[mapserver-dev] New Label Offset code

Stephen Woodbridge woodbri at swoodbridge.com
Mon Mar 12 14:58:12 EDT 2012

Hi Thomas,

After playing with the label leader code all morning here is some 
feedback. Overall it is a really nice enhancement. Based on the 
discussions in tickets it seems like there are more things you want to 
do with it for finer control.

Here are some thoughts on what is needed. I can put some or all of these 
into ticket(s) if you like.

1. I tried the follow and it did not work:

     MAXSCALEDENOM 100000000
     MINSCALEDENOM  50000000
     TEXT ('[STUSPS]')
       COLOR "#f2efe9"
       GRIDSTEP 5
         GEOMTRANSFORM 'end' # also tried 'start'
         SYMBOL 'circle-filled'
         SIZE 6.5  # make it big to besure its not hidden
         COLOR "#000000"
         COLOR "#000000"
         WIDTH 0.5
       COLOR "#675844"
       FONT "arial"
       SIZE 6
       BUFFER 1
       MINDISTANCE 200
       OUTLINECOLOR "#ffffff"

My goal was to put a symbol at the start or end of the leader, but as 
you indicated it is not supported yet. I also tried to change the WIDTH 
on the leader style to no success.

2. Label positioning.

I mentioned in another thread the it would be nice to be able to control 
the order of the label attempts, maybe by listing them in order of 
preference. I found it very hard to get a layout of labels that I felt 
looked good. Using POSITION CC ended up being the best for the example 
of placing state abbreviations on the states. Using AUTO biases the 
labels to the left as we start labeling positions with UL

In most of the Automatic Label Positioning papers I have read they seem 
to express that the cartographically preferred order to be: CR, CL, UC, 
LC, UR, LR, UL, LL or in matrix form:

7 3 5
2 X 1
8 4 6

Where X is the label point. That said, I think the order should be

6 1 5
4 X 3
8 2 7

or UC, LC, CR, CL, UR, UL, LR, LL. And often I would use CC in the first 

I bring this up because we probably need also to change the label 
positioning order dynamically (algorithmically) when there is a leader 
line and my assumption is that have some facility to do this would 
benefit both. The ideal order for positioning the label at the end of 
the leader would be to pick positions that are farthest from the object 
end of the leader. I think this will make for better looking labels and 

  5 3 1
  7 X 2
  8/6 4
  /           6 4 2
o------------8-X 1
  \           7 5 3
  8\6 4
  7 X 2
  5 3 1

Above are few examples where o is the object end of the leader, X is the 
label end of the leader and the number are the positions to try in order.

The above forces the label to minimize intersection with its own leader 
and in general pushes the label away from an already crowded area.

3. For others that are trying to use this, I can recommend putting an 
outlinecolor on your label to minimize interference with the leader line.

All in all, it is a great job. I'll continue to work with it. I know you 
have a lot on your plate trying to freeze 6.2, but keep please keep 
these suggestions in mind as you tackle related issues.

Let me know if you want tickets for any of these.

Thank you for all your effort and help with this new feature.

Best regards,
   -Steve W

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