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Draw to offscreen renderbuffer in OpenGL ES (iPhone)

I'm trying to create an offscreen render buffer in OpenGL ES on the iPhone. I've created the buffer like this:

        glGenFramebuffersOES(1, &offscreenFramebuffer);
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreenFramebuffer);

    glGenRenderbuffersOES(1, &offscreenRenderbuffer);
    glBindRenderbufferOES(GL_RENDERBUFFER_OES, offscreenRenderbuffer);
    glFramebufferRenderbufferOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_RENDERBUFFER_OES, offscreenRenderbuffer);

But I'm confused on how to render the storage. Apple's documentation says to use the EAGLContext renderBufferStorage:fromDrawable: method, but this seems to only work for one render buffer (the main one being displayed). If I use the normal OpenGL function glRenderBufferStorageOES, then I can't seem to get it to display. Here's the code:

        // this is in the initialization section:
    glRenderbufferStorageOES(GL_RENDERBUFFER_OES, GL_RGB8_OES, backingWidth, backingHeight);

    // and this is when I'm trying to draw to it and display it:
    glBindFramebufferOES(GL_FRAMEBUFFER_OES, offscreenFramebuffer);
    GLfloat vc[] = {
        0.0f, 0.0f, 0.0f,
        10.0f, 10.0f, 10.0f,
        0.0f, 0.0f, 0.0f,
        -10.0f, -10.0f, -10.0f,         
    };

    glLoadIdentity();
    glEnableClientState(GL_VERTEX_ARRAY);
    glVertexPointer(3, GL_FLOAT, 0, vc);
    glDrawArrays(GL_LINES, 0, 4);
    glDisableClientState(GL_VERTEX_ARRAY);

    glBindRenderbufferOES(GL_RENDERBUFFER_OES, offscreenRenderbuffer);
    [context presentRenderbuffer:GL_RENDERBUFFER_OES];

Doing it this way, nothing is displayed on the screen. However, if I switch out the references to "offscreen...Buffer" to the buffers that were created with the renderBufferStorage method, it works fine.

Any suggestions?

like image 646
David Ensminger Avatar asked Dec 05 '09 21:12

David Ensminger


3 Answers

Since you can't use presentRenderbuffer with an offscreen FBO, you should associate it with a texture object using glFramebufferTexture2DOES, then render a textured full-screen quad.

like image 104
prideout Avatar answered Nov 09 '22 23:11

prideout


@david good idea.. what you need to do is what @prideout said.. create a texture and render to it.. and use the texture on a quad every time. Make sure you draw to the texture only once, as in your case things are persistent.

- (void)setUpTextureBuffer 
{
 glGenFramebuffersOES(1, &texturebuffer); 
 glBindFramebufferOES(GL_FRAMEBUFFER_OES, texturebuffer);

 // create the texture
 glGenTextures(1, &canvastexture); 
 glBindTexture(GL_TEXTURE_2D, canvastexture); 
 glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA,  512, 512, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL); 
 glFramebufferTexture2DOES(GL_FRAMEBUFFER_OES, GL_COLOR_ATTACHMENT0_OES, GL_TEXTURE_2D, canvastexture, 0);

 GLenum status = glCheckFramebufferStatusOES(GL_FRAMEBUFFER_OES); 
 if(status != GL_FRAMEBUFFER_COMPLETE_OES) { 
  NSLog(@"failed to make complete framebuffer object %x", status);
 }

 glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
 glTexParameterf( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );

 glClearColor(1.0, 1.0, 1.0, 1.0);
 glViewport(0, 0, 512, 512);
 glClear(GL_COLOR_BUFFER_BIT);
}



//setTargetToTexture() function

 glBindFramebufferOES(GL_FRAMEBUFFER_OES, tbuffer);
 glBindTexture(GL_TEXTURE_2D, allbrushes);
 glViewport(0, 0, 512, 512);

//reset pointers after finishing drawing to textures

 glViewport(0, 0, BWIDTH, BHEIGHT);
 glVertexPointer(2, GL_FLOAT, 0, canvas);   //canvas vertices
 glTexCoordPointer(2, GL_FLOAT, 0, texels);
 glBindTexture(GL_TEXTURE_2D, boundtexture);   //bind to the texture which is the special render target 
 glBindFramebufferOES(GL_FRAMEBUFFER_OES, fbuffer); //back to normal framebuffer
like image 22
Rajavanya Subramaniyan Avatar answered Nov 09 '22 22:11

Rajavanya Subramaniyan


You cannot present an normal renderbuffer (created with glRenderbufferStorage), it is always offscreen. presentRenderbuffer: can only be used for renderbuffers that were created using the renderbufferStorage:fromDrawable:. If you checked the return value of that presentRenderbuffer:, you should observe it failing.

What are you trying to do?

like image 1
Frogblast Avatar answered Nov 09 '22 23:11

Frogblast