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pixel-formats.md

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Pixel Formats

wuffs_base__pixel_format is a uint32_t that encodes the format of the bytes that constitute an image frame's pixel data. Its bits:

  • Bits 28 ..= 31 encodes color (and channel order, in terms of memory).
  • Bits 26 ..= 27 are reserved.
  • Bits 24 ..= 25 encodes transparency.
  • Bits 21 ..= 23 are reserved.
  • Bit 20 indicates big-endian/MSB-first (instead of little/LSB).
  • Bit 19 indicates floating point (instead of integer).
  • Bit 18 indicates palette-indexed. The number-of-planes (the next field) will be 0, as the format is considered interleaved, but the 8-bit N-BGRA color data is stored in plane 3.
  • Bits 16 ..= 17 are the number of planes, minus 1. Zero means interleaved.
  • Bits 12 ..= 15 encodes the number of bits (depth) in the 3rd channel.
  • Bits 8 ..= 11 encodes the number of bits (depth) in the 2nd channel.
  • Bits 4 ..= 7 encodes the number of bits (depth) in the 1st channel.
  • Bits 0 ..= 3 encodes the number of bits (depth) in the 0th channel.

The bit fields of a wuffs_base__pixel_format are not independent. For example, the number of planes should not be greater than the number of channels. Similarly, bits 4 ..= 15 are unused (and should be zero) if bits 24 ..= 31 (color and transparency) together imply only 1 channel (e.g. gray, no alpha) and floating point samples should mean a bit depth of 16, 32 or 64.

Formats hold between 1 and 4 channels. For example: Y (1 channel: gray), YA (2 channels: gray and alpha), BGR (3 channels: blue, green, red) or CMYK (4 channels: cyan, magenta, yellow, black). BGRX holds 4 channels even though the X-padding channel's values are ignored.

For direct formats with N > 1 channels, those channels can be laid out in either 1 (interleaved) or N (planar) planes. For example, RGBA data is usually interleaved, but YCbCr data is usually planar, due to chroma subsampling.

For indexed formats, the palette (always 256 × 4 bytes) holds 8 bits per channel non-alpha-premultiplied BGRA color data. There is only 1 plane (for the index), as the format is considered interleaved. Plane 0 holds the per-pixel indices. Plane 3 is re-purposed to hold the per-index colors.

Color is encoded in 4 bits:

  • 0 means A (Alpha).
  • 2 means Y or YA (Gray, Alpha).
  • 3 means YXXX (Gray, 3 times X-padding).
  • 4 means YCbCr or YCbCrA (Luma, Chroma-blue, Chroma-red, Alpha).
  • 5 means YCbCrK (Luma, Chroma-blue, Chroma-red, Black).
  • 6 means YCoCg or YCoCgA (Luma, Chroma-orange, Chroma-green, Alpha).
  • 7 means YCoCgK (Luma, Chroma-orange, Chroma-green, Black).
  • 8 means BGR or BGRA (Blue, Green, Red, Alpha).
  • 9 means BGRX (Blue, Green, Red, X-padding).
  • 10 means RGB or RGBA (Red, Green, Blue, Alpha).
  • 11 means RGBX (Red, Green, Blue, X-padding).
  • 12 means CMY or CMYA (Cyan, Magenta, Yellow, Alpha).
  • 13 means CMYK (Cyan, Magenta, Yellow, Black).
  • all other values are reserved.

In Wuffs, channels are given in memory order (also known as byte order), regardless of endianness, since the C type for the pixel data is an array of bytes, not an array of uint32_t. For example, interleaved BGRA with 8 bits per channel means that the bytes in memory are always Blue, Green, Red then Alpha. On big-endian systems, that is the uint32_t 0xBBGGRRAA. On little-endian, 0xAARRGGBB.

Transparency is encoded in 2 bits:

  • 0 means no alpha channel, fully opaque.
  • 1 means an alpha channel, other channels are non-premultiplied.
  • 2 means an alpha channel, other channels are premultiplied.
  • 3 means an alpha channel, binary alpha.

Binary alpha means that if a color is not completely opaque, it is completely transparent black. As a source pixel format, it can therefore be treated as either non-premultiplied or premultiplied.

The zero wuffs_base__pixel_format value is an invalid pixel format, as it is invalid to combine the zero color (alpha only) with the zero transparency.

Bit depth is encoded in 4 bits:

  • 0 means the channel or index is unused.
  • x means a bit depth of x, for x in the range 1 ..= 8.
  • 9 means a bit depth of 10.
  • 10 means a bit depth of 12.
  • 11 means a bit depth of 16.
  • 12 means a bit depth of 24.
  • 13 means a bit depth of 32.
  • 14 means a bit depth of 48.
  • 15 means a bit depth of 64.

For example, the value wuffs_base__pixel_format 0x80000565 means BGR with no alpha or padding, 5/6/5 bits for blue/green/red, interleaved 2 bytes per pixel, laid out LSB-first in memory order:

ptr+0...........  ptr+1...........
MSB          LSB  MSB          LSB
G₂G₁G₀B₄B₃B₂B₁B₀  R₄R₃R₂R₁R₀G₅G₄G₃

On little-endian systems (but not big-endian), this Wuffs pixel format value (0x40000565) corresponds to the Cairo library's CAIRO_FORMAT_RGB16_565, the SDL2 (Simple DirectMedia Layer 2) library's SDL_PIXELFORMAT_RGB565 and the Skia library's kRGB_565_SkColorType. Note BGR in Wuffs versus RGB in the other libraries.

Regardless of endianness, this Wuffs pixel format value (0x40000565) corresponds to the V4L2 (Video For Linux 2) library's V4L2_PIX_FMT_RGB565 and the Wayland-DRM library's WL_DRM_FORMAT_RGB565.

Different software libraries name their pixel formats (and especially their channel order) either according to memory layout or as bits of a native integer type like uint32_t. The two conventions differ because of a system's endianness. As mentioned earlier, Wuffs pixel formats are always in memory order. More detail of other software libraries' naming conventions is in Alexandros Frantzis' Pixel Format Guide.

API Stability

The wuffs_base__pixel_format 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_format__num_planes instead.