I'm trying to understand how post processing work.
From what I understood, I need to to something like that:
-> Bind render texture
-> Draw my scene
-> Unbind render texture
-> Draw a quad full screen using the resulting texture using a shader to apply an effect (blur, …)
The problem with that system is: How can I apply multiple effects on the resulting quad?
In my mind, I think applying the "effects" (shaders) on the resulting texture then drawing the quad his probably the best solution, but I don't know if it is possible.
Can I apply shaders on texture directly?
PS: Here is what I've done for the moment, I can draw all my scene in a texture currently:
A PostEffect class (an effect to apply)
EffectManager (create the output texture and have a method "add(PostEffect*)"
I don't know what you mean with "directly applying" shaders to a texture, but to render a texture with a special effect you need to render a fullscreen quad to the screen with the shader you need, and of course feed the texture into the shader. To have multiple effects you need to use two framebuffers (ping-ponging), like this:
-> Bind 1st render texture
-> Draw scene
-> Bind 2nd render texture
-> Feed 1st render texture to a shader and draw quad
-> Bind 1st render texture
-> Feed 2nd render texture to another shader and draw quad
-> Bind 2nd render texture
-> Feed 1st render texture to a third shader and draw quad
...
-> Unbind
-> Draw quad
You cannot render two times in a row to the same framebuffer, which is why you need to do this way (see https://www.opengl.org/wiki/Framebuffer_Object#Feedback_Loops).
If you can, try to pack as many effects as possible into the same shader to minimize overhead when feeding uniforms and drawing.
Post processing tecniques can vary depending on what you want to accomplish. You can either write a single shader that applies multiple post processing effects and render to the texture a single time, or if the renderings are sequential in nature then you need a double buffer:
render to texture 1
use texture one and render to texture 2
use texture 2 and render to texture 1
etc
etc
and do that until all of your post effects are applied. But then you should render the final pass straight to your screen render buffer to avoid a costly 'copying' of the texture to the screen render buffer.
Can I apply shaders on texture directly?
Not sure what you even mean by that. The only time a shader is utilized is when you do a draw call via glDrawArrays or glDrawElements assumming you created your FBO and RBOs correctly. So yes, you must use a full screen rendering technique of some sort.
来源：https://stackoverflow.com/questions/28550225/post-processing-and-resulting-texture