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: 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