FBO and Render to renderbuffer

Question

Hi I am trying to use the Framebuffer object extension for offscreen rendering following the notes and example on : http://developer.apple.com/library/mac/#documentation/GraphicsImaging/Conceptual/OpenGL-MacProgGuide/opengl_offscreen/opengl_offscreen.html

I am having a problem how to display the contents i have drawn on the FBO to the window! I am getting only a fully black screen!

Also are textures preferable? I have found that renderbuffers are better in performance is that so?

Can anyone help me with this pls?

#include <stdio.h>

#include <stdlib.h>

#include <GLUT/glut.h>   


void drawfunc();
void init(int argc, char** argv);
void initialise_FBO_toRenderBuf();
void onExit();

GLuint renderbufID;
GLuint bufferID;

int main(int argc, char** argv)
{
    init(argc,argv);
    initialise_FBO_toRenderBuf();
    glutDisplayFunc(drawfunc);
    glutMainLoop();
}

void init(int argc, char** argv)
{
    //initialising window settings
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE|GL_RGBA8);
    glutInitWindowSize(250,250);
    glutInitWindowPosition(100,100);
    glutCreateWindow("Hello Leanne mine");
    /*GLubyte* versiongl;
     versiongl = glGetString(GL_VERSION);
     printf("version is: %s\n",versiongl);
     */
    //initial background colour of the window
    glClearColor(0.0, 0.0, 0.0, 0.0);

    //the coordinate system to be used
    glOrtho(0.0, 1.0, 0.0, 1.0, -10.0, 1.0);


    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

}

void initialise_FBO_toRenderBuf(void)
{


    //generate FBO name (bufferID) and create a buffer objct with the texture name (bufferID)
    glGenFramebuffersEXT(1, &bufferID);
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, bufferID);


    //generate renderbuffer name (renderbufID) and create a renderbuffer object with the renderbuffer name (renderbufID)
    glGenRenderbuffersEXT(1, &renderbufID);
    glBindRenderbufferEXT(GL_RENDERBUFFER_EXT, renderbufID);

    //Create data storage and establish the pixel format and dimensions of the renderbuffer image by calling the following function
    glRenderbufferStorageEXT (GL_RENDERBUFFER_EXT, GL_RGBA8,250, 250);

    //attach the texture to the FBO
    glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_RENDERBUFFER_EXT, renderbufID);

    switch(glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT))
    {

        case GL_FRAMEBUFFER_COMPLETE_EXT:    printf("The fbo is complete\n"); break;
        case GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT_EXT:    printf("GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT_EXT\n"); break;
        case GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT_EXT:    printf("GL_FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT_EXT\n"); break;   
        case GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS_EXT:    printf("GL_FRAMEBUFFER_INCOMPLETE_DIMENSIONS_EXT\n"); break;
        case GL_FRAMEBUFFER_INCOMPLETE_FORMATS_EXT:    printf("GL_FRAMEBUFFER_INCOMPLETE_FORMATS_EXT\n"); break;
    }

}


void drawfunc()
{
    //use the FBO with name bufferID
    glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, bufferID);

    //clears the FBO to this color when glClear is called i.e it is the texture background color
    glClearColor(0.0, 1.0, 0.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT);


    //drawing here
    glColor3f(0.0, 1.0, 1.0);
    glBegin(GL_QUADS);
    glVertex2f(0.25, 0.25);
    glVertex2f(0.25, 0.75);
    glVertex2f(0.75, 0.75);
    glVertex2f(0.75, 0.25);
    glEnd();


    glBegin(GL_QUADS);
    glColor3f(1.0, 0.0, 0.0);
    glVertex2f(0.5, 0.5);
    glVertex2f(0.5, 0.65);
    glVertex2f(0.65, 0.65);
    glVertex2f(0.65, 0.5);
    glEnd();

    GLvoid* pixels = malloc(250*250*4);

    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

    glReadBuffer(GL_COLOR_ATTACHMENT0_EXT);
    glReadPixels(0,0, 250,250, GL_RGBA8, GL_UNSIGNED_BYTE, pixels);


    glDrawBuffer(0);
    //glBindFramebufferEXT(GL_DRAW_FRAMEBUFFER, 0 );
    glRasterPos2d(0, 0);
    glDrawPixels(250, 250, GL_RGBA8,GL_UNSIGNED_BYTE,pixels);



    glFlush();

    onExit();
}

void onExit()
{
    //glDeleteTextures(1, &renderbufID);
    glDeleteFramebuffersEXT(1, &bufferID);
}

Answer 1

Here's a list of at least some of the errors:

1. You call glOrtho without first having set the matrix mode to GL_PROJECTION, will probably "work" but the projection matrix will end up in the modelview matrix.
2. You should change the line glDrawBuffer(0); to glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0) to redirect the rendering to the window.
3. At the end of the draw function you delete the render target, what do you think will happen next time drawfunc is called?

After you've fixed these thing, put a breakpoint after glDrawPixels and check in your debugger that you got the correct data.