How to draw a 3d rendered Image (perspective proj) back to another viewport with orthogonal proj. simultaniously using multiple Viewports and OpenGL [closed]

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My problem is that i want to take a kind of snapshot of a 3d scene manipulate that snapshot and draw it back to another viewport of the scene,

I just read the image using the glReadPixel method. Now I want to draw back that image to a specified viewport but with the usage of modern OpenGL.

I read about FrameBufferObject (fbo) and PixelBufferObject (pbo) and the solution to write back the FrameBufferObject contents into a gl2DTexture and pass it to the FragementShader as simple texture.

Is this way correct or can anyone provide a simple example of how to render the image back to the scene using modern OpenGL and not the deprecated glDrawPixel method?

Answer 1

The overall process you want to do will look something like this

1. Create an FBO with a color and depth attachment. Bind it.

2. Render your scene

3. Copy the contents out of its color attachment to client memory to do the operations you want on it.*

4. Copy the image back into an OpenGL texture (may as well keep the same one).

5. Bind the default framebuffer (0)

6. Render a full screen quad using your image as a texture map. (Possibly using a different shader or switching shader functionality).

Possible questions you may have:

• Do I have to render a full screen quad? Yup. You can't bypass the vertex shader. So somewhere just go make four vertices with texture coordinates in a VBO, yada yada.

• My vertex shader deals with projecting things, how do I deal with that quad? You can create a subroutine that toggles how you deal with vertices in your vertex shader. One can be for regular 3D rendering (ie transforming from model space into world/view/screen space) and one can just be a pass through that sends along your vertices unmodified. You'll just want your vertices at the four corners of the square on (-1,-1) to (1,1). Send those along to your fragment shader and it'll do what you want. You can optionally just set all your matrices to identity if you don't feel like using subroutines.

*If you can find a way do your texture operations in a shader, I'd highly recommend it. GPUs are quite literally built for this.