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 Post subject: Erratic enemy movement?
PostPosted: Wed May 16, 2018 10:42 pm 
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I’ve been having some trouble visualizing how to make enemies move in a way that’s “erratic”, but not completely obviously random. Particularly among flying / airborne enemies.

Some examples of movements similar to what I’m thinking of are things like the birds in the Airman / Crashman stages of Mega Man, and the dude from 8 Eye’s’s falcon (who I know isn’t an enemy, but the way he moves is still interesting.)

Does anyone have a general idea of how movement like this is done? Is it all just lookup tables? Trig-based?


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PostPosted: Wed May 16, 2018 11:59 pm 
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pick random direction, pick random duration. Move that direction for that long. Loop.


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PostPosted: Thu May 17, 2018 12:04 am 
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Have an array of patterns. Pick a pattern randomly. Repeat.


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PostPosted: Thu May 17, 2018 12:04 am 
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Two things I could suggest:

1. Don't make random decisions every frame. Pick a random direction and make the enemy commit to it for many frames before making another decision. This interval could be randomized as well.

2. Make smooth / filtered movement by controlling acceleration rather than velocity. Like how velocity adds to position each frame, you can add acceleration to velocity each frame as well. This extra step changes a straight line into a curve, which gives a very different impression of motion, but also may help the player anticipate the way it's going to move, because it can be seen turning in advance.

You can make a pseudo sine wave motion (e.g. Castlevania medusa head) simply by flipping the acceleration at a regular interval, maybe every 100 frames.


Edit: suggestion 1 was already given above while I was typing.


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PostPosted: Thu May 17, 2018 12:57 am 
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Oziphantom wrote:
pick random direction, pick random duration. Move that direction for that long. Loop.
I’m more or less doing this already with other enemies, but I still want there to be a sense that the enemy is moving towards the player. Maybe I can do the standard “if the player is to the left move left, etc.”, but add a bit of “noise” via the random variable.

rainwarrior wrote:
2. Make smooth / filtered movement by controlling acceleration rather than velocity. Like how velocity adds to position each frame, you can add acceleration to velocity each frame as well. This extra step changes a straight line into a curve, which gives a very different impression of motion, but also may help the player anticipate the way it's going to move, because it can be seen turning in advance.
Do you think it’s necessary to have sub-pixels for acceleration?


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PostPosted: Thu May 17, 2018 3:35 am 
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Sogona wrote:
Do you think it’s necessary to have sub-pixels for acceleration?

Definitely yes.


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PostPosted: Thu May 17, 2018 4:47 am 
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For reference, here's the enemy logic for the birds in Ninja Gaiden: http://tasvideos.org/GameResources/NES/ ... html#Birds

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PostPosted: Thu May 17, 2018 5:03 am 
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thefox wrote:
For reference, here's the enemy logic for the birds in Ninja Gaiden: http://tasvideos.org/GameResources/NES/ ... html#Birds
Thank you! I tried to disassemble what I think is the code in MM2 for the birds, but without a symbol list it was hard to understand any of it. And any RAM maps for the game I could find were pretty lacking.


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PostPosted: Wed May 23, 2018 3:56 am 
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Randomness in video game is a pretty huge subject, and you can probably find a lot of good resources out there (it's not specific for NES or old school games, after all). The general idea is that you decide on some kind of consistency in the way each enemy behaves, and let some of the factors be random.
The traditional one, usually applied to boss fights are what was already mentioned about having a bunch of set patterns and just choosing between them randomly. Another classic one could have a "patrolling" enemy that attacks the player when it gets near, aiming in the correct direction, but with a random deviance applied to the angle.
A similar effect is also typically seen in shooters with "spray patterns", where the attack feels predictable, but you still need to pay attention to the bullets in order to correctly dodge them.

For research, I recommend playing through Ghouls 'n Ghosts. Aside from the final stage, almost everything you encounter in that game is extremely random, but balanced in such a way that knowledge of the patterns will always help you be prepared with how to deal with them, and also in a way that ensures never running into completely unavoidable patterns, and maintaining a distinct identity of every type of enemy. It is one of the best examples of constant randomness in a video game ever created - barring abstract stuff like Tetris.
(for the record, SUPER GnG is a completely different game and doesn't do the same thing, in fact it's almost a complete opposite in that regard)

thefox wrote:
For reference, here's the enemy logic for the birds in Ninja Gaiden

The birds in Ninja Gaiden are 100% deterministic though.


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