How to use FrameBuffer objects (OpenGL)

Question

I want to draw a 2D scene and after the scene i want to draw some light effects.

When i draw some light, i create a FBO, draw in it and when finished with drawing, i want to create a texture where i draw the content of the FBO.

My current code:

//////////
//Init_

//Create Texture
glActiveTexture(GL_TEXTURE0);
glGenTextures(1, out _texfbo);
glBindTexture(GL_TEXTURE_2D, _texfbo);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _screenWidth, _screenHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, null);
glBindTexture(GL_TEXTURE_2D, 0);

//Create Renderbuffer
glGenRenderbuffers(1, out _rb);
glBindRenderbuffer(GL_RENDERBUFFER, _rb);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, _screenWidth, _screenHeight);
glBindRenderbuffer(GL_RENDERBUFFER, 0);

//Create Framebuffer
glGenFramebuffers(1, out _fbo);
glBindFramebuffer(GL_FRAMEBUFFER, _fbo);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _texfbo, 0);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, _rb);
glBindFramebuffer(GL_FRAMEBUFFER, 0);

//////////
//Loop:

glBindFramebuffer(GL_FRAMEBUFFER, _fbo);

//Draw Scene

glBindFramebuffer(GL_FRAMEBUFFER, 0);

glBindTexture(GL_TEXTURE_2D, _texfbo);

//Draw Rectangle with texture from Framebuffer

The Scene is not drawn on the screen, i think that means it's drawn in the right buffer. But i can't get someting on the screen. What is wrong?

Answer 1

You are missing a glDrawBuffers (...) call. You need to establish that color attachment 0 is the first buffer your fragment shader is going to output.

This should fix your problem:

glBindFramebuffer(GL_FRAMEBUFFER, _fbo);

const GLenum draw_buffer = GL_COLOR_ATTACHMENT0;
glDrawBuffers (1, &draw_buffer);

//Draw Scene

glBindFramebuffer(GL_FRAMEBUFFER, 0);

You do not have to do this in the loop, you could do it once when you initialize your FBO. But you do have to do it somewhere while your FBO is bound.