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Bigger Hitboxes in a Tile based sidescroller


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#1 Megalukes   Members   -  Reputation: 106

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Posted 01 February 2013 - 09:13 AM

Hello guys. I've been trying to make a sidescroller game. I tried a lot of different kinds of collision methods, but then I found out that a tile based system would be enough for me.

 

I'm using Tonypa tutorials. They're in Flash, so I have to change a lot of things.
I'm currently using 32x32 tiles in my game. The collisions just work fine, but if the character's hitbox is bigger than 32x32, all my code seems useless. The character's hitbox gets inside tiles when moving/jumping. I have no idea how to fix that.

 

I guess this may be a common issue regarding sidescrollers since a lot of them uses tilemaps.

I attached my current project. I'm using Ruby for now, but I intend to write everything in C++ if I can get everything working just fine.

Thanks in advance.

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#2 Milcho   Crossbones+   -  Reputation: 1177

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Posted 01 February 2013 - 09:34 AM

I haven't read through your code, and I briefly looked at the tutorial you referenced - but this sounds familiar.

Your character only stores 1 x,y location for where it located. The collision is based on this location alone - checking the below/above/left/right tiles from this location when trying to collide with something.

When your hitbox is smaller than the tile size, this is not a problem since it won't ever overlap tiles that are more than 1 distance away (diagonals are also 1, so this isn't manhattan distance)

What you need to do, is define your hitbox as something like top, left, width, height, then obtain all the tiles that overlap your large hitbox, and perform collision tests to see if you collide with *any* of those tiles.

Obtaining which tiles overlap based on the top/height and left/width should be easy enough. The ranges are

xmin = floor( left/tile_size_x ); xmax = floor( (left+width)/tile_size_x );
ymin = floor( top/tile_size_x ); ymax = floor( (top+height)/tile_size_x );

(alternatively define your hitbox as top,left,right,bottom, instead of adding, top+height, which is just bottom, every time)

Then perform the collision checking against all the tiles in the x,y range of xmin-xmax, ymin-ymax, including the boundries (that is including the tiles at xmin and xmax, ymin and ymax)

 

That should collide your bounding box with anything it can possibly collide with.



#3 Megalukes   Members   -  Reputation: 106

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Posted 01 February 2013 - 06:30 PM

Oh, I see. That's actually what I'm doing here. But it seems that's a normal issue.
I found out that there's no solution for that. According to Metanet Software:

The one problem with our approach is that all dynamic objects must be smaller than a grid cell; while this limitation can be circumvented (for instance, by representing a single large object as multiple smaller objects), in actionscript, it really helps keep things running fast. Thus, it was a self-imposed design constraint.

 

So I guess the best option I've got is using multiple hitboxes for each character object.



#4 Milcho   Crossbones+   -  Reputation: 1177

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Posted 01 February 2013 - 09:06 PM

That's one possibility yes.
I actually went through the metanet tutorials when I was writing the physics of my engine about a month ago. From my understanding, their design limitations to which you are referring to are because they keep each collision shape 'inserted' into only 1 grid cell at a time - thus they only check the 9 cells (1 + 8 surrounding) for collision.
If you started inserting the object into multiple grid cells, you could collide with it fine.

However, I too broke down my character into smaller shapes, but for totally different reasons (I wanted to have a taller but thinner collision shape, that also could 'lean' as the ninja in n does (which in their game is achieved by making him one big circle in terms of collision) - and rectangle collision shapes can't be rotated, so I went with multiple circles instead)
Example:
nvNfTQgs.png
There's a third circle between the two.

However, I do use objects in a way such that they are bigger than 1 tile size. The objects get collided with the player (well, technically the player collides with them, not the other way around), but in principle, bigger objects like that will work.
At least, I'm planning on testing that because I have to write some NPCs, which I was planning to implement as just one giant rectangle that's 0.8 wide by 1.8 wide (while my tile size is 1). When I get to that, I simply plan to use the method I described above.

If you're still experiencing issues, could you perhaps trace your collision to see which tiles the collision occurs with? I'm very interested because like I said, my understanding was that by 'inserting' the object into multiple tiles should allow it to be bigger than the tile.
Edit: Shrunk the huge img

Edited by Milcho, 01 February 2013 - 09:07 PM.


#5 Sik_the_hedgehog   Crossbones+   -  Reputation: 1836

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Posted 02 February 2013 - 03:58 AM

What you need to do, is define your hitbox as something like top, left, width, height, then obtain all the tiles that overlap your large hitbox, and perform collision tests to see if you collide with *any* of those tiles.

If you want a cheaper "workaround" (well, pretty much the same as that one but made look somewhat simpler), just check for the maximum amount of tiles it can overlap. For example, if you have a side that's 48 units, that'd mean it can overlap three tiles at most, so you would do three checks (the two extremes and one in the middle). Just make sure that the checks aren't more than a tile away in distance.


Don't pay much attention to "the hedgehog" in my nick, it's just because "Sik" was already taken =/ By the way, Sik is pronounced like seek, not like sick.

#6 Megalukes   Members   -  Reputation: 106

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Posted 02 February 2013 - 04:46 PM

I actually went through the metanet tutorials when I was writing the physics of my engine about a month ago. From my understanding, their design limitations to which you are referring to are because they keep each collision shape 'inserted' into only 1 grid cell at a time - thus they only check the 9 cells (1 + 8 surrounding) for collision. 
If you started inserting the object into multiple grid cells, you could collide with it fine.

 

Ah, now I get it. I realized what happens here. Even though the hitbox's size is bigger than the tilesize, it checks each side correctly, but it never checks the middle of the hitbox while moving, that's why it gets inside the tiles. It also occurs because, as you said, it checks just one cell at a time:

mapateste-654_zps1b9f5a4d.png

Ok then, I guess I should try to do what you said: inserting objects into multiple cells. I have to make some little changes in my code, but I guess it may work.
Thank you very much. If I succeed with that I'll tell you biggrin.png






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