The way I do this in OpenGL is pre-draw a screen full of circles, starting with the biggest diameter and working down, changing the color value by 1 for each circle, and only drawing 256 circles in total. I save this as an alpha pixmap and upload it as a GL_ALPHA texture. Then I switch off drawing to alpha, but draw a textured quad over the screen with the alpha texture map, set up so that bits greater than a given value are drawn to the stencil buffer. Then I draw the `game screen`, taking the stencil into account so that only pixels not stencilled are drawn. This way you can do per-pixel wipes (hint: distort the image containing the circles).
Circle in & circle out effect
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