Doom 2 GBA's quirks: MAP30 speedrun strat

[Searinox]

Hey all.

 

As the topic of this thread indicates, I'd like to showcase a strat for beating MAP30 quickly in Doom 2 GBA. However, more than this, I am also hoping for a greater strides within the community towards GBA-specific speedrunning strats. I've decided to make this video because what I present to you here - something I'd known since first playing the GBA port way back in the day - does not appear to be used in any of the speedruns I've seen.

 

So anyways here it is:

 

I've added a description in the video as well as a pinned comment with more observations. Basically for some reason autoaim considers the spawn cubes a target and attempts to align the shot with them and this can allow for very quickly sending a BFG projectile directly into the skull.

 

As I've said before however, I'm hoping there will be greater discussion around this port's quirks among speedrunners. Granted, I know Doom 2 GBA is something very few have speedrun. So anyway, Doom 2 GBA and Duke Nukem Advance both use a proprietary engine and share some quirks, one of those I've actually seen discussed is the ability to walk perpendicularly into a wall that is diagonal relative to the map's cardinal directions in order to slowly advance and eventually fully clip out of bounds. But these are just mine and a few others' observations. If there's any serious digging done, I'm sure there is more to uncover.

 

Part of the fun of watching speedruns is seeing shit get broken and with this post, and video, I hope to advance this cause of bug hunting and also maybe if there are any particularities you too have observed, consider discussing here because there just isn't a whole lot of documentation on Doom 2 GBA exploits and maybe not everyone is aware of all that is possible. Doom 2 on GBA may not be as popular but maybe if it has unique and interesting enough exclusive glitches, more interest will arise. Peace all!

Edited April 23, 2022 by Searinox

[Roebloz]

As a fan of Doom 2 GBA, this is really cool to see. Is this specific angle needed, or can it work with any spawn cube?

[Searinox]

9 hours ago, Roebloz said:

As a fan of Doom 2 GBA, this is really cool to see. Is this specific angle needed, or can it work with any spawn cube?

 

All spawn cubes are targeted, and the BFG even hits them with a tracer, producing the green flash of a hit, though it does not appear to do any damage to the cubes or make them destructible. The cubes being fired to the top floor are the ones that will most easily align your shot with the skull opening, though it doesn't matter which floor they're being shot at since at the very beginning of their movement they are very close to the hole. I've not been able to achieve proper alignment from any except the top floor however.

 

A BFG projectile spawn is a precise action since it takes some time for the projectile to spawn after firing, and also before it can be fired again. But it's much easier to just aim for the skull and spam rockets hoping they will sync with the cube spawn since the firing rate is much faster. Of course, this takes a few rockets as opposed to only one BFG shot.

 

The implementation of the cube spawner is different from vanilla Doom 2. The enemies spawn in with a teleport effect instead of Arch-vile flames and cannot telefrag the player. When an enemy and the player overlap in the GBA port, instead of them being stuck in eachother, they are able to freely move until they no longer overlap, so I guess the telefrag was not needed. They still telefrag other enemies however and the player remains able to telefrag enemies throughout all levels. No idea about player vs player telefrag with link match however.

 

It is possible to get a sync with the very 2nd cube the Icon ever spawns if you're fast enough, which would be the lowest possible time to align a shot into the skull. In theory this would mean that there is a clear hard limit to what time can be achieved since from level start you hold up and beeline to the teleporter, always triggering the Icon - and thus its 2nd cube spawn - at exactly the same time, and aligning the projectile with the very first frame of the 2nd cube spawn. In practice this will differ between single-level pistol start record and full run since if you pistol start, you have to strafe a little to the left in order to grab the BFG and I don't know how much that changes possibilities. It's also possible to try and fire from closer up in order to reduce the projectile travel distance, getting a faster time.

 

I was able to get the time as low as 19 seconds in the past. However, I don't intend to join speedrunning. I recorded this off a build of VBA-M which I'm sure would not qualify it for the leaderboards anyway, and as much as I like watching them, performing speedruns just isn't my thing. So I suspect 19 or 18 seconds may be a hard limit on how low the time can go given my observations. But well, let's see that happen.

