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.sstatic.net/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()
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!