topic Re: Tangent in Libraries & objects

Tangent

Red — Tue, 07 Dec 2004 15:54:58 GMT

I created a detail callout that includes editable hotspots so that you can move the circle and have the line follow. Now the problem I'm running into is when I move the circle upward in the Y direction the line crosses through the circle. (See attached picture)

I understand the reason behind the line going through the circle. The line end is determined by the radius of the circle and is added in the Y direction. Can someone please tell me if there is away for the line to stay tangent to that circle no matter what the size the circle maybe.

Re: Tangent

Red — Tue, 07 Dec 2004 16:21:26 GMT

Or is there away to have it act like the current line tool does.

(See attached picture)

Re: Tangent

James Murray — Tue, 07 Dec 2004 17:35:03 GMT

I assume you mean perpendicular to the tangent, based on your picture.

You need to know the x, y of the circle center, and the x, y of the point where the line meets the horizontal line.

Let's call them cpX, cpY, epX, epY. (ep: end point)

You need to know the angle of that line. That's ng=ATN(rise/run) -> ng=ATN(ABS(cpY-epY)/ABS(cpX-epX)).

Once you know that angle, you can adjust the start point of the line:

(sp: start point, rds: radius)

spX=cpX+rds*COS(ng)
spY=cpY-rds*SIN(ng)

This is for the quadrant you show in your first picture. For the others, you'll need to change whether you add or subtract the rds*COS(ng) & rds*SIN(ng). You can find out what quadrant you're in by seeing which is greater of cpX/epX and cpY/epY. There's probably a better way, but my trig is not great and I can only find such things by trial and error. I build such things in +X, +Y first, then see what breaks when I go to other quadrants. The basic idea is right.

Then the line is LINE2 spX, spY, epX, epY, with a circle of CIRCLE2 cpX, cpY, rds.

HTH, (and hope I didn't make a mistake!)

Re: Tangent

Red — Tue, 07 Dec 2004 18:04:37 GMT

Thanks that was very helpful. And yes you are correct I should have said perpendicular to the tangent.

Re: Tangent

Durval — Thu, 09 Dec 2004 18:23:21 GMT

Maybe it's simpler not to use the ABS inside the ATN.
The resulting angle from ATN function would be in range -90 to +90.
You then use the following code to convert it to the range -90 to 270 ('cvng' is the converted angle):

IF epX-cpX<0 THEN cvng=ng+180 ELSE cvng=ng

Notice that, for calculation of SINs and COSs, the range -90 to 270 is the same as the range 0 to 360 (e.g., SIN (-45) = SIN (315)).
This way, the script will 'know' the correct quadrant, by providing the proper negative or positive values for SINs and COSs.
But beware: you must provide a conditional calculation of 'ng' when epX=cpX, to avoid division by zero when calculating the ATN.