Draw OpenGL renderbuffer to screen

Question

I created a Renderbuffer, that's then modified in OpenCL.

//OpenGL
glGenFramebuffers(1, &frameBuffer);

glBindFramebuffer(GL_FRAMEBUFFER, frameBuffer);

glGenRenderbuffers(1, &colorRenderbuffer);

glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);

glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8, 600, 600);

glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorRenderbuffer);
//OpenCL
renderEngine = new OpenCLProgram("render.cl");
renderEngine->addArgumentGLRBuffer(colorRenderbuffer);

How would I then proceed drawing my OpenCL creation, the buffer to the screen? I could bind it to a texture and draw a quad the size of my window, but I am not that sure, if it is the most efficient way. Also, if there was a better way of drawing to the screen from OpenCL, that would help!

Answer 1

The call you're looking for is glBlitFramebuffer(). To use this, you bind your FBO as the read framebuffer, and the default framebuffer as the draw framebuffer:

glBindFramebuffer(GL_READ_FRAMEBUFFER, srcFbo);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
glBlitFramebuffer(0, 0, srcWidth, srcHeight, 0, 0, dstWidth, dstHeight,
                  GL_COLOR_BUFFER_BIT, GL_NEAREST);

Adjust the parameters for your specific use based on the linked man page.

This is preferable over writing your own shader and rendering a screen sized quad. Not only is it simpler, and requires fewer state changes, it can also be more efficient. Knowing that a blit operation needs to be performed gives the implementation a chance to use a more efficient path. For example, where present, it could use a dedicated blit engine that can run asynchronously to the general rendering functionality of the GPU.

Whether you should use a renderbuffer or texture is not as clear cut. Chances are that it won't make much of a difference. Still, I would recommend to use a renderbuffer as long as that's all you need. Because it has more limited functionality, the driver has the option to create a memory allocation that is more optimized for the purpose. Rendering to a renderbuffer can potentially be more efficient than rendering to a texture on some hardware, particularly if your rendering is pixel output limited.

Answer 2

Don't make it a renderbuffer.

OpenGL renderbuffers exist for the sole purpose of being render targets. The only OpenGL operations that read from them are per-sample operations during rendering to the framebuffer, framebuffer blits, and pixel transfer operations.

Use a texture instead. There is no reason you couldn't create a 600x600 GL_RGBA8 2D texture.