pyopengl - creating a cylinder without using gluCylinder function

Question

Using pyopengl, I am trying to create a cylinder. The top of the cylinder is not circular whilst the bottom is as shown in the image linked below. I would like to know how to fix this, if its the way I have coded it or simply the way I have done it does not work with pyopengl. I am using Pygame 1.9.2, Python 3.5 and PyOpenGL-3.1.0.

https://i.stack.imgur.com/KYPLY.jpg

import pygame
from pygame.locals import *

from OpenGL.GL import *
from OpenGL.GLU import *





def securityCamera(radius,halflength,slices):
    glBegin(GL_TRIANGLES)
    for i in range(1,slices+1):
        angleSize=(2*math.pi)/slices
        theta=i*angleSize
        nextTheta=(i+1)*angleSize

        glColor3fv((0/256,0/256,0/256))

        glVertex3f(radius*math.cos(nextTheta), halflength, radius*math.cos(nextTheta))
        glVertex3f(radius*math.cos(theta), halflength, radius*math.sin(theta))
        glVertex3f(0.0, halflength, 0.0)

        glVertex3f(radius*math.cos(nextTheta), -halflength, radius*math.sin(nextTheta))
        glVertex3f(radius*math.cos(theta), -halflength, radius*math.sin(theta))
        glVertex3f(0.0, -halflength, 0.0)

    glEnd()

    glBegin(GL_QUADS)
    for i in range(1,slices+1):
        angleSize=(2*math.pi)/slices
        theta=i*angleSize
        nextTheta=(i+1)*angleSize


        glColor3fv((256/256,256/256,256/256))
        glVertex3f(radius*math.cos(theta), halflength, radius*math.sin(theta))
        glVertex3f(radius*math.cos(nextTheta), halflength, radius*math.cos(nextTheta))
        glVertex3f(radius*math.cos(theta), -halflength, radius*math.sin(theta))
        glVertex3f(radius*math.cos(nextTheta), -halflength, radius*math.sin(nextTheta))
    glEnd()

score 1 · Accepted Answer · edited May 23 '17 at 12:24

A cylinder can be divided up into two circles and a rectangular tube/sheet that connects the two. To avoid duplicating some vertices, I used GL_TRIANGLE_FAN (for the circles) and GL_TRIANGLE_STRIP (for the tube) instead of GL_TRIANGLE and GL_QUAD. If you want to learn more about those, I suggest you look at this answer.

Below is a demo that displays a rotating cylinder with a red end, a blue end, and a green tube in the middle (important code is in the draw_cylinder function):

import pygame
from pygame.locals import *
from OpenGL.GL import *
from OpenGL.GLU import *
import math
import sys

def draw_cylinder(radius, height, num_slices):
    r = radius
    h = height
    n = float(num_slices)

    circle_pts = []
    for i in range(int(n) + 1):
        angle = 2 * math.pi * (i/n)
        x = r * math.cos(angle)
        y = r * math.sin(angle)
        pt = (x, y)
        circle_pts.append(pt)

    glBegin(GL_TRIANGLE_FAN)#drawing the back circle
    glColor(1, 0, 0)
    glVertex(0, 0, h/2.0)
    for (x, y) in circle_pts:
        z = h/2.0
        glVertex(x, y, z)
    glEnd()

    glBegin(GL_TRIANGLE_FAN)#drawing the front circle
    glColor(0, 0, 1)
    glVertex(0, 0, h/2.0)
    for (x, y) in circle_pts:
        z = -h/2.0
        glVertex(x, y, z)
    glEnd()

    glBegin(GL_TRIANGLE_STRIP)#draw the tube
    glColor(0, 1, 0)
    for (x, y) in circle_pts:
        z = h/2.0
        glVertex(x, y, z)
        glVertex(x, y, -z)
    glEnd()

pygame.init()
(width, height) = (800, 600)
screen = pygame.display.set_mode((width, height), OPENGL | DOUBLEBUF)
clock = pygame.time.Clock()
rotation = 0.0

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()

    rotation += 1.0
    glClear(GL_COLOR_BUFFER_BIT)
    glClear(GL_DEPTH_BUFFER_BIT)
    glEnable(GL_DEPTH_TEST)

    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    gluPerspective(30, float(width)/height, 1, 1000)
    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity()
    glTranslate(0, 0, -50)#move back far enough to see this object 
    glRotate(rotation, 0, 1, 0)#NOTE: this is applied BEFORE the translation due to OpenGL multiply order

    draw_cylinder(5, 10, 20)
    pygame.display.flip()
    clock.tick(60)

By the way, you are probably aware that fixed-function methods like glBegin and glVertex are really ancient and out-of-date, but I thought it would not hurt to mention. With this kind of code, you are basically sending vertices one at a time to the GPU, which is not very efficient. You might want to take a look at VBOs if you run into performance issues later down the road. Hope this helps!

pyopengl - creating a cylinder without using gluCylinder function

1 Answers1