# Formats in gallium

Gallium format names mostly follow D3D10 conventions, with some extensions.

Format names like XnYnZnWn have the X component in the lowest-address n bits and the W component in the highest-address n bits; for B8G8R8A8, byte 0 is blue and byte 3 is alpha. Note that platform endianness is not considered in this definition. In C:

    struct x8y8z8w8 { uint8_t x, y, z, w; };

Format aliases like XYZWstrq are (s+t+r+q)-bit integers in host endianness, with the X component in the s least-significant bits of the integer. In C:

    uint32_t xyzw8888 = (x << 0) | (y << 8) | (z << 16) | (w << 24);

Format suffixes affect the interpretation of the channel:

- `SINT`: N bit signed integer [-2^(N-1) … 2^(N-1) - 1]

- `SNORM`: N bit signed integer normalized to [-1 … 1]

- `SSCALED`: N bit signed integer [-2^(N-1) … 2^(N-1) - 1]

- `FIXED`: Signed fixed point integer, (N/2 - 1) bits of mantissa

- `FLOAT`: N bit IEEE754 float

- `NORM`: Normalized integers, signed or unsigned per channel

- `UINT`: N bit unsigned integer [0 … 2^N - 1]

- `UNORM`: N bit unsigned integer normalized to [0 … 1]

- `USCALED`: N bit unsigned integer [0 … 2^N - 1]

The difference between `SINT` and `SSCALED` is that the former are pure integers in shaders, while the latter are floats; likewise for `UINT` versus `USCALED`.

There are two exceptions for `FLOAT`. `R9G9B9E5_FLOAT` is nine bits each of red green and blue mantissa, with a shared five bit exponent. `R11G11B10_FLOAT` is five bits of exponent and five or six bits of mantissa for each color channel.

For the `NORM` suffix, the signedness of each channel is indicated with an S or U after the number of channel bits, as in `R5SG5SB6U_NORM`.

The `SRGB` suffix is like `UNORM` in range, but in the sRGB colorspace.

Compressed formats are named first by the compression format string (`DXT1`, `ETC1`, etc), followed by a format-specific subtype. Refer to the appropriate compression spec for details.

Formats used in video playback are named by their FOURCC code.

Format names with an embedded underscore are subsampled. `R8G8_B8G8` is a single 32-bit block of two pixels, where the R and B values are repeated in both pixels.

Index buffers do not have a natural format in Gallium structures. For purposes of `is_format_supported` queries, the formats `R8_UINT`, `R16_UINT`, and `R32_UINT` are used with `PIPE_BIND_INDEX_BUFFER` for 8-bit, 16-bit, and 32-bit index buffers respectively.

## References

DirectX Graphics Infrastructure documentation on DXGI_FORMAT enum: https://learn.microsoft.com/en-us/windows/win32/api/dxgiformat/ne-dxgiformat-dxgi_format

FOURCC codes for YUV formats: http://web.archive.org/web/20220523043110/https://www.fourcc.org/yuv/