How to Limit the Displaying of Data Using PointTheme over the map

Question

Hi,
I have the following requirement, which i'm trying to implement with <dvt:map & <dvt:PointTheme components.
*The requirement is ... I have a base map, which is being accessed via <dvt:map component. And i'm using point theme to display certain data over the map. Let us consider, over the US map ... i'm displaying 24 points(for each polygonone point .... taking the centroid of each polygon) when the map zoom level is 0.
At this zoom level '0', user can see all 24 points. But when the user start clicking on '+' to Zoom In(Now the zoom level is '1'), at this zoom level user can see only 18 points. User can see the remaining 6 points by dragging the base map to either sides.
This mean, irrespctive of the zoom level the point theme is rendering the all the points over the map. Instead, i would like to display the data specific to the polygons that are visible to the user. At zoom level '0', if the user is able to see all the polygons, then display all 24 points. And at zoom level '1'...if the user is able to see 18 polygons and then display only those 18 points. And if the user tries to see the other polygons data ... user may drag the base map either sides. In that case, the point theme should render data specific to the polygons that the user is able to see.
Similarly, if the user is at zoom level '2'...at this zoom level user is able to see 13 polygons over the IE. so the point theme should render only 13 points specific to each polygon. If the user tries to do dragging the base map either sides...then display the points, specific to the polygons that the user is able to see.
Similarly for other zoom levels ... Please let me know if you require any other information.
Thanks & Regards,
Kiran Konjeti

Answer 1

You'll need to do a little software workaround on the values you read from your counter.   But first a little tutorial to explain *why*.
The E-series boards use 24-bit counters.  When LabVIEW requests an integer count value, the count is returned in a 32-bit unsigned integer datatype, where the 24 lowest bits represent the value in the board's count register.  The upper 8 bits are *always* 0's.
The count register can increment from 0 to 16 million plus (2^24 - 1), then it must "roll over" back to 0 again just like an odometer.  Similarly, when it decrements past 0, it next goes to the max value of 2^24 -1 as you observed.
So we need a little software workaround that'll convert (2^24 -1) to (-1) and (2^24 -2) to (-2), etc.  The way to do it is to first detect whether the highest bit is a 1.  One way is to compare whether the count value is >= 2^23, making sure to use integers to do the comparison.  Whenever the value is >= 2^23, subtract 2^24 from it to create your new count value which will then decrement 3,2,1,0,-1,-2,-3, etc. as desired.
Here's a link to a similar explanation.  Couple more notes too.  First, if this is a continuously rotating encoder, you may still encounter a discontinuity when the revised count value crosses the +/- 2^23 boundary.   Second, direct connection of a quadrature encoder to an E-series board isn't recommended because of unrepeatability when measureing direction changes.
-Kevin P.