OpenGL：OpenGL-ES之外的Triangle Strip中的退化三角形是可以接受的吗？

Question

In this tutorial for OpenGL ES, techniques for optimizing models are explained and one of those is to use triangle strips to define your mesh, using "degenerate" triangles to end one strip and begin another without ending the primitive. http://www.learnopengles.com/tag/degenerate-triangles/

However, this guide is very specific to mobile platforms, and I wanted to know if this technique held for modern desktop hardware. Specifically, would it hurt ? Would it either cause graphical artifacts or degrade performance (opposed to splitting the strips into separate primatives?)

If it causes no artifacts and performs at least as well, I aim to use it solely because it makes organizing vertices in a certain mesh I want to draw easier.

Answer 1

Degenerate triangles work pretty well on all platforms. I'm aware of an old fixed-function console that struggled with degenerate triangles, but anything vaguely modern will be fine. Reducing the number of draw calls is always good and I would certainly use degenerates rather than multiple calls to glDrawArrays.

However, an alternative that usually performs better is indexed draws of triangle lists. With a triangle list you have a lot of flexibility to reorder the triangles to take maximum advantage of the post-transform cache. The post-transform cache is a hardware cache of the last few vertices that went through the vertex shader, the GPU can spot if you've re-issued the same vertex and skip the entire vertex shader for that vertex.

Answer 2

In addition to the above answers (no it shouldn't hurt at all unless you do something mad in terms of the ratio of real triangles to the degenerates), also note that the newer versions of OpenGL and OpenGL ES (3.x or higher) APIs support a means to insert breaks into index lists without needing an actual degenerate triangle, which is called primitive restart.

https://www.khronos.org/opengles/sdk/docs/man3/html/glEnable.xhtml

When enabled you can encode "MAX_INT" for the index type, and when detected that forces the GPU to restart building a new tristrip from the next index value.

Answer 3

It will not cause artifacts. As to "degrading performance"... relative to what? Relative to a random assortment of triangles with no indexing? Yes, it will be faster than that.

But there are plenty of other things one can do. For example, primitive restarting, which removes the need for degenerate triangles. Then there's using ordered lists of triangles for improved cache coherency. Will triangle strips be faster than that?

It rather depends on what you're rendering, how expensive your vertex shaders are, and various other things.

But at the end of the day, if you care about maximum performance on particular platforms, then you should profile for each platform and pick the vertex data based on what platform you're running on. If performance is really that important to you, then you're going to have to put forth some effort.