 

On the topic of GBA Doom 2 glitches, I don't know if it's been mentioned but whenever you start a new level your armor isn't applied or taken into account at all, until you pick up an armor upgrade. Yet, your total is taken into consideration when you run into one, so if you started the level with 100 armor or more you can't pick up a green armor for instance. Since it also doesn't drop if you get hit, if you're starting the level at 200 armor, your only way to get it unstuck and working again is to pick up a Megasphere or armor bonus.

 

New update:

On MAP29 I have found out that it is possible to move precisely into the right side of the square-shaped stub that blocks you from accessing the platform that raises the big bridge where you are ambushed by Cacodemons in a way that clips you on top of it, something not possible in vanilla Doom 2. This allows for skipping the part where you go back to fight some Hell Knights and activate the switch that lowers it. Clearly there is more physics jank to exploit in this game than just the Icon and wall clipping.

Edited April 24, 2022 by Searinox

[Roebloz]

3 hours ago, Searinox said:

 

All spawn cubes are targeted, and the BFG even hits them with a tracer, producing the green flash of a hit, though it does not appear to do any damage to the cubes or make them destructible. The cubes being fired to the top floor are the ones that will most easily align your shot with the skull opening, though it doesn't matter which floor they're being shot at since at the very beginning of their movement they are very close to the hole. I've not been able to achieve proper alignment from any except the top floor however.

 

A BFG projectile spawn is a precise action since it takes some time for the projectile to spawn after firing, and also before it can be fired again. But it's much easier to just aim for the skull and spam rockets hoping they will sync with the cube spawn since the firing rate is much faster. Of course, this takes a few rockets as opposed to only one BFG shot.

 

The implementation of the cube spawner is different from vanilla Doom 2. The enemies spawn in with a teleport effect instead of Arch-vile flames and cannot telefrag the player. When an enemy and the player overlap in the GBA port, instead of them being stuck in eachother, they are able to freely move until they no longer overlap, so I guess the telefrag was not needed. They still telefrag other enemies however and the player remains able to telefrag enemies throughout all levels. No idea about player vs player telefrag with link match however.

 

It is possible to get a sync with the very 2nd cube the Icon ever spawns if you're fast enough, which would be the lowest possible time to align a shot into the skull. In theory this would mean that there is a clear hard limit to what time can be achieved since from level start you hold up and beeline to the teleporter, always triggering the Icon - and thus its 2nd cube spawn - at exactly the same time, and aligning the projectile with the very first frame of the 2nd cube spawn. In practice this will differ between single-level pistol start record and full run since if you pistol start, you have to strafe a little to the left in order to grab the BFG and I don't know how much that changes possibilities. It's also possible to try and fire from closer up in order to reduce the projectile travel distance, getting a faster time.

 

I was able to get the time as low as 19 seconds in the past. However, I don't intend to join speedrunning. I recorded this off a build of VBA-M which I'm sure would not qualify it for the leaderboards anyway, and as much as I like watching them, performing speedruns just isn't my thing. So I suspect 19 or 18 seconds may be a hard limit on how low the time can go given my observations. But well, let's see that happen.

 

On the topic of GBA Doom 2 glitches, I don't know if it's been mentioned but whenever you start a new level your armor isn't applied or taken into account at all, until you pick up an armor upgrade. Yet, your total is taken into consideration when you run into one, so if you started the level with 100 armor or more you can't pick up a green armor for instance. Since it also doesn't drop if you get hit, if you're starting the level at 200 armor, your only way to get it unstuck and working again is to pick up a Megasphere or armor bonus.

 

New update:

On MAP29 I have found out that it is possible to move precisely into the right side of the square-shaped stub that blocks you from accessing the platform that raises the big bridge where you are ambushed by Cacodemons in a way that clips you on top of it, something not possible in vanilla Doom 2. This allows for skipping the part where you go back to fight some Hell Knights and activate the switch that lowers it. Clearly there is more physics jank to exploit in this game than just the Icon and wall clipping.

Oh trust me, EVERYONE that plays Doom 2 GBA knows about the armor bug. It's honestly the only big thing holding this port back. (Aside from the messed up palettes and the like) I'm not even sure there is a leaderboard for single-level Doom 2 GBA speedruns?

[Searinox]

I don't think there is one, no.