Per Pixel Collision - Code explanation

Question

I\'m currently trying to understand per pixel collision detection.

This is the code I don\'t understand:

static bool IntersectPixels(Rectangle rectan         


        

Answer 1

First up, it finds the region the two image rectangles intersect, then it iterates through each pixel in that region, and compares the alpha values of each image of each pixel. If neither has an alpha value of 0, they are both considered 'solid' and therefore colliding.

Answer 2

it's not that hard (in this case) - you give the algorithm the two bounding-boxes of your objects (so the hole object is inside this box), and a array with color-information for them. Tha algorithm assumes that a point belongs to the object IFF it is not transparent - this is important.

The first step is to calculate the intersecting rectangle - if you intersect two rectangles that have sides parallel to the axes like in this case - you will get a rectangle again or an empty set.

The next step is to iterate in this intersecting rectangle for all (x,y) -coordinates insiede - first y, then x -so you get your normal first x, then y inside, but this is minor point and not important.

Then finally the algorithm gets the color for object A and B at the current pixel (x,y) - if both colors are NOT transparent then the pixel is in both objects and the objects have to intersect at this point - so the algorithm terminates with "YES they intersect"

If all pixels in the intersection of the bounding boxes where checked and no common (e.g. not transparent) pixel was found the object don't intersect and so the algorithm terminates with "NO they don't intersect"

I hope this helps.

Answer 3

for (int y = top; y < bottom; y++) loops over the lines of the resulting rectangle from top to bottom, and for (int x = left; x < right; x++) loops over pixels inside each line, left to right.