Optimization of per-pixel collision

Question

I'm been developing my own physics engine for a school project and recently I encountered a problem: Per pixel collisions on big sprites make my FPS drop A LOT.

Here's my pixel collision code. Before entering the following code I'm using Intersects() to see if their bounding boxes collided.

private bool PerPixelCollision(IObject a, IObject b) { Color[] bitsA = new Color[a.Texture.Width * a.Texture.Height]; a.Texture.GetData(bitsA); Color[] bitsB = new Color[b.Texture.Width * b.Texture.Height]; b.Texture.GetData(bitsB); // Calculate the intersecting rectangle int x1 = Math.Max(a.Bounds.X, b.Bounds.X); int x2 = Math.Min(a.Bounds.X + a.Bounds.Width, b.Bounds.X + b.Bounds.Width); int y1 = Math.Max(a.Bounds.Y, b.Bounds.Y); int y2 = Math.Min(a.Bounds.Y + a.Bounds.Height, b.Bounds.Y + b.Bounds.Height); Color c; Color d; // For each single pixel in the intersecting rectangle for (int y = y1; y < y2; ++y) { for (int x = x1; x < x2; ++x) { // Get the color from each texture c = bitsA[(x - a.Bounds.X) + (y - a.Bounds.Y) * a.Texture.Width]; d = bitsB[(x - b.Bounds.X) + (y - b.Bounds.Y) * b.Texture.Width]; if (c.A != 0 && d.A != 0) // If both colors are not transparent (the alpha channel is not 0), then there is a collision { return true; } } } // If no collision occurred by now, we're clear. return false; }

In the example I'm using to test I'm dropping 4 sprites in another sprite that represents the floor (736x79). When I change the sprite that represents the floor to a bigger sprite (3600x270) the FPS drops. I know the problem is in the pixel collision because I'm using a profiler.

Does anyone have any idea on how to optimize the collision?

P.S.: Sorry if I wasn't clear enough about my problem. My english is not so good.

EDIT: The first problem was solved by using the solution provided by @pinckerman but I found another one related to pixel collision detection. When a sprite collide with a the transparent part of a texture, we get the part that intersected and check all the pixels of that part. The problem is: When the transparent part is big enough to cover the whole sprite, I check the whole texture of that sprite (currently using 50x50 textures which is 2500 pixels). Is there any way to not check the whole texture?

Thanks

I'm no expert, but I don't think you should do per pixel collision checks on something like a floor. If you want terrain collision do some kind of polygon collision check. Or if the floor is just flat do a rectangle or even a line intersection check. Sorry but I can't help you with optimizing your algorithm. — Jens Vossnack
– Jens Vossnack, Commented Sep 16, 2013 at 15:27
don't use this. this is perormace killer. use line, box, circle, polygon collision. or all together in combination, but not pixel collision. it's just waste of resources. — Davor Mlinaric
– Davor Mlinaric, Commented Sep 17, 2013 at 13:52
I feel it necessary to add my voice to the throng and point out that you're always going to run into performance issues doing this kind of collision detection. Calling GetData() on a texture stalls the rendering pipeline, which prevents the GPU and CPU from working together at maximum efficiency. In most cases it's going to be better to use geometric collision detection--though of course, there's nothing wrong with this sort of thing as a learning exercise. See this article for more information. — Cole Campbell
– Cole Campbell, Commented Sep 17, 2013 at 15:39

Sep Roland · Accepted Answer · 2024-02-11 20:51:00Z

I'm pretty sure that your FPS drops so much because you're doing:

Color[] bitsA = new Color[a.Texture.Width * a.Texture.Height]; a.Texture.GetData(bitsA); Color[] bitsB = new Color[b.Texture.Width * b.Texture.Height]; b.Texture.GetData(bitsB);

at the beginning of your method.
I suppose you call your PerPixelCollision every Update loop and creating and copying millions of values every game cycle is not a very efficient thing to do.

A big sprite creates a huge Color[] array: the bigger your arrays are, the slower will be this method.

EDIT:

For your second problem, I think that, if you don't know beforehand where is the transparent area of your "big" texture, you have to check the whole sprite anyway.
If your sprite is not too big, that's not a big waste of performance.

PS: I see that you get the intersecting Rectangle on your own, maybe you could find useful this method.

Not only does it have to create the arrays, the garbage collector has to clean them up later. Never create arrays in code that's being called every frame, if at all possible!
Thanks @pinckerman! The problem was solved :) but I found another one x.x. Please read the edit note on the post

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