[mapserver-dev] RFC81: offset labels with leader lines

Lime, Steve D (DNR) Steve.Lime at state.mn.us
Mon Dec 19 13:17:26 EST 2011

The symbol example does imply some interesting problems with regards to within group
collision detection. In some cases you want it (e.g. position auto or with leader lines)
but often you don't (consider the bbox on the wind barb). Any thoughts on that?


-----Original Message-----
From: mapserver-dev-bounces at lists.osgeo.org [mailto:mapserver-dev-bounces at lists.osgeo.org] On Behalf Of thomas bonfort
Sent: Monday, December 19, 2011 4:02 AM
To: Stephen Woodbridge
Cc: mapserver-dev at lists.osgeo.org
Subject: Re: [mapserver-dev] RFC81: offset labels with leader lines


First off, a bit of context on why this RFC and RFC77 have been
implemented: meteo/forecast symbols are standardised complex symbols,
an example of which can be seen here:

Such a symbol would be defined by multiple label blocks inside a
class, each having a fixed offset from the center of the symbol. As
such, to answer your first question, the leader code will offset the
classObj labels as a whole, i.e. an individual label's offset will be
kept relatively to the newly computed leader-offsetted position. A
side effect of this is that there will be a single leader line
whatever the number of labels that are defined in the classObj.

more inline...

On Sun, Dec 18, 2011 at 18:14, Stephen Woodbridge
<woodbri at swoodbridge.com> wrote:
> On 12/18/2011 11:40 AM, Stephen Woodbridge wrote:
>> On 12/16/2011 11:41 AM, thomas bonfort wrote:
>>> devs,
>>> please have a look and comment on
>>> http://mapserver.org/trunk/development/rfc/ms-rfc-81.html
>> Thomas,
>> This looks like an awesome addition to mapserver. I like that this
>> automatically searches for a location.
>> How does this interact with OFFSET X Y, and would it be possible
>> (optionally?) to draw a leader between the object point and the offset x
>> y label point using this same tools set?
> As a follow on to this, and this might work for Mike Smith also.
> If there was an option like LEADERONOFFSET that used you label and leader
> line detection, then it would be possible to have multiple labels on an
> object (ref: RCF 77) with leader lines and to explicitly place them to the
> right or left using OFFSET X Y to manual position the labels relative to the
> object label point.
>                    A
>                    |
>                    |
>        label1\    /  /r label1
>  longer label2----o----r label2
>  other label3/  /    \r label3
>                 |
>                 /
>                B

I don't understand what the "r" means in front of the right labels....
> Where A-B is a line for example. And this would require 6 label blocks each
> with an appropriate OFFSET X Y value to position and ALIGN to align the
> text.
> Or in the more simple case of labeling the state of Maryland and New Jersey,
> having OFFSET X Y to position the names out in the Atlantic Ocean. This
> might be a case where you would want east coast names only offset to the
> right and west coast names offset to the left as a separate use case to
> Mike's dual scenario labeling.

Limiting the leader offsets to specific directions might be a
subsequent evolution to this RFC. Along with another evolution to
RFC77 that would allow to decouple the classObj's multiple labels
(i.e. check the labelcache and render each label individually instead
of as a whole) I imagine that it will be possible to attain the result
you are looking for. In that case you would have:
 - each individual label can be leader-offseted on its own (possibly
limiting the leader-offseted to (a) specific direction(s).
 - each leader-offseted label would have its own leader line
 - To be determined: should an individual label's OFFSET be taken into
account once the leader offset has been applied.

All in all, your and Mike's proposal are reasonable and feasible with
the changes of RFC 77 and 81, but require subsequent work and funding.

best regards,


> -Steve W
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