[OpenLayers-Users] Re: drawing a box or circle

[wls]

Ulao,

I ran into this a while back, and here's my understanding of what's going on
with OpenLayers, though I'm willing to defer to any experts that state
otherwise.

In order to manipulate an OpenLayers map, you need to "
http://dev.openlayers.org/docs/files/OpenLayers/Map-js.html#OpenLayers.Map.addControl
add " a  http://dev.openlayers.org/docs/files/OpenLayers/Control-js.html
Control  to it.  Controls sit dormant until 
http://dev.openlayers.org/docs/files/OpenLayers/Control-js.html#OpenLayers.Control.activate
activated , then they do their thing, and stop when 
http://dev.openlayers.org/docs/files/OpenLayers/Control-js.html#OpenLayers.Control.deactivate
deactivated .

The control you're interested in is the 
http://dev.openlayers.org/docs/files/OpenLayers/Control/DrawFeature-js.html
DrawFeature  control which adds 
http://dev.openlayers.org/docs/files/OpenLayers/Feature-js.html features  to
a map.

A map consists of 
http://dev.openlayers.org/docs/files/OpenLayers/Layer-js.html layers , so
you'll need to tell the control which layer the feature will be drawn to,
usually done by specifying the layer in the constructor.

What kind of shape is determined by the 
http://dev.openlayers.org/docs/files/OpenLayers/Handler-js.html Handler  you
pass to the DrawFeature control.  While it is possible to write your own,
it's more than likely one already exists that does what you want.

A handler watches mouse events and interprets them a certain way.  Two
separate clicks could be a line segment, a rectangle's opposite corners, or
several ways to define a circle.  Often the same handler can perform
different tasks just based on the options passed to it.

This  http://openlayers.org/dev/examples/modify-feature.html drawing example 
has a superb number of shapes, all produced fairly simply.  You'll note that
a "circle" is actual a 
http://dev.openlayers.org/docs/files/OpenLayers/Handler/RegularPolygon-js.html
regular polygon  with 40+ sides.

Hope this helps,

Walt Stoneburner


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