wuffs_base__pixel_subsampling is a uint32_t that encodes whether samples cover one pixel or cover multiple pixels. RGBA or BGRA pixel formats are typically one pixel per sample. For YCbCr pixel formats (i.e. the native formats of JPEG and WebP lossy images), the luma channel (Y) is also typically one pixel per sample but the chroma channels (Cb, Cr) are often 2 or 4 pixels per sample. In JPEG terminology, 4:4:4, 4:2:2 or 4:2:0 subsampling correspond to 1, 2 or 4 pixels per chroma sample.
Equivalently, wuffs_base__pixel_subsampling encodes the mapping of pixel space coordinates (x, y) to sample space coordinates (i, j), also known as pixel buffer indices. That mapping can differ for each plane p. A Wuffs image can have up to 4 planes or channels. For a depth of 8 bits (1 byte), the p‘th plane’s sample starts at (planes[p].ptr + (j * planes[p].stride) + i).
For interleaved pixel formats, which are always one pixel per sample, the mapping is trivial: i = x and j = y. For planar pixel formats, the mapping can differ due to chroma subsampling. For example, consider a three plane YCbCr pixel format with 4:2:2 subsampling. For the luma (Y) channel, there is one sample for every pixel, but for the chroma (Cb, Cr) channels, there is one sample for every two pixels: pairs of horizontally adjacent pixels form one macropixel, i = x / 2 and j == y. In general, for a given p:
i = (x + bias_x) / denominator_x.j = (y + bias_y) / denominator_y.where biases and denominators are in the range 0 ..= 3 and 1 ..= 4 respectively.
In general, the biases will be zero after decoding an image. However, making a sub-image may change the bias, since the (x, y) coordinates are relative to the sub-image‘s top-left origin, but the backing pixel buffers were created relative to the original image’s origin.
For example, consider a 10×3 image with what JPEG calls 4:1:1 subsampling, where each Chroma sample covers a macropixel (a block of pixels) 4 wide and 1 high. For a Chroma channel, there are 30 pixels and 9 samples. bias_x = 0 and denominator_x = 2, so that the 6th pixel column (x = 5) maps to the 2nd sample column (i = 1).
Pixel space: +---------------------------------------+ |l00 l01 l02 l03 m04 m05 m06 m07 n08 n09| | | |p10 p11 p12 p13 q14 q15 q16 q17 r18 r19| | +-----------+ | |t21 t21 t22|t23 u24 u25|u26 u27 v28 v29| +-----------+-----------+---------------+ Chroma sample space: +-----+ |l m n| | | |p q r| | | |t u v| +-----+
Now consider the 3×1 sub-image shown above. Even though its pixel width (3) is less than the macropixel width (4 columns per sample), as that sub-image shares the sample buffer with its containing image, it still straddles 2 sample columns (t and u). denominator_x = 2, the same as before, but now bias_x = 3.
Pixel space: +-----------+ |t23 u24 u25| +-----------+ Chroma sample space: +---+ |t u| +---+
For each plane p, the adjusted denominator is defined to be one less than the denominator. The plane's four numbers (two biases and two adjusted denominators) are each in the range 0 ..= 3, and are each encodable in two bits. Four groups of two bits combine to form an 8 bit unsigned integer:
e_p = (bias_x << 6) | (adj_denom_x << 4) | (bias_y << 2) | (adj_denom_y << 0)
Those e_p values (e_0 for the first plane, e_1 for the second plane, etc) combine to form a wuffs_base__pixel_subsampling value:
pixsub = (e_3 << 24) | (e_2 << 16) | (e_1 << 8) | (e_0 << 0)
For example, visualizing 4:2:2 YCbCr (with no bias):
Pixel space: +-------------------+ |l00 l01 m02 m03 n04| | | |p10 p11 q12 q13 r14| +-------------------+ Chroma sample space: +-----+ |l m n| | | |p q r| +-----+
This corresponds to a denominator_x of 1, 2 and 2 for the three planes (Y, Cb, Cr) and a denominator_y of 1, 1 and 1. The uint32_t encoding is 0x101000.
The wuffs_base__pixel_subsampling bit packing is documented for explanation and to assist in debugging (e.g. printfing the bits in %x format). However, do not manipulate its bits directly; they are private implementation details. Use functions such as wuffs_base__pixel_subsampling__bias_x instead.