Selection / glRenderMode(GL_SELECT)

Question

In order to do object picking in OpenGL, do I really have to render the scene twice?

I realize rendering the scene is supposed to be cheap, going at 30fps.

But if every selection object requires an additional gall to RenderScene()

then if I click at 30 times a second, then the GPU has to render twice as many times?

Answer 1

One common trick is to have two separate functions to render your scene. When you're in picking mode, you can render a simplified version of the world, without the things you don't want to pick. So terrain, inert objects, etc, don't need to be rendered at all.

The time to render a stripped-down scene should be much less than the time to render a full scene. Even if you click 30 times a second (!), your frame rate should not be impacted much.

Answer 2

First of all, the only way you're going to get 30 mouse clicks per second is if you have some other code simulating mouse clicks. For a person, 10 clicks a second would be pretty fast -- and at that, they wouldn't have any chance to look at what they'd selected -- that's just clicking the button as fast as possible.

Second, when you're using GL_SELECT, you normally want to use gluPickMatrix to give it a small area to render, typically a (say) 10x10 pixel square, centered on the click point. At least in a typical case, the vast majority of objects will fall entirely outside that area, and be culled immediately (won't be rendered at all). This speeds up that rendering pass tremendously in most cases.

Answer 3

There have been some good suggestions already about how to optimize picking in GL. That will probably work for you.

But if you need more performance than you can squeeze out of gl-picking, then you may want to consider doing what most game-engines do. Since most engines already have some form of a 3D collision detection system, it can be much faster to use that. Unproject the screen coordinates of the click and run a ray-vs-world collision test to see what was clicked on. You don't get to leverage the GPU, but the volume of work is much smaller. Even smaller than the CPU-side setup work that gl-picking requires.

Select based on simpler collision hulls, or even just bounding boxes. The performance scales by number of objects/hulls in the scene rather than by the amount of geometry plus the number of objects.