how to draw a spiral using opengl

Question

Hey guys I was starting to study OpenGL, and i wanted to know how to draw a spiral. I wrote this code:

void RenderScene(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    GLfloat x,y,z = -50,angle;
    glBegin(GL_POINTS);

    for(angle = 0; angle < 360; angle += 1)
    {   
        x = 50 * cos(angle);
        y = 50 * sin(angle);
        glVertex3f(x,y,z);
        z+=1;
    }
    glEnd();
    glutSwapBuffers();
}

If I don't include the z terms I get a perfect circle but when I include z, then I get 3 dots that's it. What might have happened?

I set the viewport using glviewport(0,0,w,h) To include z should i do anything to set viewport in z direction? Thanks

Answer 1

You see points because you are drawing points with glBegin(GL_POINTS). Try replacing it by glBegin(GL_LINE_STRIP).

NOTE: when you saw the circle you also drew only points, but drawn close enough to appear as a connected circle.

Also, you may have not setup the depth buffer to accept values in the range z = [-50, 310] that you use. These arguments should be provided as zNear and zFar clipping planes in your gluPerspective, glOrtho() or glFrustum() call.

NOTE: this would explain why with z value you only see a few points: the other points are clipped because they are outside the z-buffer range.

UPDATE AFTER YOU HAVE SHOWN YOUR CODE:

glOrtho(-100*aspectratio,100*aspectratio,-100,100,1,-1); would only allow z-values in the [-1, 1] range, which is why only the three points with z = -1, z = 0 and z = 1 will be drawn (thus 3 points).

Finally, you're probably viewing the spiral from the top, looking directly in the direction of the rotation axis. If you are not using a perspective projection (but an isometric one), the spiral will still show up as a circle. You might want to change your view with gluLookAt().

EXAMPLE OF SETTING UP PERSPECTIVE

The following code is taken from the excellent OpenGL tutorials by NeHe:

glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION);      // Select The Projection Matrix
glLoadIdentity();       // Reset The Projection Matrix

// Calculate The Aspect Ratio Of The Window
gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,100.0f);

glMatrixMode(GL_MODELVIEW);      // Select The Modelview Matrix
glLoadIdentity();       // Reset The Modelview Matrix

Then, in your draw loop would look something like this:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);  // Clear The Screen And The Depth Buffer
glLoadIdentity(); 
glTranslatef(-1.5f,0.0f,-6.0f);     // Move Left 1.5 Units And Into The Screen 6.0
glBegin(GL_TRIANGLES);      // Drawing Using Triangles
 glVertex3f( 0.0f, 1.0f, 0.0f);    // Top
 glVertex3f(-1.0f,-1.0f, 0.0f);    // Bottom Left
 glVertex3f( 1.0f,-1.0f, 0.0f);    // Bottom Right
glEnd();

Of course, you should alter this example code your needs.

Answer 2

No sir not that, i used points itself and i got the circle in points....thats fine but when i incude z also i get just 3 points wheres i shud b getting more than 30

Answer 3

sir, i am sorry but i dont know glu perspective shall i give my entire code here?

#include <iostream>
#include "Xlib.h"
#include "gl.h"
#include "glu.h"
#include "glut.h"
#include <math.h>

#define GL_PI 3.1415f
void SetupRC()
{
    // Black background
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f );
    // Set drawing color to green
    glColor3f(0.0f, 1.0f, 0.0f);
    glPointSize(4);
}

// Called to draw scene
void RenderScene(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    GLfloat x,y,z = -50,angle;
    glBegin(GL_POINTS);

    for(angle = 0; angle < 360; angle += 1)
    { 
     x = 50 * cos(angle);
     y = 50 * sin(angle);
     glVertex3f(x,y,z);
     z+=1;
    }
    glEnd();
    glutSwapBuffers();
}

void ChangeSize(GLint w ,GLint h)
{
    if(h==0)
     h=1;

    glViewport(0,0,w,h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    GLfloat aspectratio = (GLfloat)w/(GLfloat)h;
    if(w<=h)
     glOrtho(-100,100,-100/aspectratio,100/aspectratio, 1,-1);
    else
     glOrtho(-100*aspectratio,100*aspectratio,-100,100,1,-1);

    glMatrixMode(GL_MODELVIEW); 
    glLoadIdentity();
}

int main(int argc , char **argv)
{
    glutInit(&argc ,argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowSize(600,600);
    glutInitWindowPosition(0,0);
    glutCreateWindow("Chelsea");
    SetupRC();
    glutDisplayFunc(RenderScene);
    glutReshapeFunc(ChangeSize);
    glutMainLoop();
    return 0;
}

Can you point out the mistake? thanks