[Mesa-dev] [PATCH 02/14] gallium: Document format name conventions

[Adam Jackson]

v2:
- Fix a channel name thinko (Michel Dänzer)
- Elaborate on SCALED versus INT
- Add links to DirectX and FOURCC docs

Signed-off-by: Adam Jackson <ajax at redhat.com>
---
 src/gallium/docs/format.rst | 54 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)
 create mode 100644 src/gallium/docs/format.rst

diff --git a/src/gallium/docs/format.rst b/src/gallium/docs/format.rst
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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.
+
+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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1.8.2.1