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| -rw-r--r-- | src/gallium/docs/source/tgsi.rst | 44 |
1 files changed, 26 insertions, 18 deletions
diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst index ecab7cb8097..f5678c744b5 100644 --- a/src/gallium/docs/source/tgsi.rst +++ b/src/gallium/docs/source/tgsi.rst @@ -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 """"""""""""""""" |