Supply 4 vertices to geometry shader in OpenGL

Question

I currently have some VAOs and an index list. The idea is to process groups of 4 elements in the geometry shader. The way of picking the groups is if I have the list (1,2,3,4,5,6,7,8,...) then the groups would be ((1,2,3,4),(5,6,7,8),...).

After spending several hours of coding I realized that geometry shaders don't accept 4 vertex primitives. I would like to know if there is a way around it. Maybe choosing a "provoking vertex" and appending the other 3 to it as added properties in the vertex shader stage (I doubt that's possible, though).

Answer 1

Geometry shaders do accept primitives with 4 vertices as input: GL_LINES_ADJACENCY.

This primitive mode might be meant for lines where you need the two neighboring segments for each segments, but it is not limited to any particular use case - it just provides 4 vertices as input, and can be used as such (it could also be used to emulate the deprecated GL_QUADS primitive mode with geometry shaders).