Suppose you have a mapfile containing a background layer and a layer with a lot of overlapping linestrings.
Now, for the overlapping linestrings, it might be good to visualize their density, thus at places where a lot of lines do overlap they might be drawn more "solid" than at places where only a few lines are present.
Using QGIS, for example, I can do this by just turning the layer opacity to some small value (say five percent). In Mapserver, however, setting the opacity of the layer, the opacity will just make the whole layer see through, but will not add up for each single element.
How can I archive the desired effect?