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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
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