HEVC / H265 Tutorial: Source, decoded, and output picture formats

This clause specifies the relationship between source and decoded pictures that is given via the bitstream. The video source that is represented by the bitstream is a sequence of pictures in decoding order. The source and decoded pictures are each comprised of one or more sample arrays:
– Luma (Y) only (monochrome).
– Luma and two chroma (YCbCr or YCgCo).
– Green, Blue and Red (GBR, also known as RGB).
– Arrays representing other unspecified monochrome or tri-stimulus colour samplings (for example, YZX, also
known as XYZ).
For convenience of notation and terminology, the variables and terms associated with these arrays are referred to as luma (or L or Y) and chroma, where the two chroma arrays are referred to as Cb and Cr; regardless of the actual colour representation method in use. 

The variables SubWidthC, and SubHeightC are specified as follows, depending on the chroma format sampling structure, which is specified through chroma_format_idc and separate_colour_plane_flag. Other values of
chroma_format_idc, SubWidthC, and SubHeightC may be specified in the future by ITU-T | ISO/IEC.

In monochrome sampling there is only one sample array, which is nominally considered the luma array.

In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array.

In 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array.

In 4:4:4 sampling, depending on the value of separate_colour_plane_flag, the following applies:

– If separate_colour_plane_flag is equal to 0, each of the two chroma arrays has the same height and width as the luma array.

– Otherwise (separate_colour_plane_flag is equal to 1), the three colour planes are separately processed as monochrome sampled pictures.

The number of bits necessary for the representation of each of the samples in the luma and chroma arrays in a video sequence is in the range of 8 to 14, inclusive, and the number of bits used in the luma array may differ from the number of bits used in the chroma arrays.

When the value of chroma_format_idc is equal to 1 (4:2:0), the nominal vertical and horizontal relative locations of luma and chroma samples in pictures are shown as follows. Alternative chroma sample relative locations may be indicated in video usability information. 

When the value of chroma_format_idc is equal to 2 (4:2:2), the chroma samples are co-sited with the corresponding luma samples and the nominal locations in a picture are as shown as follows.

When the value of chroma_format_idc is equal to 3 (4:4:4), all array samples are co-sited for all cases of pictures and the nominal locations in a picture are as shown as follows.