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