Circumference Points
In Category GeometryArticle published 29/09/2009 10:44:56pm

Finding and plotting points on the circumference of a circle: This article explains how to write a simple function that provides cartesian co-ordinates for a point based on a circles centre co-ordinates, its radius, and the degree of rotation.
Finding a point on the circumference of a circle can be a pleasantly easy affair. To find any point, we will need the following information:
- The co-ordinates of the centre of the circle (nXpos,nYpos)
- The radius of the circle (nRadius), and
- The offset of the point, in degrees of rotation (dDegrees)
dDegrees = ( dDegrees - 90 ) % 360;
We can now easily find the x,y co-ordinates of the point
we want using sine and cosine:
x = nRadius x cos( dDegrees x π / 180 );
y = nRadius x sin( dDegrees x π / 180 );
Easy! A quick example of this function written in
Javascript would work as follows:
y = nRadius x sin( dDegrees x π / 180 );
function GetPoint(nRadius,dDegrees,nXpos,nYpos) {
dDegrees = (dDegrees-90)%360; // Account for 90.deg. offset
x = nRadius*Math.cos(dDegrees*Math.PI/180);
y = nRadius*Math.sin(dDegrees*Math.PI/180);
x = Math.round(x);
y = Math.round(y);
return Array((x+nXpos),(y+nYpos));
}
dDegrees = (dDegrees-90)%360; // Account for 90.deg. offset
x = nRadius*Math.cos(dDegrees*Math.PI/180);
y = nRadius*Math.sin(dDegrees*Math.PI/180);
x = Math.round(x);
y = Math.round(y);
return Array((x+nXpos),(y+nYpos));
}