Draw to offscreen renderbuffer in OpenGL ES (iPhone) - iphone

Question

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?

Answer 1

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.

Answer 2

#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

Answer 3

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?