Some GLSL shading questions

Game Engine Game Engine Support and Discussion
1
  • is it possible to access and edit the vertex colours in the vertex shader?
  • is it possible to store vertex colour value in vertex shader for later use?
  • is it possible to use 2 shaders in one material? i.e.: (first shader -> render -> second shader)
2
  1. Yes
  2. No: shaders are (essentially) completely renewed every frame
  3. No, but you can have two objects or play with RenderToTexture

You may want to manipulate vertex colours via python instead. It is slower, but will allow easier control (probably)
I wrote a resource on Render Passes

3

*2. Theoretically there are ways to store values that are generated in a shader and read it back in the next frame (for example with fbos) but it is more complicated than just simply create a variable or something like that, so I wouldn’t use it for simple stuff.
(Also the bge hasn’t a right way (API) to create fbos or something like this, there are only hacky ways with pyopengl/bgl)… so I would go the way that sdfgeoff has mentioned.

4

@Maujoe:
There is now a proper way to create FBO’s (or FBO-like objects) I believe:
https://www.blender.org/api/blender_python_api_2_76_2/gpu.offscreen.html

But I’ve never used them.

5

Hi, the correct API for FBOs inside bge has been commited after 2.77 release->https://developer.blender.org/rB40f1c4f34337d7dfb3fa5bcbd2daa2f602e12011 (VideoTexture module)

6

Interesting…
I never heard of FBO before, can someone please explain me more about it? Or some example .blend file will be so helpful :slight_smile:

7

Hi, I tried to make an fbo example with the new FBO API (to be opened with last blender buildbot version): http://www.pasteall.org/blend/42900

  • On the renderedplane I made a GLSL shader to render what is around mouse cursor in red. Then I copied the result into a FBO (fbo.py).

  • On the other plane I used the image rendered into fbo in a second shader (mainshader.py) and I superimposed fbo color (the red cursor) with the colored grid of Plane.001.

The goal was to use the output (the rendered texture into the fbo) in another shader… This is a bit complex and I’m wondering if it can be simplified (avoiding copyTex as I did) but the goal is to use the output of a shader as input in a second shader.

A more entertaining example: http://www.pasteall.org/blend/42902 (I guess this is a kind of “ping pong” technique but with copyTex it is very slow)(I made it in upbge so you have to check GLSL in render tab)