I got portals rendering on quads and that's about as far as I think I got: It had trouble rendering multiple portals at once and recursive portals, so it was basically with the same limitations as basejka misc_portal entities. The engine source code wasn't out at this point but in the Q3 source code I believe there's a single if statement that limits how many portals can be rendered at once to 1 (which also prevents recursion). This rendering experiment wasn't done with the portalmod source code, so there was no moving through portals here. Also I wasn't sure how I'd fix issues like this, where the result of one portal renders through another: In the 21st century, you'd probably do this with stencil buffers to limit the rendering of each portal to within their "viewport". Maybe this already happens and there needs to be multiple "levels" of stencil to handle distinct portals.