Framebuffer and the use of shaders in opengl

Question

I was a little confused about framebuffers. I want to use a framebuffer with many textures, fill each texture, and then use a shader to combine (mix) all the textures to create a new output. Does that sound simple? Yes, that I also thought, but I do not understand this.

How to pass the current bound texture to a shader?

Answer 1

You need to put the texture in a specific slot, and then use the sampler to read from it. In your application:

GLuint frameBuffer;
glGenFramebuffersEXT(1, &frameBuffer); //Create a frame buffer
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, frameBuffer); //Bind it so we draw to it
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, yourTexture, 0); //Attach yourTexture so drawing goes into it

//Draw here and it'll go into yourTexture.

glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0); //Unbind the framebuffer so that you can draw normally again

//Here we put the texture in slot 1.
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, yourTexture);
glActiveTexture(GL_TEXTURE0); //Don't forget to go back to GL_TEXTURE0 if you're going to do more drawing later

//Now we tell your shader where to look.
GLint var = glGetUniformLocationARB(yourShaderProgram, "yourSampler");
glUniform1i(var, 1); //We use 1 here because we used GL_TEXTURE1 to bind our texture

And in your fragment shader:

uniform sampler2D yourSampler;

void main()
{
    gl_FragColor = texture2D(yourSampler, whateverCoordinatesYouWant);
}

GL_TEXTURE2 .., .