '''4:1:1''' || colspan="2" rowspan="7"|
| rowspan="11" style="width:3ex;" | || colspan="4" style="font-size:larger; width:100px;" | '''4:2:0''' || colspan="2" rowspan="7"|
| rowspan="11" style="width:3ex;" | || colspan="4" style="font-size:larger; width:100px;" | '''4:2:2''' || colspan="2" rowspan="7"|
| rowspan="11" style="width:3ex;" | || colspan="4" style="font-size:larger; width:100px;" | '''4:4:0''' || colspan="2" rowspan="7"|
| rowspan="11" style="width:3ex;" | || colspan="4" style="font-size:larger; width:100px;" | '''4:4:4''' || colspan="2" rowspan="7"|
|-
| rowspan="2" style="height:50px;" | '''Y'CrCb'''
| style="background:#6666cc;" | &nbsp; || style="background:#666699;" |  || style="background:#6666cc;" |  || style="background:#666699;" |
| style="background:#6666cc;" |        || style="background:#666699;" |  || style="background:#66cc66;" |  || style="background:#669966;" |
| style="background:#6666cc;" |        || style="background:#666699;" |  || style="background:#66cc66;" |  || style="background:#669966;" |
| style="background:#6666cc;" |        || style="background:#669999;" |  || style="background:#66cc66;" |  || style="background:#666666;" |
| style="background:#6666cc;" |        || style="background:#669999;" |  || style="background:#66cc66;" |  || style="background:#666666;" |
|-
| style="background:#996699;" | &nbsp; || style="background:#cc66cc;" |  || style="background:#996699;" |  || style="background:#cc66cc;" |
| style="background:#666699;" |        || style="background:#6666cc;" |  || style="background:#669966;" |  || style="background:#66cc66;" |
| style="background:#996699;" |        || style="background:#cc66cc;" |  || style="background:#999966;" |  || style="background:#cccc66;" |
| style="background:#666699;" |        || style="background:#66cccc;" |  || style="background:#669966;" |  || style="background:#999999;" |
| style="background:#996699;" |        || style="background:#cccccc;" |  || style="background:#999966;" |  || style="background:#cc6666;" |
|-
| 
| colspan="4" style="font-size:larger;" | '''='''
| colspan="4" style="font-size:larger;" | '''='''
| colspan="4" style="font-size:larger;" | '''='''
| colspan="4" style="font-size:larger;" | '''='''
| colspan="4" style="font-size:larger;" | '''='''
|-
| rowspan="2" style="height:50px;" | '''Y'<nowiki/>'''
| style="background:#cccccc;" | &nbsp; || style="background:#999999;" |  || style="background:#cccccc;" |  || style="background:#999999;" |
| style="background:#cccccc;" |        || style="background:#999999;" |  || style="background:#cccccc;" |  || style="background:#999999;" |
| style="background:#cccccc;" |        || style="background:#999999;" |  || style="background:#cccccc;" |  || style="background:#999999;" |
| style="background:#cccccc;" |        || style="background:#999999;" |  || style="background:#cccccc;" |  || style="background:#999999;" |
| style="background:#cccccc;" |        || style="background:#999999;" |  || style="background:#cccccc;" |  || style="background:#999999;" |
|-
| style="background:#999999;" | &nbsp; || style="background:#cccccc;" |  || style="background:#999999;" |  || style="background:#cccccc;" |
| style="background:#999999;" |        || style="background:#cccccc;" |  || style="background:#999999;" |  || style="background:#cccccc;" |
| style="background:#999999;" |        || style="background:#cccccc;" |  || style="background:#999999;" |  || style="background:#cccccc;" |
| style="background:#999999;" |        || style="background:#cccccc;" |  || style="background:#999999;" |  || style="background:#cccccc;" |
| style="background:#999999;" |        || style="background:#cccccc;" |  || style="background:#999999;" |  || style="background:#cccccc;" |
|-
| 
| colspan="4" style="font-size:larger;" | '''+'''
| colspan="4" style="font-size:larger;" | '''+'''
| colspan="4" style="font-size:larger;" | '''+'''
| colspan="4" style="font-size:larger;" | '''+'''
| colspan="4" style="font-size:larger;" | '''+'''
|-
| 
| 1 || 2 || 3 || 4 || style="padding-left:1ex;" | ''J''|| =&nbsp;4
| 1 || 2 || 3 || 4 || style="padding-left:1ex;" | ''J''|| =&nbsp;4
| 1 || 2 || 3 || 4 || style="padding-left:1ex;" | ''J''|| =&nbsp;4
| 1 || 2 || 3 || 4 || style="padding-left:1ex;" | ''J''|| =&nbsp;4
| 1 || 2 || 3 || 4 || style="padding-left:1ex;" | ''J''|| =&nbsp;4
|-
| rowspan="2" style="height:50px;" | '''(Cr, Cb)'''
| colspan="4" style="background:#6666ff;" | '''1'''
| style="padding-left:1ex;"   | ''a''|| =&nbsp;'''1'''

