Clipping

[Livo]

In most games, when you clip out of a level, you go into a black space. However, in Doom, when you clip out of the level, all you see is a repeating wall blurring (sort of blurred anyway).

Why is this?

[myk]

HOM!!

[Quasar]

This is because DOOM doesn't rasterize anything to the screen by default. When you're operating in an OpenGL or Direct3D environment, there's generally a background color, which can be set to anything. In Quake II, this was a pukey orange color for some unknown reason. The default is generally black.

DOOM, however, not being a true 3D renderer, draws everything to the screen as columns. If there are no source columns mapping to some area on the screen, then nothing is drawn there, and what you get is whatever was there before. Hence the blurring effect of HOM.

If you've ever noticed, in the original DOOM.EXE, HOM was a bit different because it shimmered, as opposed to being a static blur like it is in most ports now. This is an effect of mode x page flipping, and slight discrepancies between the screen and the back buffer. When the pages are flipped, the HOM shimmers slightly, giving it a sort of magical feel -- heheheh

[Bloodshedder]

Interesting. I always wondered why HOM looked different in the original engine. On a related subject, how does the Doom engine handle "wraparound" at ±32768 units?

[Linguica]

Bloodshedder said:

Interesting. I always wondered why HOM looked different in the original engine. On a related subject, how does the Doom engine handle "wraparound" at ±32768 units?

http://groups.google.com/groups?dq=&hl=en&lr=&ie=UTF-8&safe=off&th=de251dad895007f1&seekm=3ddf09d4.1592763%40news.earthlink.net

[Use]

Quasar said:

DOOM, however, not being a true 3D renderer, draws everything to the screen as columns. If there are no source columns mapping to some area on the screen, then nothing is drawn there, and what you get is whatever was there before. Hence the blurring effect of HOM.

If you've ever noticed, in the original DOOM.EXE, HOM was a bit different because it shimmered, as opposed to being a static blur like it is in most ports now. This is an effect of mode x page flipping, and slight discrepancies between the screen and the back buffer. When the pages are flipped, the HOM shimmers slightly, giving it a sort of magical feel -- heheheh

The first part is old news, but i knew nothing of that mode x page flipping thingy. I noticed a difference in HOM with Hexen and Doom back in the DOS days. hexen does the static images repeating, but Doom flickers, neat.