Viewports

Why would you want a non-rectangular viewport - that would require an entirely different 3D->2D mapping from that which the graphics card uses. Besides, your monitor is rectangular and therefore doesn''t directly "support" non-rectangular viewports.

You can hack it though: blend on oval (or circular) region over the screen to only leave a non-rectangular area visible.

Otherwise, the technique you''re looking for is probably the fisheye lens effect (linear transformation onto a rectangular surface such as the screen is also a lens-derived effect). Since the graphics card doesn''t support fisheye-mapping directly, you might want to use some extensions or grab the frame buffer and tessellate it with modified vertex coordinates.

That is, if I understood your question correctly.

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glViewport only use width and height to describe the shape of the viewport, so it can only be a rectangle. But you can simulate any viewport shape using the stencil buffer. And using the destination alpha channel, you can not only get an arbitrary shape, but also an arbitrary degree of "visibility" (like a blurry edge you see in the movies when someone looks in a pair of binoculars).