[mkgmap-dev] combining two img covering other polygon below

Fri Nov 12 23:01:03 GMT 2010

On 11/11/2010, maning sambale wrote:
> The problem, all other polygons and lines are "under" the contour
> polys including the boundingbox of the test contour map.  POIS and
> labels are visible.

I see you are using RoadTrip on Mac. Your map has a white background.
Therefore you will have specified a land polygon type in the style file,
and you will have set that polygon type to white with a .TYP file. What
you are seeing is the white background of your contour overlay map,
covering the road map below. The labels are visible because of a bug in
RoadTrip. Mapsource is similar.

In a Garmin map, pixels can only be 100% transparent or 100% opaque. You
cannot have partial transparency - you cannot blend two colours. This is
because many Garmin devices have a palette of only 256 colours.

You need to have different processing of the base map and the overlay
map, so that only one has a white background. This will only partly
solve the problem, however, because you have applied colours to areas
above a certain height. These coloured areas will still cover the map
below. You have three options:
1. Accept that the coloured areas will cover the map below.
2. Have the contour layer below, with a white background, and the road
layer above, transparent with no background. Then you have to accept
that areas in the road map, such as woods and lakes, will cover the
coloured areas in the contour layer.
3. You could experiment with a stipple (a mixture of transparent and
opaque pixels) for the colour in the contour layer, so that some of the
map detail below will show through, and see if you can get a
satisfactory result.

I have successfully produced maps with a road map layer below, and a
transparent contour layer above, using standard Garmin line types for
the contours. These maps display correctly on the GPS device. But
Mapsource and RoadTrip do not display these maps correctly because of a
bug. The contours are covered by some of the areas in the map.

You may know how to control which map is above, and which is below, both
in Mapsource and on the GPS device. If not, these may help:
http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2009q4/006026.html
http://www.mkgmap.org.uk/pipermail/mkgmap-dev/2010q1/006746.html
There is a more detailed description somewhere but I could not find it
when I looked just now. Perhaps someone could kindly point us towards
it? For above and below, RoadTrip behaves similarly to Mapsource.