View Full Version : Creating cylinder

enne87

03-28-2009, 03:38 PM

Hi!

I'm absolute new to OpenGl and have the following question:

I'd like to draw a cylinder and specify the normal vectors for the lighting. I want to do this without the gluCylinder()-Methode.

Now I'm not sure how to start defining the coordinates for the cylinder.

Thanks for your help!

enne

todayman

03-28-2009, 04:14 PM

A perfect cylinder is two circles (the ends) separated by some distance with the middle space filled in. The best way to render one is to approximate the circles with triangles. Pick the number of slices of a circle you want to use (think slices of a pie), pick a radius, and pick a length of the cylinder.

Now, the coordinates of a circle are r*sin(theta) and r*cos(theta), so I'm going to use a for loop, increment theta, and use those values to construct the cylinder.

Something like this:

float radius, halfLength;

int slices;

for(int i=0; i<slices; i++) {

float theta = ((float)i)*2.0*M_PI;

float nextTheta = ((float)i+1)*2.0*M_PI;

glBegin(GL_TRIANGLE_STRIP);

/*vertex at middle of end */ glVertex3f(0.0, halfLength, 0.0);

/*vertices at edges of circle*/ glVertex3f(radius*cos(theta), halfLength, radius*sin(theta));

glVertex3f (radius*cos(nextTheta), halfLength, radius*sin(nextTheta));

/* the same vertices at the bottom of the cylinder*/

glVertex3f (radius*cos(nextTheta), -halfLength, radius*sin(nextTheta));

glVertex3f(radius*cos(theta), -halfLength, radius*sin(theta));

glVertex3f(0.0, -halfLength, 0.0);

glEnd();

}

enne87

03-28-2009, 04:31 PM

Perfect, thanks todayman :)

Cheers,

enne

enne87

03-28-2009, 05:12 PM

One last question: I'm not sure how to create the normal-vectors. Can I use the same parameters as for the vertices?

todayman

03-28-2009, 05:36 PM

The sides of the cylinder have normals that point away from the center, along what would be a radius, so those are (cos(theta), 0.0, sin(theta)), and the top/bottom faces straight up/straight down so those normals are (0.0, (-)1.0, 0.0).

enne87

03-28-2009, 06:10 PM

Ok, thanks again!

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