However, much like geometry shaders, they can “see” all the verts in a single primitive.
Tessellation triangle serial#
They both execute on a per-vertex basis, so their programs typically don’t have serial loops like the old-fashioned geometry programs did.
So, we now have two additional programmable stages at our disposal: Tessellation Control and Tessellation Evaluation. Now is the time for the obligatory diagram: (I used a red pen for the stages that are new to OpenGL 4.0) The New OpenGL 4.0+ PipelineĪlthough adding multiple GS invocations was helpful, performing highly-efficient subdivision demanded brand new stages in the pipeline. You can figure out which vert you’re on by looking at the built-in gl_InvocationID variable. This tells the GPU that your GS should run three times on a single primitive. Nowadays you can do a bit better by specifying an invocations count at the top of your GS, like so: They didn’t do a great job of leveraging the massive parallelism in GPUs. Most GS programs had a serial loop that simply pushed out the verts of the new primitive(s), one vertex after another.
The GS processing for a single primitive was initially limited to a single processing unit. It was not designed for large-scale amplification of vertex data. The GS unit turned out to be convenient for certain effects, but overall it was somewhat disappointing. And finally, the GPU could produce more primitives than it consumed. When Geometry Shaders (GS) first came out, we were all excited because we could finally write a shader that could “see” all the verts in a triangle at once. This entry gives an overview of tessellation and walks through an example of simple triangle subdivision in the next entry, we’ll focus on quad subdivision. This is the first of a two-part article on tessellation shaders with OpenGL 4.0+.
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