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# collision detection, 2d platformer

## 9 posts in this topic

Hey guys, new to the forum and to game development in general.

I'm an engineering/physics student getting my feet wet with game development to get better at programming and have som fun along the way. I'm making a 2d platformer, but want to make my engine fairly physics based (eventually adding rotations, angular momentum, maybe even elasticity etc).

Looking for some input on how to best handle collision detection for games with continuous movement (ie not chopping up the screen into 20x20 grids or whatever). My problem is that when updating player coordinates with hero.x = hero.x + dx, collisions are a bit tricky. My previous solution was to keep track of the distance between each object and the hero, and comparing the distance between and after resolving x = x + dx. If the distance had switched sign, that means the player has went through a wall which triggers a collision "fix".

This works, but it's buggy sometimes. I'm considering another solution: When generating the objects, I could keep track of all the space coordinates that are occupied by these objects in some sort of list. Then, before updating player position, I could check if this list of occupied coordinates contains any of the points between hero.x and hero.x + dx. That would mean a collision has occured. Sounds fairly quick computationally, especially if I happen to have some sort of idea what the thickness of the objects are - say they're all atleast 5 pixels thick. Then I only need to check every fourth or fifth point on the line connecting x and x + dx.

Is this viable? Are there better solutions out there? Any help is appreciated :)

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Usually, people start with simple boxes, that have functions to check if they're colliding with another box. Then the scene holds a list of boxes, or classes which are some kind of box, you get the idea.

Then, you could do something like this (using C++ in this case):

if (placeFree(x + dx))
move(dx, 0);


There are also problems with this but I think this is a much better place to start than looking for sign changes. Checking collisions between boxes is usually done by checking if a box is not colliding with the other, instead of the other way around (just a hint).

Welcome to the forums! You seem to be starting good, remember you can always post here if you want feedback on your code. Good luck!

Edit: as you seem to be working with small collision objects (or boxes), you will have to check multiple positions depending on the size of your player. If you're going to use the delta time (like you should) to adjust for lower update rates, this is even more important. If you don't, the player could teleport through walls if the game froze for a small time.

Edited by ProtectedMode
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Usually, people start with simple boxes, that have functions to check if they're colliding with another box. Then the scene holds a list of boxes, or classes which are some kind of box, you get the idea.

Then, you could do something like this (using C++ in this case):

if (placeFree(x + dx))
move(dx, 0);


There are also problems with this but I think this is a much better place to start than looking for sign changes. Checking collisions between boxes is usually done by checking if a box is not colliding with the other, instead of the other way around (just a hint).

Thanks for the reply! The problem with checking if x + dx is free is that x + dx might be free, but x + 0.5*dx might not be! That would send my guy straight through the block!

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Thanks for the reply! The problem with checking if x + dx is free is that x + dx might be free, but x + 0.5*dx might not be! That would send my guy straight through the block!

See my last edit. You will have to check all different possible positions where a collision could have happened between the players position and the obstacles one. How much checks you will have to do depends on size of the player. This will probably become slow if you add a lot of objects, but there is a lot of room for optimalization, but this method is good enough when you're just starting. If you're curious however, you could improve collision checking performance using quad trees, or one of the many similar methods.

Edited by ProtectedMode
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What ProtectedMode is trying to say (I think), just use bounding boxes to start and do the simple rectangle intersection test to check for collision.  Have a list of collideable objects, and check your moving objects against each of them.  If a collision occurs, the objects would be move back to where they weren't colliding and, typically, you would notify each object of what it has collided with, if reasonable.

For example, a player hitting an enemy monster, the player needs to know he's hit an enemy monster (so his health will decrease), but if the player hits a wall, neither the player nor the wall needs to know this, and no notification would be required.

Later, if you get to the point where you have thousands of collideable objects, you may want to look into spatial hashing so you're only checking objects that live within certain areas of your world or screen.

Edited by BeerNutts
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That's indeed what I meant, I guess my English is not very good.

For example, a player hitting an enemy monster, the player needs to know he's hit an enemy monster (so his health will decrease), but if the player hits a wall, neither the player nor the wall needs to know this, and no notification would be required.

You can easily integrate this. The most basic solution would be to add the collision to a list of collisions if it happens by the placeFree() object, and then handle the collisions after the regular updates. This solution is however obviously not very clean because you don't expect a placeFree() function to que a collision for you...

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The above solutions are the common ones used. If your objects are moving extremely fast you might want to consider casting a ray from the 4 corners of your bounding box from the old position to the corresponding corner in the new position. The collision ray with the smallest magnitude (if any)  is where you will need to resolve your collision and impart any forces on the colliding objects.

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The above solutions are the common ones used. If your objects are moving extremely fast you might want to consider casting a ray from the 4 corners of your bounding box from the old position to the corresponding corner in the new position. The collision ray with the smallest magnitude (if any)  is where you will need to resolve your collision and impart any forces on the colliding objects.

You're right. Isn't this what other physics engines like Box2D do when you tell the engine your object is a bullet?

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The above solutions are the common ones used. If your objects are moving extremely fast you might want to consider casting a ray from the 4 corners of your bounding box from the old position to the corresponding corner in the new position. The collision ray with the smallest magnitude (if any)  is where you will need to resolve your collision and impart any forces on the colliding objects.

Sounds like what I'm trying to achieve. Thanks guys, I'll definitely update with my progress. This is so much fun hehe

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I'll definitely update with my progress

It could also be fun and useful to keep a journal here on gamedev. (Just a tip...)

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