Here's a loooong beta version of a glide tutorial I wrote in case someone new to the game wants to figure out how this stuff works without wading through billion doomworld posts in random threads. It's probably more confusing than helpful though so read at your own risk. Would probably be more helpful with video tutorials but oh well. Chances are there's also tons of errors and typos.
Normally in doom, you can not go through a 32 unit wide gap. However, as doomguy is exactly 32x32 map units in size, with perfect positioning, luck, tons of practice and some arcane voodoo magics it's possible to squeeze through a gap of that size. That is called a "glide". Often very useful in speedrunning because it lets you skip large parts of levels. Glides work very differently depending on what sourceport and compatibility level you use. I remember speding over an hour wondering why I couldn't do doom2 map16 glide on prboom-plus then realising I had it on some weird complevel. I'd imagine especially the sourceports with freelook and whatnot will be very different from vanilla compatible stuff. I, and I guess most people that have put work into glides, only really have experience with glides in vanilla compatible sourceports (doom.exe, doom2.exe, chocolate-doom, prboom-plus with complevel 2/3/4). Since glides are mostly a speedrunning thing, they are usually done while using the built in demo recording (-record demoname). Doing glides with -record makes a HUGE difference because it basically reduces the amount of directions you can face, making finding the correct direction easier (or even possible). http://doomwiki.org/wiki/Turning_re...recording_demos for a better explanation. While there are demo formats which remove this problem/feature, I don't think I've ever seen a vanilla demo without the reduced turning resolution. Demos recorded for .wads that can't even be played on the vanilla .exes usually use this reduced turning resolution! For some reason "longtic" demos never caught on. Some ports have command line options for using the lower turning resolution without having to record a demo, like prboom-plus with -shorttics.
What seems to makes glides happen is getting in the correct fraction of a map unit. Checking coordinates with idmypos it seems like whenever I get inside the gap either x or y coordinate is 0x****0000, depending on which direction the glide is in. Obviously you can't visually judge if you're in the correct position or not. Best you can do is figure out when you're fairly close (less than one whole map unit from the gap close). Then you need to find a way of very slowly inching yourself towards the gap, which is probably best done with mouse movement - either playing with vertical mouse movement on (better for glides, at least on vanilla/chocolate doom) or using strafing with the mouse (much less consistent on vanilla/chocolate).
Difference between mouse strafing and vertical movement is that vertical movement can give you the smallest movement tic there is (GF1 if you check on a replay editor) while smallest movement tic mouse strafing can give is SR2 / SL2 (if you don't know what these mean, bigger number kinda makes you go faster... but doom movement code is very weird and there's a lot more to it than that). The bigger movement tics can lead to getting stuck one fraction of a map unit from the glide spot which will require repositioning, but more about that later. Now, with prboom-plus it's possible to get SR1 and SL1 tics with mouse strafing which is a massive advantage for anyone playing without vertical mouse move. It is still "vanilla compatible" in the sense that it will play back on original .exe, but as far as I know getting SR1/SL1 input with original .exes or chocolate doom is impossible.
For doing any of this stuff you'll want to have autorun off. In some ports it's a toggle and that's probably nice and all, but vanilla doesn't have that luxury. There having autorun off means you'll have to almost constantly hold down run. With run on, mouse strafing will always give SR50/SL50 tics which makes these tricks impossible. Not running also seems to increase the probability of getting "stuck" when positioning for east-west glides. tl;dr walking is good running is bad
Easiest category of glides is the guided glides, where you can use a wall for positioning yourself in front of the gap. For practice I'd recommend using vertical movement, even if you're not going to use it in a real run it should make learning how this works much easier. Doom2 map16 has one of the better known ones. Loopers 30uv run has a good example of this that isn't terribly fast, he does it with strafing though. Looper and xepop actually used a weird way of strafing towards the wall with keyboard and away from the wall with mouse at the same time but that's just weird, I'll let them explain that crap themselves if they show up in this thread. Individual level compet-n records are usually crazy flukes where the glides are done almost instantly and it's hard to even tell what happened in them. Just run against the "corner" of the black wall and the red skull pillar. Then facing directly east (this is where that reduced turning accuracy comes in handy) just move forward with mouse very slightly. For strafing you'll obviously want to face north or south, otherwise it's the same, just with the possible difficulty of not having as precise movement as with vertical movement. Note that you need to do this glide from east to west, gliding from the other side of the pillar from south to north is much more difficult. Why? Who knows. Doing glides, even guided ones, from south to north is obscenely difficult for some reason.
Sort of second easiest category is east to west guideless glide, as popularised by I guess cack_handed. In these positioning is slightly trickier as you can't use a wall to get close the gap, you need to use the gap itself. His pn08-019 demo ( doomedsda.us/lmps/946/3/pn08-019.zip ) has a text file explaining his method. Personally I do these by facing west, going to the right side of the gap, and then sloooowly moving left towards the gap by walking against the wall. Normal keyboard walk is fine, no need to mess with mouse movement yet. With luck, you'll end up stopping right at the gap (or with even more luck getting instantly through it, that's fairly rare though) - not running seems to increase chances of getting "stuck" like this. Then just push through with mouse movement. This is where novert makes a huge difference because it's possible to get stuck one fraction of a map unit from the glide spot (at 0x****0001 or something) when trying to get through with strafing, unless you're using prboom which changes the way mouse strafing works (even on complevel 2/3/4). If you get stuck like that reposition and try again. Vertical movement will never get you stuck like that which is why it makes learning this stuff easier, no need to worry about the completely random factor in strafe glides.
Last and maybe not least is the north to south guideless glide mostly done by yours truly I guess. The positioning for these is fairly similar to east-west ones, but you will almost never get stuck at the gap when running against the wall like in east-west situation. Visually the correct glide spot looks the "same" as east-west, there will be one column of pixels on the left side of the screen from the wall on the left side of the gap showing (assuming vanilla FOV, things will look weird if you play on some widescreen resolution with prboom). However you can't judge this only visually, there's a timing element in knowing when to stop after seeing that column of pixels show up. Then by walking back and forth using keyboard with a certain rhythm you can glide through the gap sooner or later... Note that this method is doable even with keyboard only play, mouse isn't used at all. I'm not 100% on how this even works but I believe there's timing in both knowing when to stop when positioning (so you're close to the glide spot) and to the rhythm in doing the back/forward movement. It's a very weird trick that took a lot of practise for me to figure out and I still can't pull it off that consistently.
Guideless glides in other directions just don't seem to happen manually. Same thing with guideless glides with normal turning. I've done at least east-west and west-east with longtics so it's possible, but it wasn't very fast. I haven't really put much thought into that though since demos are shorttics anyway. Some TASes also do west-east guideless glides, but I can't think of any TAS that did a south-north guideless glide. Aleksander Nikulin's Hell Revealed TAS had at least one guided south-north glide though. After you first figure out guided glides which, assuming they work at all non TAS, work the same for every direction, good place for getting used to how guideless glides in different directions work is plutonia map19 exit room as you can glide to the exit teleporter from every direction.