Color Formats

To represent color values, a format is agreed upon. Color formats are made up of three things, the order of the components, the bit depth, and whether it is a packed or a planar format. In some cases, endianness may be important.

Component order

The order in which the components (which come from a color model) are arranged is simply represented by writing them out. For example, RGB for red first, then green, then blue, or BGR for blue, green, red.

Bit depth

A bit depth is how many bits are available to store the sample value. There are two main ways to specify the bit depth in a :

• bits per component. Here, RGB888 reads as RGB color model, with 8 bits for the red component, 8 bits for the green component, and 8 bits for the blue component and RGB565 reads as RGB color model, with 5 bits for the red component, 6 bits for the green component, and 5 bits for the blue component.
• bits per sample. Here, RGB24 reads as RGB color model, with 24 bits in total for the red, green, and blue components. This is ambiguous, because one does not know exactly how many bits are allocated to each component. RGB565, RGB556, and RGB655 (even though the latter ones do not make much sense as the eye is most sensitive to green light) all become RGB16.

Packed vs planar

Components can be stored either packed, where all components are interleaved (here, RGB):

Sample number:   1   2   3   4   5
Data:          RGB RGB RGB RGB RGB

or stored separately for each component:

Sample number: 1 2 3 4 5
Data:          R R R R R...
Data:          G G G G G...
Data:          B B B B B...

In planar formats, many operations can be easier to implement, as it is possible to implement the algorithm once and then operate on all planes. On the other hand, packed formats are simpler and often used in hardware.¹

Endianness

Different computer architectures store numbers differently. For more information, visit the Wikipedia article on endianness. There are two main ways to store numbers with more than 8 bits (1 is the least significant byte and 4 is the most significant byte, here 4 bytes):

• Most significant byte first, little endian, 4321. This is what x86-family processors use.
• Least significant byte first, big endian, 1234. This is what PowerPC-family processors use.

This can be important for color formats, as some computers might store it in their native endianness. VapourSynth doesn't seem to care about endianness, but FFmpeg does.

For example, RGB565 might store its two bytes in 12 or 21 order, and if they are read in the wrong order, it will produce garbage.

Chroma subsampling

In Y'CbCr signals, there are three widely used variants of chroma subsampling:

• 4:2:0 which has half the vertical and horizontal chroma resolution
• 4:2:2 which has half the horizontal chroma resolution but full vertical resolution
• 4:4:4 which has full chroma resolution (no subsampling)

4:2:2 is not particularly useful over the other options, so this guide will focus on 4:2:0 and 4:4:4.

4:2:0 is the most commonly used format for videos. Nearly every DVD, blu-ray, YouTube video, etc. uses 4:2:0 subsampling. This is because, in the majority of cases, human eyes do not notice the reduction in chroma resolution. There is very little benefit to using 4:4:4 in the average case.

However, there are some exceptions. The most notable is screen recordings. Things like text overlays, video game UI overlays, etc. have very fine, color-dependent detail that can be destroyed by chroma subsampling and result in an aliased look to the video. Therefore, it is recommended to use 4:4:4 subsampling when recording your desktop, and 4:2:0 subsampling in most other cases.

Common formats

References