Drop chances in Super Metroid

Gromba:

registered on 2008-09-27 03:35:07 am.

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Gromba

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Gromba: 2009-04-22 08:25:52 am

2009-04-22 06:35:07 am

Hey,

I've got a question about the drops in Super Metroid when TASsing. Since in a old post of moozooh about a Top-10 (ish) he puts a 6 super missile drop ([url=http://www.youtube.com/watch?v=2ewgBGXFR8A#t=2m25]seen here[/url]) on place 1 and Kriole recently posted a [url=http://www.youtube.com/watch?v=b3PYzxoMvEE]video[/url] about a 0.08% chance 'luck' video on youtube... it seems like a real issue.
How does the game choose the drops and why isn't this 'cheatable'?

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Cpadolf:

registered on 2007-02-14 03:32:46 am.

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Location: Sweden

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Cpadolf

2009-04-22 06:54:16 am

Hurr Durr

The reason that you can't cheat it is that the game decides what to drop based on an RNG that only updates every new frame (as far as I know), so the only way you can manipulate is by waiting frames, and for things that have a really low chance of happening the waiting can become far too long.

It seems that there is some more to it though. As some drops also changes depending on where you stand when the drop appears, and if you enter a room 1 frame later (or add a frame of lag to the door), sometimes the luck seems to change completely, sometimes it seems to just act as if you delayed a frame normally and sometimes the luck seems dependant only for the time you have spent in that specific room.

Kejardon:

registered on 2004-12-07 07:21:36 pm.

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Kejardon

2009-04-22 11:44:32 am

Embarrasing Fact: Power suit made by lowest bidder

The RNG has a couple quirks to it.

*Every* processed frame, the game will run the current seed through the RNG algorithm. Many random events will also run the RNG through another algorithm, but these are relatively uncommon. The most common event is item drops, then behind that would be certain enemy actions (the Gold Pirates come to mind, I forget what it's for but they have a 1/4 chance of doing something periodically), and aside from that.... I can't even really think of anything. A few room events like the falling panels in Ceres, a couple enemy death explosions... I'm almost certain nothing Samus does can directly affect the RNG though. CPAdolf, can you give an example of where changing where Samus stands affects the RNG?

Every normal game-frame, I think it is (I'm actually fairly unsure of when to be honest), it will XBA (Swap the high and low bytes of) the RNG seed. This doesn't happen going through doors though.

Because it has two *different* steps, if the game suddenly runs one of the steps multiple times, all following calculations are going to end up different. So even if you know the current seed, you can't reliably predict the following values of the seed.

The RNG has a couple quirks to it.

*Every* processed frame, the game will run the current seed through the RNG algorithm. Many random events will also run the RNG through another algorithm, but these are relatively uncommon. The most common event is item drops, then behind that would be certain enemy actions (the Gold Pirates come to mind, I forget what it's for but they have a 1/4 chance of doing something periodically), and aside from that.... I can't even really think of anything. A few room events like the falling panels in Ceres, a couple enemy death explosions... I'm almost certain nothing Samus does can directly affect the RNG though. CPAdolf, can you give an example of where changing where Samus stands affects the RNG?

Every normal game-frame, I think it is (I'm actually fairly unsure of when to be honest), it will XBA (Swap the high and low bytes of) the RNG seed. This doesn't happen going through doors though.

Because it has two *different* steps, if the game suddenly runs one of the steps multiple times, all following calculations are going to end up different. So even if you know the current seed, you can't reliably predict the following values of the seed.

Cpadolf:

registered on 2007-02-14 03:32:46 am.

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Cpadolf

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Cpadolf: 2009-04-22 04:44:33 pm

2009-04-22 12:28:28 pm

Hurr Durr

I picked two cases of the top of my head and recorded examples. In both comparisons the two different movies start out from the same savestate and the enemies die at the same frame. At first I thought that it was something exclusive to enemies who will act differently depending on where you stand (as I had only encountered it with the fireflies in the second example, as well as some hoppers, though not all (I think)). But during the last TAS I made I also noticed the same thing with the crawlers (or at least one of them) in the first Pink Brinstar room.

Test 1 V1
Test 1 V2
Test 2 V1
Test 2 V2

Kejardon:

registered on 2004-12-07 07:21:36 pm.

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Kejardon

2009-04-23 02:29:36 am

Embarrasing Fact: Power suit made by lowest bidder

For Test 1, it's the Reo's... each one on screen (well, each one PROCESSING) will run step 1 of the RNG every frame, whether they need a random number or not. So do the Holtz, Gerutas, and Squeewpts (The 2 Reo-act-alikes, and the lava enemy that jumps up to a random height in the room before Norfair's bubble room). For Test 2 it's the Mellas, with something similar.

Other misc. info I've found while poking at enemy data just now:
When you enter a room with volcanic rocks (D1FF), they set the RNG to 11. Similar thing with all sidehoppers/desgeegas.
Kagos and Beetoms also set the RNG to a specific value, not sure in (might be on room entry too).
The following enemies call the RNG (not noting when they use it though):
Stoke, Puyo, Cacatac, Owtch, Volcanic Rocks (three calls), Kago bugs(two calls), all Sidehoppers and Desgeegas, Skree, Metroids, Draygon (8 calls), Ridley (only 1 call, surprisingly), ... something in Ceres I don't have in my notes, steam on Ceres, the doors on Ceres??, Phantoon (8 calls), Beetoms, electric sparks, Mother Brain (3 calls), Golden Torizo (4 calls)

For Test 1, it's the Reo's... each one on screen (well, each one PROCESSING) will run step 1 of the RNG every frame, whether they need a random number or not. So do the Holtz, Gerutas, and Squeewpts (The 2 Reo-act-alikes, and the lava enemy that jumps up to a random height in the room before Norfair's bubble room). For Test 2 it's the Mellas, with something similar.

Other misc. info I've found while poking at enemy data just now:
When you enter a room with volcanic rocks (D1FF), they set the RNG to 11. Similar thing with all sidehoppers/desgeegas.
Kagos and Beetoms also set the RNG to a specific value, not sure in (might be on room entry too).
The following enemies call the RNG (not noting when they use it though):
Stoke, Puyo, Cacatac, Owtch, Volcanic Rocks (three calls), Kago bugs(two calls), all Sidehoppers and Desgeegas, Skree, Metroids, Draygon (8 calls), Ridley (only 1 call, surprisingly), ... something in Ceres I don't have in my notes, steam on Ceres, the doors on Ceres??, Phantoon (8 calls), Beetoms, electric sparks, Mother Brain (3 calls), Golden Torizo (4 calls)

Cpadolf:

registered on 2007-02-14 03:32:46 am.

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Cpadolf

2009-04-23 01:25:12 pm

Hurr Durr

Quote from Kejardon:

When you enter a room with volcanic rocks (D1FF), they set the RNG to 11. Similar thing with all sidehoppers/desgeegas.

So that would be why it was impossible for example to luck manipulate a sidehopper in the big green brinstar room just before red brinstar in any other way but to wait frames in the room itself instead of just adding lag frames to the door transition right?

Kejardon:

registered on 2004-12-07 07:21:36 pm.

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Kejardon

2009-04-23 05:34:38 pm

Embarrasing Fact: Power suit made by lowest bidder

Most likely, yes.