I've manually edited maps using GIMP and had great success with keeping planners from making paths through unwanted areas. I used black instead of gray as that corresponds to "very" occupied. From the wiki:
When comparing to the threshold parameters, the occupancy probability of an image pixel is computed as follows: occ = (255 - color_avg) / 255.0
, where color_avg
is the 8-bit value that results from averaging over all channels, e.g. if the
image is 24-bit color, a pixel with the color 0x0a0a0a
has a probability of 0.96, which is very occupied. The color
0xeeeeee
yields 0.07, which is very unoccupied.
As black is 0x000000
it should yield 100% (or very) occupied. Gray may not result in an area being considered occupied.
Edit:
According to the wiki that I linked to, you can use a PNG instead of PGM, but PNG is not supported on OS X.
As @Procópio stated in the comments
painting the map black will have the
side-effect of degrading localization,
as the real LIDAR measurements will
not match those of the edited map.