[Mesa-dev] LAYER_PROVOKING_VERTEX and VIEWPORT_INDEX_PROVOKING_VERTEX queries

[Roland Scheidegger]

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