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Re: Locator Appearance mystery in LocatorPane

• To: mathgroup at smc.vnet.net
• Subject: [mg80009] Re: [mg79984] Locator Appearance mystery in LocatorPane
• From: John Fultz <jfultz at wolfram.com>
• Date: Fri, 10 Aug 2007 06:38:55 -0400 (EDT)
• Reply-to: jfultz at wolfram.com

On Fri, 10 Aug 2007 01:45:30 -0400 (EDT), Murray Eisenberg wrote:
> Here's a LocatorPane inside a DynamicModule with a Locator initialized
>
> at the origin and where additional Locator points will be created at
> mouse clicks:
>
> dot=Automatic;
>
> DynamicModule[{pts = {{0, 0}}},
> LocatorPane[Dynamic[pts],
> Graphics[Point /@ pts],
> Appearance -> dot,
> LocatorAutoCreate -> All]
> ]
>
> This works as expected. (The setting Automatic for Appearance is the
> default.)
>
> Now change the definition of dot to:
>
> dot=Graphics[Disk[],ImageSize->5]
>
> Still OK.  But, finally, change the defintion of dot to:
>
> Graphics[{PointSize[Medium],Point[{0,0}]}]
>
> Now it does NOT work as expected: Although the initialized points do
> appear correctly, and a new dot appears at the first click in the
> output, upon each additional click a new dot does not appear but,
> instead, that one new dot moves to the location of the click.
>
> Diagnosis?  Cure?

This is correct behavior.  If you change dot to be...

dot = Graphics[{PointSize[Medium], Point[{0, 0}]}, Background -> Red, 
  Frame -> True]

...and play around with the result, the reason for the behavior should start to 
be come obvious.  The default size of a Graphics[] object is the size of the
graphic.  And the entire graphics object represents the Locator, so all you're 
doing is moving the existing Locator (as you should correctly do when clicking 
directly on a Locator), rather than creating a new one.  The only reason the
Disk[] example worked as you expected is because you set the ImageSize to be so 
small, which affected the size of not just the disk, but the Locator itself.

Sincerely,
 
John Fultz
jfultz at wolfram.com
User Interface Group
Wolfram Research, Inc.

• Follow-Ups:
  • Re: Re: Locator Appearance mystery in LocatorPane
    • From: Murray Eisenberg <murray@math.umass.edu>