MS RFC 60: Labeling enhancement: ability to skip ANGLE FOLLOW labels with too much character overlap¶
Daniel Morissette (dmorissette at mapgears.com)
Alan Boudreault (aboudreault at mapgears.com)
- Last Edited:
Adopted on 2010-09-23
At the moment, ANGLE FOLLOW labels on very sharp curved lines can result in labels on which some characters overlap, resulting in either bad looking or sometimes completely unreadable labels.
This RFC proposes a mechanism to detect overlapping characters in ANGLE FOLLOW labels and simply skip the labels, leaving room for other/better labels to fall around the same spot, leading to better looking maps.
In ticket #2221, several options were discussed to improve problematic ANGLE FOLLOW labels, including line smoothing, or increasing the spacing between the characters when overlaps are detected, those approaches may have potential but will be dealt with separately (i.e. in their own RFCs).
This RFC deals specifically with skipping bad labels with too much character overlap and ticket #3523 has been created for it.
This issue was found as part of the FOSS4G 2010 Benchmarking exercise where one of the layers to render was contours with labels. We've seen that MapServer did a poor job of labeling some contours with sharp curves using ANGLE FOLLOW, resulting in overlapping characters and unreadable labels.
OTOH, we found that GeoServer for instance didn't have this type of problematic labels because it simply detected the overlapping chars and skipped those labels. This is the approach we propose to implement here.
Here are two maps of the same area, the first produced by MapServer with the bad labels and the second by GeoServer without the bad labels:
We have found that it is of no use to test the character bboxes for overlap since most of them overlap in normal situations as we can see in the following images:
OTOH we found that we could compare the angles of consecutive characters within a given label and use that as a better indicator of possible overlap. In most cases, a overlap of more than 22.5 degrees is a good threshold to use to decide to skip a given label.
The following image shows the same contours in which labels with character overlap larger than 22.5 degrees are skipped:
Here is another example with a street map:
A new MAXOVERLAPANGLE keyword will be added to the LABEL object, whose value is the angle threshold to use in filtering out ANGLE FOLLOW labels in which characters overlap (floating point value in degrees).
This filtering will be enabled by default starting with MapServer 6.0. The default MAXOVERLAPANGLE value will be 22.5 degrees, which also matches the default in GeoServer. Users will be free to tune the value up or down depending on the type of data they are dealing with and their tolerance to bad overlap in labels.
Setting MAXOVERLAPANGLE == 0 will completely disable filtering of labels based on this criteria, restoring the pre-6.0 behavior.
This feature is enabled by default with a default value of 22.5 degrees, so no mapfile changes are required to enable it.
This example will increase the MAXOVERLAPANGLE to 30 degrees, resulting in less labels being skipped:
MAP ... LABEL ANGLE FOLLOW ... MAXOVERLAPANGLE 30 END ... END
Keep in mind that this option can be combined with REPEATDISTANCE and MINDISTANCE to produce maps with even more labels. Here is an example:
MAP ... LABEL ANGLE FOLLOW ... MAXOVERLAPANGLE 22.5 REPEATDISTANCE 400 MINDISTANCE 100 END ... END
... and here is the resulting contour map with the above settings:
Backwards Compatibility Issues¶
This new feature will be enabled by default with a default MAXOVERLAPANGLE value of 22.5 degreees. Existing mapfiles will work without any change, but may end up producing maps with less labels if they contain several bad labels with overlapping characters. It is always possible to come back to the old behavior (disabling filtering) using MAXOVERLAPANGLE 0 or to tune the value up or down to get better results.
There is no other backwards compatibility issue.
Adopted on 2010-09-23 with +1 from DanielM, SteveL, FrankW, TomK, PericlesN, ThomasB, SteveW, HowardB, AssefaY, JeffM and TamasS.