# Using Trigonometry to draw concentric circles following a path

Hey Guys!

First Post and hopefully an interesting problem - at least one I am trying to get my head around!

I have a small OpenGL application, which uses a series of 3D points to draw a line. I am trying to implement an algorithm that allow me to generate a tube from the line, by drawing a cirle of points around each vertex in my list of points. I have got my head around OpenGL for drawing the line and at each point drawing a circle of points, as shown:

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float angle = 6.283185307f / 15;

for (int j = 0; j<10; j++){ //Each point on the line
for (int j = 0; j< 15; j++){ //Each point on the circle
pX[pT] = 0 + (sin(angle * i));
pY[pT] = (0-(float)pointsY[j] + (cos(angle * i));
pZ[pT] = (0-(float)pointsX[j]);
}
pT++;
}

I am however struggling to extend this so that based on the previous point and the next point in the list from where I am, I perform a rotation so that the circle I draw is perpendicular to the line. I am trying to do this using trigonometry and I am sure that it must be relitively straightforward, I just canâ€™t get my head around it today! Any suggestions or pointers are greatly appreciated.

Chris

This is not as straightfoward as you may think checkout this library GLE Tubing and Extrusion Fact Sheet