"superclark" <x@no.email> wrote in message news:1161726008095-431912@exchange.ni.com...
I am trying to use the Draw Arc VI to draw arcs (filled in) at different degree values, starting from zero, but can't get it to work like I expect it should at 1 or 2 degrees. Rather than drawing a 1 or 2 degree sliver, the VI draws an entire semicircle. It seems to work fine at any other degree value, including -1 and -2 degrees. Attached is a simple VI to demonstrate what I am seeing. Just enter a 1 or 2 in the numeric control and run it. The VI was built in LV 8.2.
Draw Arc Confusion.vi:
http://forums.ni.com/attachments/ni/170/211983/1/Draw Arc Confusion.vi
Hi,
This is a bug. Notice that it only happens if the size is less then a certain value, or slightly bigger and even!
A size 114, 112, 110, 108 shows the bug, a size of 109, 111 or 112 doesn't, nor do sizes bigger then 115. (This is in LV7.1, but I imagine not much has changed in LV8.2, 0 to 1 degree).
For 0 - 2 degree, the problem occurs at roughly half the size (only horizontal size seem to matter):
0 - 5 degree :sizes <12, even numbers from 12 up to 22
0 - 4 degree :sizes <14, even numbers from 14 up to 28
0 - 3 degree :sizes <20, even numbers from 20 up to 38
0 - 2 degree :sizes <28, even numbers from 28 up to 56
0 - 1 degree :sizes <58, even numbers from 58 up to 114
So it has to be some sort of rounding bug. 1 degree gets rounded to 0 in some cases (if scale is to small, with some even/uneven side effects), and 0 - 0 is then regarded as the default value, so a full circle is used.
As a simple workaround you could make the arc size larger (larger then 114). Or draw the arc flipped. No problem if you draw from 0 with arc size -1.
If this is not possible, you'll have to check (with a lookup table?) if the problem occurs, and if so, draw a line instead.
You could try to find a formula that has the some effect to tell if the problem occurs, but it won't be easy (probably with conversion from degrees to radians, or from angle to x possition). It might be easier to just always draw a line if drawing from 0 to 4 and size <29, or 0 to 3 <39 and so on.
It's not the only bug. A full circle is also drawn if start angle = 1 and arc size = -1, 2 and -2 (but not 1, -2 or 2, -1!). I tried to draw an arc from 1, size -1 instead of an arc from 0 to 1, but than won't fix the problem.
Regards,
Wiebe.
