Recent discussion about the perils of doors in gamedev reminded me of a bug caused by a door in a game you may have heard of called "Half Life 2". Are you sitting comfortably? Then I shall begin.

Once upon a time, I worked at Valve on virtual reality. This was in 2013 around the time the Oculus DK1 emerged, and Joe Ludwig and I decided that the best way to figure out how VR would work in a real game context was to port a real game to it and see what happened.

We picked Team Fortress 2 as the game - the reason why is a whole different story I won't go into here. TF2 used the Source 1 engine, and as it happened two Valve games also using that same version of the engine were Half Life 2 and Portal 1. So as a side-effect they also got to work in VR.

Well, Portal 1 "worked" - but all the tricks with perspective when you went through a portal were of course a nauseating disaster - it was pretty unplayable.

But HL2 did actually work pretty well. Joe spent a lot of time making the boat sequences work in a reasonable way.

There's a sequence of stacking boxes near the start that is somewhat infuriating in the original - the stack keeps falling over - but in VR it's really easy to place them well.

Also, whacking the manhacks with your crowbar goes from being a panicked flailing in flatscreen, to being an elegant one-swing home-run hit in VR.

Luckily, there was some other excuse to reissue HL2 anyway (see https://developer.valvesoftware.com/wiki/Source_2013) and the VR version worked pretty well, so we put the VR support on a command-line, labelled it "beta", rebuilt the whole of HL2 and prepared to ship it.

Of course we've played a bunch of HL2 by this point, testing all the VR stuff works. But we just skipped to the relevant chapters - we never actually played through from the start. And I hadn't played it through in a while, so I thought I'd do that in VR, start to finish. If I discovered anything that still didn't work, I could at least document it in the release notes.

So I started it up, selected new game, played the intro section. It's a fairly well-known section - you arrive at the train station with a message from Breen, a guard makes you pick up a can, and then you have to go into a room and... uh... I got stuck. I wasn't dead, I just couldn't go anywhere. I was stuck in a corridor with a guard, and nowhere to go. Bizarre.

What is meant to happen is a guard (spoiler alert - it's actually Barney in disguise) bangs on a door, the door opens, he says "get in", and then the game waits for you to enter the room before the script proceeds.

But in this case the door sort of rattled, but didn't open, and then locked shut again. So you can't get in the room, and the gate closed behind you, so you can't go do anything else. The guard waits forever, pointing at the locked door, and you're stuck.

I checked a video online, wondering if my memory was faulty - nope, the door's meant to open automatically, and you walk in. https://www.youtube.com/watch?v=y_3vMUOayyc&t=215s (at 3:40). But... now it doesn't!

Oh dear. We can't ship this. I get some other folks, including some folks who worked on HL2 originally, and yep - it's broken. And it's broken when you're not in VR either - so it's not something Joe and I broke. But nobody knows why - none of the relevant code has changed.

Someone even goes back in the source history and compiles the original game as it shipped - nope, that original version is also broken. How can this possibly be? At this point people are freaking out - this isn't a normal bug - it appears to have traveled backwards in time and infected the original!

After about a day remembering how to use the debugging and replay tools, someone smart (sorry, I don't remember who) figured out what was going wrong.

If you watch the video, when the door unlocks and then opens, there's a second guard standing inside the room to the left of the opening door. That guard is actually standing very slightly too close - the very corner of his bounding box intersects the door's path as it opens. So what's happening is the door starts to open, slightly nudges into the guard's toe, bounces back, closes, and then automatically locks. And because there's no script to deal with this and re-open the door, you're stuck.

Once we'd figured this out, the fix was simple - move the guard back about a millimeter. Easy. But it took a lot of work to find because people had to dust off old memories of how the debugging tools worked, etc.

OK cool now we can ship the game phew. But why did this EVER work? The guard's toe was in the way in the original version as well. As I say, we went back in time and compiled the original as-shipped source code - and the bug happened there as well. It's always been there. Why didn't the door slam closed again? How did this ever ship in the first place?

So this kicked off an even longer bug-hunt. The answer was (as with so many of my stories) good old floating point. Half Life 2 was originally shipped in 2004, and although the SSE instruction set existed, it wasn't yet ubiquitous, so most of HL2 was compiled to use the older 8087 or x87 maths instruction set. That has a wacky grab-bag of precisions - some things are 32-bit, some are 64-bit, some are 80-bit, and exactly which precision you get in which bits of code is somewhat arcane.

But ten years later in 2013, SSE had been standard in all x86 CPUs for a while - the OS depended on it being there, so you could rely on it. So of course by default the compilers use it - in fact you have to go out of your way to make them emit the old (slightly slower) x87 code. SSE uses a much more well-defined precision of either 32 or 64 bit according to what the code asks for - it's much more predictable.

So problem solved, right? 80 bits of precision means the collision didn't happen, but in 32 bits of precision it does, and that's your problem, more bits better, QED, right? Well not quite.

The guard's toe overlaps in both cases - a few millimeters is still significantly larger than ANY of the possible precisions. In both the SSE and x87 versions, the door hits the guard's toe. So far, both agree.

This collision is actually properly modelled - a big innovation of HL2 was the extensive use of a real physics engine. The door and the guard are both physical objects, both have momentum, they impart an impulse on each other, and although the door hinge is frictionless, the guard's boots have some amount of friction with the floor.

On both versions, the door has just enough momentum to rotate the guard very slightly. The guard's friction on the floor is not quite enough to oppose this, and he rotates a tiny fraction of a degree. On the x87 version, this tiny rotation is enough to move his toe out of the way, the collision is resolved, and the door continues to swing open. All is well.

But on the SSE version, a whole bunch of tiny precisions are very slightly different, and a combination of the friction on the floor and the mass of the objects means the guard still rotates from the collision, but now he rotates very slightly less far.

So on the next frame of simulation, his toe is still in the way of the door. The door isn't allowed to just pass through his toe, so it does the only other option - it bounces back. I think by default it's set to do so completely elastically, so the door bounces back with exactly the speed it came in at, slams shut, and locks again. And you're stuck.

And that's why the bug went "back in time" - because yes it's the old code, but we were using a newer compiler with new default settings. In the original build, the compiler defaulted to x87, but in the newer compilers the default is SSE. It's not that one is "better" - the fundamental bug is that the guard was too close to the door, and that had always been there. But in the original the problem "self corrected" and so was never spotted, whereas in the newer compile it became a showstopper.