[Trigonometric Knowledge Useful!]

Alright, I'm trying to create my own version of a circle by using triangles, I guess right now my intended circle will be an octagon because it has 8 sides, well it will.

So far I have this one triangle:



The two sides on the left and right is known as r for radius which is equal to half of the window's height. Since I was going to try to deal with a circle with 8 "slices" the angle measure of the bottom angle has to be 45 degrees. and I use that to find the length of the top side which is done with this function:


double FindMissingSide(double a, double b, double C)
{
//ToRadians is a variable which is PI/180.0
double c = sqrt(pow(a, 2.0) + pow(b, 2.0) - 2 * a * b * cos(C * ToRadians));
return c;
}



I use it in my drawing code (Yes I know I'm calculating the unknown side every time, but this is a small project so I'm not worried):


glPushMatrix();
glColor3f(1.0f, 1.0f, 1.0f);
glTranslated((double)ScreenWidth/2.0, (double)ScreenHeight/2.0, 0.0);
glBegin(GL_TRIANGLES);
glVertex2d(0.0 - (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);
glVertex2d(0.0 + (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);
glVertex2d(0.0, 0.0);
glEnd();
glPopMatrix();


Now before I went and created a for loop, I decided I'd create another triangle and rotate 45 degrees with glRotated(45.0, 0.0, 0.0, 1.0);, which I would expect the two triangles to share one side, but it has a gap of about 3 degrees:



Now I said it seemed to have a gap of 3 degrees because if I set the x rotation to 42, it shares the side, but then it's off by 3 degrees, and the next triangle I would have to subtract 6 degrees because I already subtracted 3 from the last rotation.

I don't understand, I must be doing something plainly stupid and I don't see it. I would think that I must be calculating the missing side wrong somehow...

Any ideas?

Because I'm stupid and I don't know how. I actually haven't even learned trigonometric math, I had to teach myself some stuff because I'm still in high school in my first semester of geometry. I'm trying to teach myself how this works that's why I'm working on it the only way I know how. I mean I could do this using GLU\GLUT or whatever to take the easy way out, but I'm just trying to figure out it works.

glVertex2d(0.0 - (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);
glVertex2d(0.0 + (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);[/quote]

I think your y values might be the problem.
Your y values are 0-0 - r (the circumRadius)
But what you want is 0-0 - h (the Apothem) - the height of the triangle in your case.

Where h is calculated using pythagoros as:
h = sqrt(pow(r, 2.0) + pow(FindMissingSide(r, r, 360.0/slices)/2, 2.0)

Wikipedia's got a good description of regular polygon's which I found pretty useful. http://en.wikipedia.org/wiki/Regular_polygon

Hope this works.

Matt

Here's the algorithm I'm using to perform the same kind of thing you're doing but closer to the way suggested by #2 Sik_the_hedgehog. I've inlined some support methods so it's all in one place so hopefully it still works .

(Java code)


public static Polygon createRegularPolygon(int numberOfSides,
double sideLength, double dblStartAngleInDegrees)
{
// slice angle
double dblSliceAngle = (2 * Math.PI) / numberOfSides;

// calculate circum radius
double dblRad = sideLength / (2 * Math.cos( (180 - dblSliceAngle) / 2);

// calculate the start angle in radians for repeated use in the for loop
double startAngleInRadians = Math.toRadians(dblStartAngleInDegrees);

// now define the polygon
for (int i = 0; i < numberOfSides; ++i)
{
double dblAngle = startAngleInRadians + (dblSliceAngle * i);
int x = (int) Math.round(Math.cos(dblAngle) * dblRad);
int y = (int) Math.round(Math.sin(dblAngle) * dblRad);

// store x and y somewhere for later use...
}
}


Matt

glVertex2d(0.0 - (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);
glVertex2d(0.0 + (FindMissingSide(r, r, 360.0/slices)/2), 0.0 - r);

I think your y values might be the problem.
Your y values are 0-0 - r (the circumRadius)
But what you want is 0-0 - h (the Apothem) - the height of the triangle in your case.

Where h is calculated using pythagoros as:
h = sqrt(pow(r, 2.0) + pow(FindMissingSide(r, r, 360.0/slices)/2, 2.0)

Wikipedia's got a good description of regular polygon's which I found pretty useful. http://en.wikipedia....Regular_polygon

Hope this works.

Matt
[/quote]

Thank you! I could've stared at my work for a year and probably never have noticed that. Since I was thinking circle I thought radius, which would be the same distance from the center in any direction, but because I'm using triangles, that's not true. I guess I accidentally created a faulty syllogism by thinking that way. Again thank you very much! Also, thank you for the extra information, although there was a bug in that one line you gave me: h = sqrt(pow(r, 2.0) + pow(FindMissingSide(r, r, 360.0/slices)/2, 2.0) That plus sign should be a subtraction one. That's at least how I did it...