It's not particularly hard to implement.
Go over each pixel starting in the upper left corner. You just scan your image top-to-bottom, left-to-right and check the left, top and top-left pixels of non-zero pixels. If those neighbours are 0, you assign a new value to your pixel, Otherwise you assign the value of those neighbours to your pixel.
The only tricky bit happens when the left and top pixel both have a value that differ. This means you need to merge them at a later stage, so just record somewhere (in a map for example) both values and assign one of them to your pixel.
Once you've assigned a value to each non-zero pixel, you go back over them to merge the numbers in your map.
