Precise sprite collisions…without the default methods

[late oriole]

For my custom hitbox system, I disable Game Maker’s default collision checks (all you have to do is turn on the physics world!) and supplement them with my own, which works shockingly well. I have several different shapes like circles, AABB’s, lines, etc.. but I realized I don’t have an equivalent for precise sprite collisions. Is there potentially a way to get two sprites and simulate place_meeting with them somehow?

[ripe tendon]

the fastest, most optimal solution, would be to use place_meeting with precise masks heh. otherwise you will need to either cache the pixels into truth tables and compare cells as pixels, or divide them into complex arrangements of shapes and test in-shape functions

you could also test pixel color but thats extremely slow

any specific reasons you dont wanna use the built in functions?

[late oriole]

I use this: https://www.gafferongames.com/post/fix_your_timestep/

Which basically means I have to only check for collisions whenever a “game” frame runs

Idk what truth tables are but I’m guessing that would be super slow right

[ripe tendon]

so you're turning off the collision events by flipping on physics, and then using your own collision functions on each of your own game steps? why not just use the built in collision functions like place_meeting, instance_place, etc. they only run when you call them and you can't make collisions faster than gm's built in functions

[late oriole]

Part of it is I can have multiple hitboxes per instance too

[ripe tendon]

you can do that using mask_index

like this is valid:

var hold = mask_index;
mask_index = check_a;
if (place_meeting(x, y, obj) == true) {
}
mask_index = check_b;
if (place_meeting(x, y, obj) == true) {
}
mask_index = hold;```

if you're looking for performance benefits then definitely use what gm offers. if yer looking to learn and dont mind the performance costs, then thats a whole other radical thing

(cannot overstate it tho, its not possible to make collisions in gm better or faster than what gm has built in)

[late oriole]

A little of both yeah

You gave me a great idea

Can you like, repeatedly turn on/off physics per frame if need be

Like turn it off just to check a single place meeting and then turn it back on immediately

[ripe tendon]

not sure about that but its not something i'd do personally

i would only turn physics on for a single thing and that would be because i am using the built in physics system

[late oriole]

True yeah

I just wish there was a better way to turn off automatic collisions

[ripe tendon]

collisions do not happen automatically though

collision checks are only made if you give an instance a collision event or you call a collision function

[late oriole]

I know they get skipped if two

Yeah

[ripe tendon]

so if you only check for collisions, using the step event and collision functions (not the collision events), when your game updates then you're already winning

[late oriole]

Yeah, you’re right

I am super proud of this hitbox system regardless though, even if I didn’t need to do it I feel like I learned a lot

[ripe tendon]

oh you absolutely should be haha

[late oriole]

I got it reasonably fast too somehow

[ripe tendon]

nice

[late oriole]

I might’ve just figured out an idea based on what you just told me

You can set the bounding boxes of instances during gameplay now can’t you

[ripe tendon]

no

[late oriole]

I thought you could, I’m probably remembering something wrong

[ripe tendon]

bboxes are just boxes that surround the collision mask, regardless of shape its always the tightest possible rectangle around the mask. except when the mask is precise, then the bbox is the full size of the sprite image

[late oriole]

Ohhhh ok, I see what I was thinking of now

sprite_set_bbox

[ripe tendon]

ah yeah, that changes the bbox on the sprite on an asset level if i recall, generally dont wanna do that

[late oriole]

Yeah, I figured

Unless you’re doing some really crazy hacks

I’m trying to figure out a potential way to combine my hitbox system with more of GM’s basic behaviors

I was thinking maybe changing the instances’ masks to AABB’s that would fit around where their hitboxes are as a simple broad phase

[ripe tendon]

i mean thats what the bbox is by default; are you talking about for precise masks tho?

[late oriole]

Not for this particular thing

I mean more like

Say I have multiple hitboxes for an instance right

The idea would be to get the area surrounding all of them, and then somehow stretch the instance’s mask over them to act as a broad phase

Then within the actual collision event if I need to for the instances, I can run a script that properly checks their hitboxes to see which ones actually collide

The broad phase is what I’ll need to figure out, and then I think I can figure out the rest

[ripe tendon]

youre using the collision event and your custom collision system?

[late oriole]

Kinda meshing them together yeah

[ripe tendon]

👀

[late oriole]

Though this is just me spitballing how to improve it in a future iteration

If I can do that without, say, having to change the x and y positions mid collision event to make it work or something I’d be golden

[ripe tendon]

well, like i said, the best possible improvement is to just cycle through mask_index and using the built in collision functions and just all together ignore the collision events heh

since youre working on a time step solution, you prolly got something like:

if (update == true) {

}```or something similar. you just use place_meeting or instance_place or whatever in that update