Trying to fix some draw bugs with layer/vp outputs (and clipping), I was wondering if GL actually guarantees sane results if the layer/vp index isn't the same on all vertices. And sure it seems it does, albeit it's implementation-dependent. Specifically (from gl 4.4 core, page 388)
"viewport index is implementation-dependent and thus portable applications will assign the same layer and viewport index for all vertices in a primitive. The vertex conventions followed for gl_Layer and gl_ViewportIndex may be determined by calling GetIntegerv with the symbolic constants LAYER_PROVOKING_VERTEX and VIEWPORT_INDEX_PROVOKING_VERTEX, respectively. For either query, if the value returned is PROVOKING_VERTEX, then vertex selection follows the convention specified by ProvokingVertex (see section 13.4). If the value returned is FIRST_VERTEX_CONVENTION, selection is always taken from the first vertex of a primitive. If the value returned is LAST_VERTEX_CONVENTION, the selection is always taken from the last vertex of a primitive. If the value returned is UNDEFINED_VERTEX, the selection is not guaranteed to be taken from any specific vertex in the primitive." However, what mesa does is just return Light.ProvokingVertex. This means that if you switch the provoking vertex, the result will be different, which seems quite inappropriate for this implementation-dependent query. Albeit I'm not sure what the result really should be, that is if all drivers will do the same - I guess though UNDEFINED_VERTEX would always be correct. Roland _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
