Irrespective of any spec lawyering you might do, we have some piglit tests which basically assume that things like
gl_Layer = foo; gl_Position = ... EmitVertex(); gl_Position = ... EmitVertex(); will always work. I believe this was based on the theory that some applications actually do this, although tbh I don't remember the details. We ended up adding a "latch" in nouveau/codegen to just always re-emit the previously-computed layer at EmitVertex() time. -ilia On Tue, Dec 1, 2015 at 12:43 PM, Roland Scheidegger <srol...@vmware.com> wrote: > 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 _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