| colspan="2" style="background:#6666ff;" | '''1'''
| colspan="2" style="background:#66ff66;" | '''2'''
| style="padding-left:1ex;"   | ''a''|| =&nbsp;'''2'''

| colspan="2" style="background:#6666ff;" | '''1'''
| colspan="2" style="background:#66ff66;" | '''2'''
| style="padding-left:1ex;"   | ''a''|| =&nbsp;'''2'''

| style="background:#6666ff;" | '''1'''
| style="background:#66ffff;" | '''2'''
| style="background:#66ff66;" | '''3'''
| style="background:#999999;" | '''4'''
| style="padding-left:1ex;"   | ''a''|| =&nbsp;'''4'''

| style="background:#6666ff;" | '''1'''
| style="background:#66ffff;" | '''2'''
| style="background:#66ff66;" | '''3'''
| style="background:#999999;" | '''4'''
| style="padding-left:1ex;"   | ''a''|| =&nbsp;'''4'''
|-
| colspan="4" style="background:#ff66ff;" | '''''1'''''
| style="padding-left:1ex;"   | ''b''|| =&nbsp;'''''1'''''

| colspan="2" style="background:#6666ff; border-top:1px dotted;" | 
| colspan="2" style="background:#66ff66; border-top:1px dotted;" | 
| style="padding-left:1ex;"   | ''b''|| =&nbsp;'''''0'''''

| colspan="2" style="background:#ff66ff;" | '''''1'''''
| colspan="2" style="background:#ffff66;" | '''''2'''''
| style="padding-left:1ex;"   | ''b''|| =&nbsp;'''''2'''''

| style="background:#6666ff; border-top:1px dotted;" | 
| style="background:#66ffff; border-top:1px dotted;" | 
| style="background:#66ff66; border-top:1px dotted;" | 
| style="background:#999999; border-top:1px dotted;" | 
| style="padding-left:1ex;"   | ''b''|| =&nbsp;'''''0'''''

| style="background:#ff66ff;" | '''''1'''''
| style="background:#ffffff;" | '''''2'''''
| style="background:#ffff66;" | '''''3'''''
| style="background:#ff6666;" | '''''4'''''
| style="padding-left:1ex;"   | ''b''|| =&nbsp;'''''4'''''
|-
| 
| colspan="6" | <small>¼ horizontal resolution,<br />full vertical resolution</small>
| colspan="6" | <small>½ horizontal resolution,<br />½ vertical resolution</small>
| colspan="6" | <small>½ horizontal resolution,<br />full vertical resolution</small>
| colspan="6" | <small>full horizontal resolution,<br />½ vertical resolution</small>
| colspan="6" | <small>full horizontal resolution,<br />full vertical resolution</small>
|}

The mapping examples given are only theoretical and for illustration. Also the diagram does not indicate any chroma filtering, which should be applied to avoid [[aliasing]]. To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present).