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.

References

DirectX Graphics Infrastructure documentation on DXGI_FORMAT enum: http://msdn.microsoft.com/en-us/library/windows/desktop/bb173059%28v=vs.85%29.aspx

FOURCC codes for YUV formats: http://www.fourcc.org/yuv.php