How to convert asphalt01_weight road image to grass removing fruit_density image, assuming fruit_density is all RGB 130,0,0 painted as grass. This guide assumes you have already converted fruit_density.gdm to png image format.
- load fruit_density.png and asphalt01_weight.png in gimp
- set asphalt01_weight image -> mode -> RGB
- SHIFT-O select by color, click the road white pixel, all white pixels are now selected
- select -> grow -> 5
- select -> shink -> 3, use any numbers here to make the selected area match your road as much as possible
- CTRL-C, select fruit_density image and CTRL-V, RMB pasted selection -> to new layer
- unselect the selection (roads), colors -> map -> color exchange, set white RBG 255,255,255 to black RGB 0,0,0
- pasted layer RMB -> merge down
- file -> overwrite fruit_density.png
All done