1 Formats in gallium 2 ================== 3 4 Gallium format names mostly follow D3D10 conventions, with some extensions. 5 6 Format names like XnYnZnWn have the X component in the lowest-address n bits 7 and the W component in the highest-address n bits; for B8G8R8A8, byte 0 is 8 blue and byte 3 is alpha. Note that platform endianness is not considered 9 in this definition. In C:: 10 11 struct x8y8z8w8 { uint8_t x, y, z, w; }; 12 13 Format aliases like XYZWstrq are (s+t+r+q)-bit integers in host endianness, 14 with the X component in the s least-significant bits of the integer. In C:: 15 16 uint32_t xyzw8888 = (x << 0) | (y << 8) | (z << 16) | (w << 24); 17 18 Format suffixes affect the interpretation of the channel: 19 20 - ``SINT``: N bit signed integer [-2^(N-1) ... 2^(N-1) - 1] 21 - ``SNORM``: N bit signed integer normalized to [-1 ... 1] 22 - ``SSCALED``: N bit signed integer [-2^(N-1) ... 2^(N-1) - 1] 23 - ``FIXED``: Signed fixed point integer, (N/2 - 1) bits of mantissa 24 - ``FLOAT``: N bit IEEE754 float 25 - ``NORM``: Normalized integers, signed or unsigned per channel 26 - ``UINT``: N bit unsigned integer [0 ... 2^N - 1] 27 - ``UNORM``: N bit unsigned integer normalized to [0 ... 1] 28 - ``USCALED``: N bit unsigned integer [0 ... 2^N - 1] 29 30 The difference between ``SINT`` and ``SSCALED`` is that the former are pure 31 integers in shaders, while the latter are floats; likewise for ``UINT`` versus 32 ``USCALED``. 33 34 There are two exceptions for ``FLOAT``. ``R9G9B9E5_FLOAT`` is nine bits 35 each of red green and blue mantissa, with a shared five bit exponent. 36 ``R11G11B10_FLOAT`` is five bits of exponent and five or six bits of mantissa 37 for each color channel. 38 39 For the ``NORM`` suffix, the signedness of each channel is indicated with an 40 S or U after the number of channel bits, as in ``R5SG5SB6U_NORM``. 41 42 The ``SRGB`` suffix is like ``UNORM`` in range, but in the sRGB colorspace. 43 44 Compressed formats are named first by the compression format string (``DXT1``, 45 ``ETC1``, etc), followed by a format-specific subtype. Refer to the 46 appropriate compression spec for details. 47 48 Formats used in video playback are named by their FOURCC code. 49 50 Format names with an embedded underscore are subsampled. ``R8G8_B8G8`` is a 51 single 32-bit block of two pixels, where the R and B values are repeated in 52 both pixels. 53 54 References 55 ---------- 56 57 DirectX Graphics Infrastructure documentation on DXGI_FORMAT enum: 58 http://msdn.microsoft.com/en-us/library/windows/desktop/bb173059%28v=vs.85%29.aspx 59 60 FOURCC codes for YUV formats: 61 http://www.fourcc.org/yuv.php 62
