topic Re: GDL Trigonometry question in GDL

GDL Trigonometry question

Mats_Knutsson — Wed, 12 Jan 2022 13:11:57 GMT

Hi,

I have a label where i move the marker around freely through hotspots (xins and yins).

I don't understand how to code the line to stop at the perimeter of the circle. If I knew how ATAN works I could do this but i get an error when trying to use ATAN.

Br,

Mats

Re: GDL Trigonometry question

Jochen Suehlo — Wed, 12 Jan 2022 13:46:24 GMT

Gegenkathete / Ankathete = TAN(alfa)

alfa = ATN(Gegenkathete / Ankathete)

Re: GDL Trigonometry question

Zsuzsanna Bori — Wed, 12 Jan 2022 13:52:39 GMT

Hi!

If I understood correctly, you would like to get the arc tangent of x, but you get an error message, correct? The function uses the argument "ATN" instead of "ATAN", see if you get the desired outcome with that.

Re: GDL Trigonometry question

Mats_Knutsson — Wed, 12 Jan 2022 15:06:52 GMT

Ah...I read a doc where it was ATAN. Will try ATN instead. Thx

Re: GDL Trigonometry question

DGSketcher — Wed, 12 Jan 2022 16:07:24 GMT

@Mats_Knutsson See if this works after your hotspots...

dLine = SQR(xIns^2 + yIns^2) - rsp IF xIns > 0 THEN aLine = ATN(yIns / xIns) ELSE aLine = ATN(yIns / xIns) + 180 LINE2 0,0, dline * COS(aLine), dLine * SIN(aLine) CIRCLE2 xIns, yIns, rsp TEXT2 xIns, yIns, stName

Re: GDL Trigonometry question

Mats_Knutsson — Wed, 12 Jan 2022 20:17:57 GMT

Gr8 thanks 🙂

https://youtu.be/NPs0vPolWhs

Re: GDL Trigonometry question

Peter Baksa — Tue, 25 Jan 2022 14:30:20 GMT

Also be extra careful not to divide by 0.

Re: GDL Trigonometry question

Mats_Knutsson — Tue, 25 Jan 2022 15:24:04 GMT

Living on the edge...

Re: GDL Trigonometry question

DGSketcher — Tue, 25 Jan 2022 23:15:27 GMT

@Peter Baksa I did think about it & it only fails at exactly (?) 90 / 270 degrees. I know GDL has a problem with real numbers and zero comparison checks e.g. If A = B is replaced with ABS ( A - B ) < eps. What tolerance / formula would be relevant to

ATN ( A / B ) to error trap when B = 0.0?

Re: GDL Trigonometry question

Peter Baksa — Wed, 26 Jan 2022 08:11:00 GMT

Yes, there are circumstances when you can be sure the input isn't on the Y axis because some parameters forbid that, and it wouldn't be sensible architecturally. But there could be circumstances where a draggable hotspot would be set to 90° deliberately.

Maybe this is where the ATN / ATAN confusion came from: most programming languages have a non-trigonometric ATAN2 function taking an x and an y coordinate and returning the points' polar coordinate, handling all the cases where ATN could go wrong. GDL doesn't have such, but it can be written as a short subroutine containing a few ifs.

EPS can be avoided by using > from the other way you would use <=.

A working example returning degrees in range (-180...180]:

if abs(_x) > 0 then
    _atan2 = atn(_y / _x) + 90 * (1 - sgn(_x)) * (1 - 2 * (sgn(_y) < 0))
else
    _atan2 = sgn(_y * (abs(_y) > 0)) * 90
endif

Re: GDL Trigonometry question

DGSketcher — Wed, 26 Jan 2022 14:59:18 GMT

Thank you. I will tuck that one away for future use. 👍

Re: GDL Trigonometry question

Mats_Knutsson — Thu, 27 Jan 2022 07:03:46 GMT

Great snippet!!! Could you at HQ arrange a site with clever code snippets like this?