Thanks to Mike Griffiths.
This program fills areas on a picture using VB code. See the code for details.
Note that it is much faster to use the Polygon API function to fill areas. However, the method used by this example lets you do a few other things. The original application used it to count the number of pixels colored.
This method also lets you fill areas between other polygons. For example, inside a rectangle and outside a circle. You can use the FloodFill API function but that won't tell you how big the area is, either.