clockwise & counter-clockwise arc drawing algorithm

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how to draw arc ("real-time" like in CAD) from 3 points - precisely, how check (from cursor)it arc will be clockwise or counter-clockwise

UPDATE:

http://a.imageshack.us/img230/2429/arcs.gif

i know how to draw arc like this, but i dont know how to find when mouse cross the segment between points and when i must change direction :-O

i try use soluction from this article OpenS-CAD, a simple 2D CAD application[^]but it not work proertly

Posted 31-Aug-10 11:26am

HassBlass

Updated 1-Sep-10 1:20am

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HassBlass 1-Sep-10 10:44am

i had problem, because i dont remember the angle can be -360 to 360, not 0 to 360 :D

2 solutions

Solution 1

you work out if it's clockwise or counterclockwise from the relative position of the points, surely ?

Posted 31-Aug-10 11:27am

Christian Graus

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helianthus87 · Accepted Answer · 2010-08-31T13:29:00

Solution 2

Do you asking about some math to calculate angle between 2 vectors to check the arc? You mentioned about 3 points and i asume there is something like this:

[how do i see this]

then you need to calculate angle or check dot product between 2 vectors and according to result you will be able to say if the arc is clockwise or counter-clockwise.

I'm not sure if this is such low level problem but if you need any other help with math-drawing feel free to ask. Stuff like this is kind of my hobby ;)

Posted 31-Aug-10 13:29pm

helianthus87

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HassBlass 1-Sep-10 5:14am

not exactly what I want, i corrected my question

helianthus87 1-Sep-10 7:38am

So youre mooving cursor according the arrow and need to find yout if the cursor is below or above line defined by 2 red points, right? If that so all you need to do is get equation of line y=a*x+b then having a,b (you can get it solving 2 equations). Lets say you have cursor P4(cx,cy), then calculate y=a*cx+b and if y