It might me easier to use a 256 greyscale mask image. Basically, you load and display the background image normally, but you load another image which isn't displayed with white as normal sections and black lined up to the obstructions, then you check colisions on the mask instead of the 32 bit color image. You can also use various shades of grey to identify the degree of obstruction, for instance white = roads, light grey = dirt paths, med grey = swamps, black = boulders, etc...