OpenGL getting perspective pixel data

I am working with a 3D rendering of a large field containing various objects. My program views this field in perspective from a high point.

I need to access the pixel data of (only) a rectangle in the field as viewed from above. I have the coordinates of this rectangle in the field, and would开发者_开发技巧 like to: (a) Find the pixels corresponding to my desired rectangle. (b) (Ideally) write the corresponding pixel matrix to a file.

Does anyone know a simple way of doing this?

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You could use something like gluPerspective to set your viewpoint to the desired rectangle, then render the scene, and use glReadPixels to get the result back. That will give you a rectangle of pixel values -- it'll be up to you to convert them to an image format of your choice.

If you just want a single, static picture that's probably the simplest way to go. If you need/want to do it more often (e.g., you really want something that looks like a video), you could consider setting it up to render to a texture or a frame buffer object. This will (at least usually) improve efficiency at the expense of extra complexity.