When starting with stage, you already have your map in raster image format, like JPG, GIF, PNG, BMP, or anything else your graphics editor can read.
Your aim on this stage is one of two following:
Of course, the second choice requires more effort and peering down to pixels of the map.
If you have color handheld, all you need is to convert your map to 16 colors and save it in uncompressed Windows BMP format. If you own grayscale device, you will need to run saved image through supplied map_color_replace utility.
Below these steps described in detail, using Adobe Photoshop (7.0, previous versions may have minor differences in choices available) as example. Using any other graphics editor should be very close, refer to its documentation for details.
map_color_replace -g --map=auto img-in.bmp --outfile=img-out.bmp
To support as much as possible devices and minimize map size, MapMap offers only 16 colors for map image. This is usually enough to represent map content without any information loss comparing to original map. However, making 16-bit map look pretty requires some effort. The basic approach to it is of course "merging" two or more colors together, possibly tweaking the resulting color either, in such a way to both avoid detailedness loss and ensure pretty look. Per my experience, graphics editors are not quite good at this, fading important objects into background and/or making map look bad. So, the approach is following: use graphics editor to get rid of extra colors to the extent possible, and then analyze remaining colors one-by-one seeking those which should be merged with some other.
You start with reducing color set to some managable extent. This should be 24-32 colors (start with 24; if it leads to bad result, increase number of colors). After that, you should take a look at color distribution ("histogram") of the image. Just a chart will not be enough - you will need detailed and exact figure for each color. That is why I written map_histogram utility.
Once you have histogram table, you analyze it. If you think that you should look for colors with least frequencies, it is not the case. In fact, you should look for:
While first and second type of suspect colors can be picked from produced histogram, your graphics editor will help greatly with the third type. In fact, I do not recommend deciding on killing a color until you really checked what it denotes. Photoshop is particularily good at this. You go Image->Color Table... and see all the colors available in image. Make sure that picker icon is pressed and Preview is checked. Now, click a color with picker cursor and it will be replaced with "checker board" pattern. Or vise-versa, you can click any color on image, it will be marked in pallette and on all pixels of that colow will be checker-boarded on the screen. Use good zoom factor - you can scroll image without closing the color table dialog. So, you can see and describe each color.
Say, you have found that:
From the decisions above, we can create color map file for map_color_replace:
# Lakes and Streams go Rivers 0000e0=>0000d0 000080=>0000d0 # Coniferous forests go Deciduous 074E0A=>33BE3A # All subway stations are red 00FF00=>FF0000 0000FF=>FF0000
So, it's simple: left-hand side is which color to map, right-hand - to which; right-hand color must exist, left-hand should be too, but map_color_replace will just warn if it does not, to allow you to get rid of colors incrementally (i.e. you can just add new mapping pait to the same mapping file and run map_color_replace on result image from its previous pass). How do you know which hex RGB values to use in map? It's simple, first of all, you can lookup them with picker tool in your graphics editor. Or, you just know that Nth color in palette shown in graphics editor corresponds to Nth color in distribution created by map_histogram (and the latter has RGB hex triplets for each).
When you have map file, you run map_color_replace like:
map_color_replace -c --mapfile=file.map img-in.bmp --outfile=img-out.bmp
When you have done with creating map which will leave only 16 active colors in the image, you will need to open resulting image in graphics editor once to save it as 4-bit (vs 8-bit as it was before) BMP (map_color_replace is not yet smart enough to do that itself).
You've done! Just optimize it for greyscale devices as described above and congratulations with creating your first really cool map. Consider contributing it to MapMap collection then ;-) (of course, if you know it will not infringe somebody's copyright).