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gallium/docs: fix definitions of TGSI_SEMANTIC_POSITION, COLOR

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Brian Paul
2010-06-23 17:00:10 -06:00
parent bf2d2772a2
commit 50b3f2e789
1 changed files with 26 additions and 18 deletions
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src/gallium/docs/source/tgsi.rst
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@@ -1306,38 +1306,46 @@ Declaration Semantic
TGSI_SEMANTIC_POSITION
""""""""""""""""""""""
Position, sometimes known as HPOS or WPOS for historical reasons, is the
location of the vertex in space, in ``(x, y, z, w)`` format. ``x``, ``y``, and ``z``
are the Cartesian coordinates, and ``w`` is the homogenous coordinate and used
for the perspective divide, if enabled.
For vertex shaders, TGSI_SEMANTIC_POSITION indicates the vertex shader
output register which contains the homogeneous vertex position in the clip
space coordinate system. After clipping, the X, Y and Z components of the
vertex will be divided by the W value to get normalized device coordinates.
As a vertex shader output, position should be scaled to the viewport. When
used in fragment shaders, position will be in window coordinates. The convention
used depends on the FS_COORD_ORIGIN and FS_COORD_PIXEL_CENTER properties.
For fragment shaders, TGSI_SEMANTIC_POSITION is used to indicate that
fragment shader input contains the fragment's window position. The X
component starts at zero and always increases from left to right.
The Y component starts at zero and always increases but Y=0 may either
indicate the top of the window or the bottom depending on the fragment
coordinate origin convention (see TGSI_PROPERTY_FS_COORD_ORIGIN).
The Z coordinate ranges from 0 to 1 to represent depth from the front
to the back of the Z buffer. The W component contains the reciprocol
of the interpolated vertex position W component.
XXX additionally, is there a way to configure the perspective divide? it's
accelerated on most chipsets AFAIK...
Position, if not specified, usually defaults to ``(0, 0, 0, 1)``, and can
be partially specified as ``(x, y, 0, 1)`` or ``(x, y, z, 1)``.
XXX usually? can we solidify that?
TGSI_SEMANTIC_COLOR
"""""""""""""""""""
Colors are used to, well, color the primitives. Colors are always in
``(r, g, b, a)`` format.
For vertex shader outputs or fragment shader inputs/outputs, this
label indicates that the resister contains an R,G,B,A color.
Several shader inputs/outputs may contain colors so the semantic index
is used to distinguish them. For example, color[0] may be the diffuse
color while color[1] may be the specular color.
This label is needed so that the flat/smooth shading can be applied
to the right interpolants during rasterization.
If alpha is not specified, it defaults to 1.
TGSI_SEMANTIC_BCOLOR
""""""""""""""""""""
Back-facing colors are only used for back-facing polygons, and are only valid
in vertex shader outputs. After rasterization, all polygons are front-facing
and COLOR and BCOLOR end up occupying the same slots in the fragment, so
all BCOLORs effectively become regular COLORs in the fragment shader.
and COLOR and BCOLOR end up occupying the same slots in the fragment shader,
so all BCOLORs effectively become regular COLORs in the fragment shader.
TGSI_SEMANTIC_FOG
"""""""""""""""""