View Full Version : Circles, ellipses with stipple pattern

Bumbala

09-05-2005, 06:32 AM

Hi!

I found samples for drawing circles or ellipses on the web. What I want to do is draw them with stipple pattern, just like glLineStipple. What is the easiest way to do this?

Thanks.

basszero

09-05-2005, 12:14 PM

Imagine a 4-sided figure. Doesn't look like a circle. Imagine an 8-sided figure (like a US stop sign). It's looking more like a circle, but still rought. Now imagine a 36-sided figure. Looks pretty much like a circle.

Really you're connecting each of the 36 points with a line (which will obey line stipple). So your quest now is to calculate the 36 points you need for a circle.

There are many resources to find this information out. If you still need help I'll try to post some more details.

ppeterson

09-05-2005, 10:59 PM

Are you having trouble drawing a circle/ellipse or is setting the right line style your problem?

RigidBody

09-05-2005, 11:00 PM

this should work:

int i, steps = 36;

float x = 0.0, y = 0.0, r = 1.0, phi, dphi = 2.*M_PI / (float)(steps);

glEnable(GL_LINE_STIPPLE);

glLineStipple(1, 0xff);

glBegin(GL_LINE_LOOP);

for(i = 0, phi = 0.0; i < steps; i ++, phi += dphi)

glVertex3f(x+r*cos(phi), y+r*sin(phi), 0.0);

glEnd();

glDisable(GL_LINE_STIPPLE);draws a circle in the x-y-plane at center x/y,

radius r. increasing "steps" will result in a

smoother curve.

a stipple pattern is defined with glLineStipple.

the pattern is a 16-bit short (values from 0x0000

- 0xffff), so "0xff" means: 8 bits sets in the

pattern, the following 8 bits are cleared. the

value "1" defines how often each bit in the

pattern is repeated.

Bumbala

09-05-2005, 11:34 PM

That's the code I needed ;) Thank you RigidBody!

basszero

09-06-2005, 02:39 AM

If you plan on drawing many circles, consider pre-calculating your sin/cos values. It will save some time in a tight loop with lots of circles.

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