How to restore a GL_RENDERBUFFER?

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];
}

Probably easier to use a texture instead of a renderbuffer. Then you can use `glTexImage2D()` to fill it with data. — Reto Koradi, Sep 11 '16 at 04:43
Problem with that approach, is I think it would take much more memory. It would be a rather large texture - around 67MB (4096x4096x4), whereas the render buffer is already allocated. — Jeshua Lacock, Sep 11 '16 at 05:07
My suggestion is to use a texture *instead of* a renderbuffer. So never allocate a renderbuffer in the first place, and use a texture in its place. — Reto Koradi, Sep 11 '16 at 05:14
To clarify, its a drawing application, so I need a render buffer to accumulate the brush strokes per frame. — Jeshua Lacock, Sep 11 '16 at 05:17
You can use a texture as a FBO attachment, and render to it just like you render to a renderbuffer. — Reto Koradi, Sep 11 '16 at 06:12

score 3 · Accepted Answer · answered Sep 11 '16 at 13:30

3

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.

answered Sep 11 '16 at 13:30

Reto Koradi

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Thanks - I am trying to implement this now. Currently before drawing to the framebuffer, I am binding it with: `glBindFramebuffer(GL_FRAMEBUFFER, fboTextureBufferData.framebuffer);` Since the frame buffer isn't generated, the pointer for `fboTextureBufferData.framebuffer` doesn't exist. So how I do bind the texture for rendering? Note that I have a 2nd frame buffer, so binding at runtime is required. – Jeshua Lacock Sep 11 '16 at 23:36
Just use `glFramebufferTexture2d` instead of `glFramebufferRenderbuffer`. – solidpixel Sep 12 '16 at 14:20
I can't get anything to draw with this. Can you please look at my edit above and look at how I am initializing and drawing? – Jeshua Lacock Sep 12 '16 at 22:20
It's not clear how to restore the texture still. Do I provide my populated "data" variable (from my sample code above) for the `glTexSubImage2D` "pixels" argument? Also note, according to Apple's [docs](https://developer.apple.com/library/ios/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/WorkingwithEAGLContexts/WorkingwithEAGLContexts.html), the `glTexImage2D` and `glFramebufferTexture2D` calls are required *in addition* to generating a `GL_FRAMEBUFFER`, so that part of my code might be right. – Jeshua Lacock Sep 13 '16 at 04:46
@Reto Koradi can you please see edit 3 above? I can get it to draw restored pixels or fresh rendered pixels, but not both together. I'm really stumped! – Jeshua Lacock Sep 14 '16 at 23:58

How to restore a GL_RENDERBUFFER?

1 Answers1