Frame buffer not working

This is a discussion on Frame buffer not working within the Game Programming forums, part of the General Programming Boards category; I am trying to render to an off-screen texture using OpenGL = for post processing effects - but the frame ...

  1. #1
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    Nov 2008
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    41

    OpenGL: Rendering to off-screen texture

    I am trying to render to an off-screen texture using OpenGL = for post processing effects - but the frame buffer doesn't seem to work. If I just render straight to the back buffer like normal the scene is rendered fine. But when I try to do the simplest use of frame buffers - draw to a frame buffer and then just read out of that frame buffer - I just get a black screen. This has been bugging me for hours.

    So here's the setup stuff at the start of the program for the FBOs:
    Code:
    void init(){
    // Setup GL etc....
    
        //Buffer textures
        glGenTextures(2, ppTexture);
    	glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, ppTexture[0]);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, textureWidth, textureHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
        glBindTexture(GL_TEXTURE_2D, ppTexture[1]);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, textureWidth, textureHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, 0);
    
        //Render buffer for depth buffer
        glGenRenderbuffersEXT(1, &renderbufferID);
        glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderbufferID);
        glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT32, textureWidth, textureHeight);
    
        //FBOs
        glGenFramebuffersEXT(6, framebufferID);
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID[0]);
        const GLenum bufs[2] = {GL_COLOR_ATTACHMENT0_EXT, GL_COLOR_ATTACHMENT1_EXT};
        glDrawBuffers(2, bufs);
        glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, renderbufferID);
        glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, ppTexture[0], 0);
        glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT1_EXT, GL_TEXTURE_2D, ppTexture[2], 0);  
    }
    Yes I do generate and attach a second texture, but I don't use it.

    And here's the display function:
    Code:
    //GLUT displays the scene with this function
    void display(){
        glEnable(GL_DEPTH_TEST);
        
        glBindFramebuffer(GL_FRAMEBUFFER, framebufferID[0]);
        glViewport(0,0,textureWidth, textureHeight);
        glUseProgram(GM->getShaderProgram());
        
        //Set projection
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluPerspective(45, static_cast<float>(SCREEN_WIDTH) / SCREEN_HEIGHT, Z_NEAR_CLIP, Z_FAR_CLIP);
        
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();
        
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    
        //.... draw the scene
        
        //Now read back from the frame buffer
        glUseProgram(0);
        glBindFramebuffer(GL_FRAMEBUFFER, 0);
        glDisable(GL_LIGHTING);
        glDisable(GL_DEPTH_TEST);
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, ppTexture[0]);
        
        glClear(GL_COLOR_BUFFER_BIT);
        
        glBegin(GL_QUADS);
            glMultiTexCoord2f(GL_TEXTURE0, 0.0f, 0.0f);
            glVertex2f(-1.0f, -1.0f);
            glMultiTexCoord2f(GL_TEXTURE0, 1.0f, 0.0f);
            glVertex2f(1.0f, -1.0f);
            glMultiTexCoord2f(GL_TEXTURE0, 1.0f, 1.0f);
            glVertex2f(1.0f, 1.0f);
            glMultiTexCoord2f(GL_TEXTURE0, 0.0f, 1.0f);
            glVertex2f(-1.0f, 1.0f);            
        glEnd();
        
        //And flush
        glutSwapBuffers();
    }
    My shader does use glFragData[0] and glFragData[1] instead of glFragColor.

    There are no OpenGL errors and the frame buffer has no error status.

    Can anyone see a problem here?

    I am using Windows XP and Dev-C++, old I know but I've gotten used to it and VC++ doesn't want to work for some reason.
    Last edited by Noise; 02-15-2009 at 06:09 PM.

  2. #2
    Super Moderator VirtualAce's Avatar
    Join Date
    Aug 2001
    Posts
    9,590
    What you are doing is rendering to an off-screen texture or a simple render to texture. Calling it this in your thread may lead more people to helping you since that is the standard terminology. I do not use OGL and so I cannot assist you. However changing this title to something like 'problems with render to texture in OGL' or something along those lines will definitely bring in more people to assist.

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