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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

Posted 31-Aug-10 11:26am
Updated 1-Sep-10 1:20am
v5
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

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## 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 ;)
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<cy youre above line, else-below. If thats what u need i can post exact way to solve equations.
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## Solution 1

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

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