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

Answer 1

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.