How to restore a GL_RENDERBUFFER? - ios

Question

I am working on storing and restoring my OpenGL ES based application's state.
I have a function to save the GL_RENDERBUFFER to dump the data with the following code:
glBindFramebuffer(GL_FRAMEBUFFER, fboTextureBufferData.framebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, fboTextureBufferData.colorbuffer);
GLint x = 0, y = 0, width2 = backingWidth, height2 = backingHeight;
NSInteger dataLength = width * height * 4;
GLubyte *data = (GLubyte*)malloc(dataLength * sizeof(GLubyte));
// Read pixel data from the framebuffer
glPixelStorei(GL_PACK_ALIGNMENT, 4);
glReadPixels(x, y, width, height, GL_RGBA, GL_UNSIGNED_BYTE, data);
I don't see a glWritePixels function. What is the best way to repopulate the GL_RENDERBUFFER with the GLubyte data populated above? An example would be greatly appreciated.
EDIT 3:
Here is how I am attempting to configure the texture render buffer, and the function used to draw it. As noted in the code, if I specify GL_COLOR_ATTACHMENT1 for the glFramebufferTexture2D parameter, the stored pixel data is restored but I can't get any updates to draw. But if I use GL_COLOR_ATTACHMENT0 instead, I get drawing updates but no pixel data restored.
I have tried various combinations (for instance also using GL_COLOR_ATTACHMENT1 for the glFramebufferRenderbuffer parameter) but then I get an invalid frame buffer error when attempting to render. It seems I am so close, but can't figure out how to get them both restoring and rendering working together.
- (bool)configureRenderTextureBuffer {
[EAGLContext setCurrentContext:context];
glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];
glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];
glGenFramebuffers(1, &fboTextureBufferData.framebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, fboTextureBufferData.framebuffer);
glGenRenderbuffers(1, &fboTextureBufferData.colorbuffer);
glBindRenderbuffer(GL_RENDERBUFFER, fboTextureBufferData.colorbuffer);
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8, backingWidth, backingHeight);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, fboTextureBufferData.colorbuffer);
// Generate texture name, stores in .textureID
glGenTextures(1, &fboTextureBufferData.textureID);
glBindTexture(GL_TEXTURE_2D, fboTextureBufferData.textureID);
////////////////// Read Existing texture data //////////////////
NSString *dataPath = [TDTDeviceUtilitesLegacy documentDirectory]; //This just returns the app's document directory
NSData *data = [NSData dataWithContentsOfFile:[NSString stringWithFormat:#"%#/buffer.data", dataPath]];
GLubyte *pixelData = (GLubyte*)[data bytes];
// If I use GL_COLOR_ATTACHMENT1 here, my existing pixel data is restored
// but no drawing occurs. If I use GL_COLOR_ATTACHMENT0, then data isn't
// restored but drawing updates work
glFramebufferTexture2D ( GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, fboTextureBufferData.textureID, 0 );
// Populate with existing data
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, backingWidth, backingHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, &pixelData[0]); //&image[0]
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER) ;
if(status != GL_FRAMEBUFFER_COMPLETE) {
NSLog(#"failed to make complete render texture framebuffer object %x", status);
return false;
}
return true;
}
Here is the code for rendering. The viewFramebuffer is attached to GL_COLOR_ATTACHMENT0 and is used so the texture frame buffer can be zoomed and positioned inside the view.
- (void)renderTextureBuffer {
//Bind the texture frame buffer, if I don't use this, I can't get it to draw
//glBindFramebuffer(GL_FRAMEBUFFER, fboTextureBufferData.framebuffer);
//If I use this instead of binding the framebuffer above, I get no drawing and black background
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, fboTextureBufferData.textureID, 0);
renderParticlesToTextureBuffer();
//Bind the view frame buffer.
glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, viewRenderbuffer);
drawFboTexture();
[context presentRenderbuffer:GL_RENDERBUFFER];
}

Answer 1

If you need to write data directly to a render target, using a renderbuffer is not a good option. In this case, it's much better to use a texture instead.
Using a texture as a FBO attachment works very similarly to using a renderbuffer. Where you currently use glRenderbufferStorage() to allocate a renderbuffer of the needed dimensions, you create a texture instead, and allocate its storage with:
GLuint texId = 0;
glGenTextures(1, &texId);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0,
GL_RGBA, GL_UNSIGNED_SHORT_5_6_5, 0);
Then you attach it to the framebuffer with:
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, texId, 0);
Now, if you later want to fill your render target with data, you can simply call glTexImage2D() again, or even better glTexSubImage2D() if the size is unchanged, to do that.