Here's something that off the top of my head sounds like it might work:
Create a dictionary with a double key, and a list of rectangle+boolean values, like this:
Dictionary< Double, List< KeyValuePair< Rectangle, Boolean>>> rectangles;
For each rectangle in your set, find the corresponding list for the x0 and the x1 values, and add the rectangle to that list, with a boolean value of true for x0, and false for x1. This way you now have a complete list of all the x-coordinates that each rectangle either enters (true) or leaves (false) the x-direction
Grab all the keys from that dictionary (all the distinct x-coordinates), sort them, and loop through them in order, make sure you can get at both the current x-value, and the next one as well (you need them both). This gives you individual strips of rectangles
- Maintain a set of rectangles you're currently looking at, which starts out empty. For each x-value you iterate over in point 3, if the rectangle is registered with a true value, add it to the set, otherwise remove it.
- For a strip, sort the rectangles by their y-coordinate
- Loop through the rectangles in the strip, counting overlapping distances (unclear to me as of yet how to do this efficiently)
- Calculate width of strip times height of overlapping distances to get areas
Example, 5 rectangles, draw on top of each other, from a to e:
aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbb
aaaaaaaaaaaaaaaa bbbbbbbbbbbbbbbbb
aaaaaaaadddddddddddddddddddddddddddddbbbbbb
aaaaaaaadddddddddddddddddddddddddddddbbbbbb
ddddddddddddddddddddddddddddd
ddddddddddddddddddddddddddddd
ddddddddddddddeeeeeeeeeeeeeeeeee
ddddddddddddddeeeeeeeeeeeeeeeeee
ddddddddddddddeeeeeeeeeeeeeeeeee
ccccccccddddddddddddddeeeeeeeeeeeeeeeeee
ccccccccddddddddddddddeeeeeeeeeeeeeeeeee
cccccccccccc eeeeeeeeeeeeeeeeee
cccccccccccc eeeeeeeeeeeeeeeeee
cccccccccccc
cccccccccccc
Here's the list of x-coordinates:
v v v v v v v v v
|aaaaaaa|aa|aaaa | bbbbbbbbbb|bb|bbb
|aaaaaaa|aa|aaaa | bbbbbbbbbb|bb|bbb
|aaaaaaa|aa|aaaa | bbbbbbbbbb|bb|bbb
|aaaaaaa|aa|aaaa | bbbbbbbbbb|bb|bbb
|aaaaaaaddd|dddddddddd|ddddddddddddddbb|bbb
|aaaaaaaddd|dddddddddd|ddddddddddddddbb|bbb
| ddd|dddddddddd|dddddddddddddd |
| ddd|dddddddddd|dddddddddddddd |
| ddd|ddddddddddeeeeeeeeeeeeeeeeee
| ddd|ddddddddddeeeeeeeeeeeeeeeeee
| ddd|ddddddddddeeeeeeeeeeeeeeeeee
ccccccccddd|ddddddddddeeeeeeeeeeeeeeeeee
ccccccccddd|ddddddddddeeeeeeeeeeeeeeeeee
cccccccccccc eeeeeeeeeeeeeeeeee
cccccccccccc eeeeeeeeeeeeeeeeee
cccccccccccc
cccccccccccc
The list would be (where each v is simply given a coordinate starting at 0 and going up):
0: +a, +c
1: +d
2: -c
3: -a
4: +e
5: +b
6: -d
7: -e
8: -b
Each strip would thus be (rectangles sorted from top to bottom):
0-1: a, c
1-2: a, d, c
2-3: a, d
3-4: d
4-5: d, e
5-6: b, d, e
6-7: b, e
7-8: b
for each strip, the overlap would be:
0-1: none
1-2: a/d, d/c
2-3: a/d
3-4: none
4-5: d/e
5-6: b/d, d/e
6-7: none
7-8: none
I'd imagine that a variation of the sort + enter/leave algorithm for the top-bottom check would be doable as well:
- sort the rectangles we're currently analyzing in the strip, top to bottom, for rectangles with the same top-coordinate, sort them by bottom coordinate as well
- iterate through the y-coordinates, and when you enter a rectangle, add it to the set, when you leave a rectangle, remove it from the set
- whenever the set has more than one rectangle, you have overlap (and if you make sure to add/remove all rectangles that have the same top/bottom coordinate you're currently looking at, multiple overlapping rectangles would not be a problem
For the 1-2 strip above, you would iterate like this:
0. empty set, zero sum
1. enter a, add a to set (1 rectangle in set)
2. enter d, add d to set (>1 rectangles in set = overlap, store this y-coordinate)
3. leave a, remove a from set (now back from >1 rectangles in set, add to sum: y - stored_y
4. enter c, add c to set (>1 rectangles in set = overlap, store this y-coordinate)
5. leave d, remove d from set (now back from >1 rectangles in set, add to sum: y - stored_y)
6. multiply sum with width of strip to get overlapping areas
You would not actually have to maintain an actual set here either, just the count of the rectangles you're inside, whenever this goes from 1 to 2, store the y, and whenever it goes from 2 down to 1, calculate current y - stored y, and sum this difference.
Hope this was understandable, and as I said, this is off the top of my head, not tested in any way.