Cylinder arrangements

[Sindhu]

Hi,

I need to place cylinder in horizontal location i,e hollow spaces of the cylinder is placed on the top side and bottom side.

So that i don't want to see the hollow spaces of the cylinder on my window.
To do this what coordinates i need to use ?

please sort me from this problem.

[Sindhu]

Hi anyone,

Is it possible to divide the cylinder surface into Triangular strips.

If possible means How it is ?

please guide me

Advance thanks.

[ZbuffeR]

Cylinders seem to be all the rage on these forums :)

Have a read here, a lot of code available to copy/paste :
http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=267916&page=all

[Sindhu]

Hi Sir,

Thanks for the reply.

The link which you sent is to help how to draw Cylinder using gluCylinder() function.

Is it possible to draw cylinder Using Triangle strips ?

or
Is it possible to do cylinder surface like triangle strips ?

please Help me.

[ZbuffeR]

Yes of course it is possible.
Does this wireframe image help you to understand how :
http://blog.thingiverse.com/wp-content/uploads/2009/04/blender_quicktip_05_03.png

In this picture, quad are used, but with 2 triangles you can easily draw 1 quad.

For the vertex coordinates, some trigonometry is needed, basically create frist vertex, add a vector v, place second vertex, rotate vector by 180-360/sides degres, etc until all sides are created.

[Sindhu]

Hi Sir,

Thanks for your replying.


As i am new to openGL, will you explain how the vectors are added?

will you give idea through Snippet of code.

Please help me from this problem.

[ZbuffeR]

The trig part is not really an opengl question, it is classic math.
Does this pseudo code helps ?


double x = y = z = 0.0;
double vx = vy = vz = 0.0;
vx = 1;
double theta = (180-360.0/SIDES);
for (int i = 0;i<=SIDES;i++) {
double theta = i * (180-360.0/SIDES);
double tempvx = SLENGTH * (vx*cos(theta) + vy*sin(theta));
double tempvy = SLENGTH * (vy*cos(theta) - vx*sin(theta));
vx = tempvx;
vy = tempvy;
x += vx;
y += vy;
glVertex3d(x,y,z);
}

First make it work to draw a circle with GL_LINES, then adapt this to store yourself each vertex and build a correct tristrip.

[Sindhu]

Hi Sir,

Thank you so much for reply.

By using the above pseudo code i wrote the code which is shown below :




#include<stdlib.h>

#include<GL/glut.h>

#include<GL/glu.h>

#include<GL/gl.h>

#include<math.h>


int angle=0;

void cylinder()

{

GLint SIDES=20,i;

GLfloat SLENGTH=30.0;
double x, y , z;
x = y = z = 0.0;
double vx,vy,vz;
vx = vy = vz = 0.0;
vx = 1;
double theta = (180-360.0/SIDES);
glBegin(GL_LINES);
for (i = 0;i<=SIDES;i++) {

double theta = i * (180-360.0/SIDES);
double tempvx = SLENGTH * (vx*cos(theta) + vy*sin(theta));
double tempvy = SLENGTH * (vy*cos(theta) - vx*sin(theta));
vx = tempvx;
vy = tempvy;
x += vx;
y += vy;
glVertex3d(x,y,z);
}


glEnd();

}
void display()
{

glClear(GL_COLOR_BUFFER_BIT);

glLoadIdentity();

gluLookAt(45.0,45.0,25.0,-45.0,-45.0,-40.0,0.0,0.0,10.0);


cylinder();

glFlush();

angle+=1;

glutSwapBuffers();

}



int main(int argc,char **argv)
{

glutInit(&amp;argc,argv);

glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);

glutInitWindowSize(500,500);

glutInitWindowPosition(100,0);

glutCreateWindow("CYLINDER");

glutDisplayFunc(display);

glMatrixMode(GL_PROJECTION);

glLoadIdentity();

glOrtho(-100.0,100.0,-100.0,100.0,-100.0,100.0);

glMatrixMode(GL_MODELVIEW);

glClearColor(1.0,1.0,1.0,1.0);

glutMainLoop();

}




This code is not generating the Cylinder with lines. i am not getting the problem

please help me from this Sir.