GLSL How to retrieve vertex position after a shade

问题:

I wrote a program that simulates soft bodies using springs. It looks nice but the problem is it consumes a lot of CPU time. So I can not run it on my laptop or any not high end PC.

I thought It would be a good idea to write a vertex shader and move the logic to the GPU. I've read some tutorials and made a toon shader so I thought (wrong) I was ready to go.

The big problem I have is that I need to know the old position of a vertex to calculate the new one. I don't know how could I retrieve a vertex position so I could send it back to the shaders each frame?

I'm not really sure is this even possible to do and maybe I'm trying to do something that shaders are never meant to do. I am still researching but I thought I could ask an see if maybe someone can help.

回答1:

You can use the transform feedback mechanism if your hardware supports OpenGL 3.0 or above. There are also other techniques for getting the vertex position back, like carefully arranging your rendering so that you're writing a triangle (or point primitive) to each separate pixel on the screen. This is fairly difficult, and you need to render to a floating-point buffer, which requires FBO support.