Question

Is it possible to achieve flat shading in OpenGL when using glDrawElements to draw objects, and if so how? The ideal way would be to calculate a normal for each triangle only once, if possible.

The solution must only use the programmable pipeline (core profile).

Answer 1

There are indeed ways around this without duplicating vertices, with some limitations for each one (at least those I can think of with my limited OpenGL experience).

I can see two solutions that would give you a constant value for the normal over each triangle :

• declare the input as flat in your shader and pick which vertex gives its value via glProvokingVertex; fast but you’ll get the normal for one vertex as the normal for the whole triangle, which might not look right
• use a geometry shader taking triangles and outputing triangles to calculate a single normal per face. This is the most flexible way, allowing you to control the resulting effect, but it might be slow (and required geometry shader capable hardware, obviously)