How to see the generated edges after tessellation?

Question

I am trying to see how my mesh is being transformed by the tessellation shader. I have seen multiple images of this online so i know it is possible.

Reading the khronos wiki it seems that to generate the same behaviour as GL_LINES I should set the patch vertices to 2 like this:

glPatchParameteri(GL_PATCH_VERTICES, 2)

However this results in the exact same output as

glPatchParameteri(GL_PATCH_VERTICES, 3)

In other words, I am seeing filled triangles instead of lines. I am drawing using GL_PATCHES and I am not getting compilation nor runtime errors.

How can I see the generated edges?

Answer 1

If you cannot use the polygon mode, you can employ the geometry shader. The geometry shader is a stage that is executed after tessellation. So, you can have a geometry shader that takes a triangle as input and produces a line strip of three lines as output. This will show the wireframe. It will also draw inner edges twice, though.

Answer 2

Just use glPolygonMode( GL_FRONT_AND_BACK, GL_LINE ); in your initialization code. Also you can call this based on some key so that you can toggle between wireframe and polygon mode.

Answer 3

I would like to point out that this question represents a stark misunderstanding of how Tessellation works.

The number of vertices in a patch is irrelevant to "how my mesh is being transformed by the tessellation shader".

Tessellation is based on an abstract patch. That is, if your TES uses triangles as its abstract patch type, then it will generate triangles. This is just as true whether your vertices-per-patch count is 20 or 2.

The job of the code in the TES is to figure out how to apply the tessellation of the abstract patch to the vertex data of the patch in order to produce the actual tessellated output vertex data.

So if you're tessellating a triangle, your TES gets a 3-element barycentric coordinate (gl_TessCoord) that determines the location in the abstract triangle to generate the vertex data for. The tessellation primitive generator's job is to decide which vertices to generate and how to assemble them into primitives (triangle edge connectivity).

So basically, the number of patch vertices is irrelevant to the edge connectivity graph. The only thing that matters for that is the abstract patch type and the tessellation levels being applied to it.