Hey there all, I am using RMagick to draw some circles. what i need though is a math formula to give me a pair of plot points for an angle. Say a circle is 40 x 40 on a sheet of graph paper. this would put the center of the circle at x=20, y=20 and it would make the radius = 20. now, i need to be able to pass in a number like say, 20 degrees and return the x and y plots of this on the circle. i know, more a math problem than ruby, but i could use any suggestions. thanks, shawn

on 2008-11-29 20:51

on 2008-11-29 21:40

Hey, I am not sure what you want. If you only want one point it is easy: x = x_0 + radius * Math.cos(angle * 2.0 * Math::PI / 360.0) y = y_0 + radius * Math.sin(angle * 2.0 * Math::PI / 360.0) This is for the angle in degrees. Like this you can get any point on the circle. Now if you want to get the points of an arc you could just increase the angle a bit at a time and plot the points. But that would not be a very nice circle and would be slow. An option would be to get a starting point with the formula above and then iterate with x**2 + y**2 = r**2. For example in the first part of the circle between 0 and 45 degrees y is growing faster than x. You would start with an y and increase it once by one. ( leave away the translation for easiness ) y' <- y + 1 x' <- sqrt( r**2 - (y+1)**2 ) = sqrt( r**2 - y**2 -2*y - 1) = sqrt( x**2 - 2 * y - 1) Hope it gives some ideas. Oliver Am Sonntag, den 30.11.2008, 04:45 +0900 schrieb shawn bright:

on 2008-11-29 21:54

This is great, thanks Oliver. One question, what is meant by x_0 and y_0, are those the plot points for the center of the graph? thanks. shawn

on 2008-11-30 00:41

Thanks much tinkered around with my numbers in your formula, and it worked. shawn

on 2008-11-30 02:14

Yes, x_0 and y_0 are the coordinates of the center of the circle. I am glad if I could help you. :) Oliver Am Sonntag, den 30.11.2008, 08:35 +0900 schrieb shawn bright: