It really doesn't. I would argue that convex sectors with portals could be quicker than casting rays across a grid for every pixel column, except for some degenerate cases. This is because for any pixel column in-between the boundaries of wall segments, there's virtually no work to be done (O(1)), compared to the work of casting a ray along the grid O(n).
Btw, 286 played wolfenstein rather poorly. The 386 is rather more appropriate.
Wolfenstein was built in a couple of months by a then 21-year-old Carmack. He didn't focus on optimizing levels until Doom.
Yes, I played it at the time on both a 286 and a 386. You might be right, although you're describing an algorithm more like what Duke3d used rather than raycasting in that case. I was talking purely about raycasting. So it sounds like a misunderstanding on my part.
It's true that Carmack has said several times, including in the readme to the source release of Wolf3d [0], that a BSP-based renderer might be faster than raycasting. He used a BSP tree based renderer for the SNES port of Wolf3d because the CPU was even slower, so I suppose that answers it! There's also a note in the GEBB page 164 from Carmack where he says it was slower than "looping through a few long wall segments". [1]
Early alpha versions of DOOM used a sector-based rendering technique, maybe like what you're describing, which ultimately was too slow [2]][3]. Then Carmack switched to BSP trees after doing the Wolf3d SNES port.
I think it would be pretty interesting to implement the same game with both a raycasting approach and a BSP or Build-style portal/sector approach and compare performance on a 386. DOOM ran terribly on a 386 but it did a lot more than Wolf3d, so it's not a great comparison. Catacomb 3D didn't use raycasting (it used a wall span rendering techniqe) and ran better than Wolf3d on similar hardware, but it had a bunch of glitches. But Carmack says they were due to lack of experience rather than the technique itself.
Anyway thanks for challenging my assumptions. This'll go on my todo list!
Thank you for all the interesting references. I had almost forgotten about the BSP-SNES port.
I think they main concern when doing sectors + portals for a wolf3d-type 3d engine is how much word is done when 'stepping into' portals. basically it requires changing the clipping range [x0, x] of the portal, and quickly finding the next visible wall segment. the clipping could actually be done in 'angle space', not even using vectors. That is, wall edges/"vertices" should be represented as [angle,distance]. I have also been thinking about how much of that math pipeline could be done using specialized 8-bit values and LUTs.
I have been thinking about trying to get something running on very slow hardware, although at some point the problem is more drawing on the screen quickly, not the renderer.
Btw, 286 played wolfenstein rather poorly. The 386 is rather more appropriate.
Wolfenstein was built in a couple of months by a then 21-year-old Carmack. He didn't focus on optimizing levels until Doom.