For those who have played Quake extensively, adding portals seems unnecessary, as teleporters are already a core part of the game mechanics. What [Matthew Earl] accomplishes is more of the Portal style of portal by rendering what is on the other side of the portal with a seamless teleportation transition.
Of course, Quake is an old game with a software renderer. Just throwing another camera into the scene, rendering to another texture, and then mapping that texture to the scene isn’t an option. Quake uses an edge rasterizer and generates spans along scanlines that track where edges intersect the current scanline. Rather than making expensive per-pixel comparisons, [Matt] stashes the portal spans and renders them in a second render, so even with multiple portals, only a single screen’s worth of pixels are rendered.
However, this technique has no near clipping plane, which means objects can appear in the portal that don’t make any sense as they are in front of the portal’s viewpoint. Luckily, Quake has an ingenious method for polygon occlusion: the BSP. While [Matt] is manually checking polygons, the BSP is the perfect tool for bisecting a room along a plane. It’s an incredible hack, and we’re excited to see Quake expand into a puzzle game. [Matt] dives into greater detail on how the software renderer works in another video that’s well worth a watch.
Perhaps the most incredible aspect of this technique is that it could run on original hardware. If you want to bring a little more Quake to life, why not get the Quake light flicker in your house? Video after the break.