line not being drawn

OpenGL OpenGL: Basic Coding
1

in following code written in python, the background color is being set but the lines and polygons are not being drawn.please help out

try:
  from OpenGL.GLUT import *
  from OpenGL.GL import *
  from OpenGL.GLU import *
  
except: print '''ERROR: PyOpenGL not installed properly.'''

import sys

def init():
    glClearColor(0.0,0.0,1.0,0.0)
    glColor3d(1.0,1.0,1.0)
    
			
def main():
  glutInit(sys.argv)
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB)
  glutInitWindowSize (640,480)
  glutInitWindowPosition (0,0)
  window=glutCreateWindow ("Solar")
  glutDisplayFunc(mydisplay)
  
  #glutKeyboardFunc(mydisplay)
  init()
  glutMainLoop()
  exit()


def mydisplay():
  glClear (GL_COLOR_BUFFER_BIT)
  glBegin(GL_LINES)
  glVertex2d(100.0,200.0)
  glVertex2d(200.0,400.0)
  glEnd()
  glBegin(GL_POLYGON)
  glVertex2d(100.9,33.0)
  glVertex2d(50.9, 50.0)
  glVertex2d(51.0, 123)
  glVertex2d(123.1, 200)
  glEnd()
  glFlush()
	
main()
2

You need to define your projection matrix. See http://www.songho.ca/opengl/gl_projectionmatrix.html

3

i added the following code

glMatrixMode(GL_PROJECTION)
    glLoadIdentity()

so the complete code becomes


try:
  from OpenGL.GLUT import *
  from OpenGL.GL import *
  from OpenGL.GLU import *
  
except: print '''ERROR: PyOpenGL not installed properly.'''

import sys

def initialize():
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glClearColor(0.0,1.0,1.0,0.0)
    glColor3d(1.0,1.0,1.0)
    
			
def main():
  glutInit(sys.argv)
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB)
  glutInitWindowSize (640,480)
  glutInitWindowPosition (0,0)
  window=glutCreateWindow ("Solar")
 
  glutDisplayFunc(mydisplay)
  #glutKeyboardFunc(mydisplay)
  initialize()
  
  glutMainLoop()
  exit()


def mydisplay():
  initialize()
  glClear (GL_COLOR_BUFFER_BIT)
  glBegin(GL_LINES)
  glVertex2d(100.0,200.0)
  glVertex2d(200.0,400.0)
  glEnd()
  glBegin(GL_POLYGON)
  glVertex2d(100.9,33.0)
  glVertex2d(50.9, 50.0)
  glVertex2d(51.0, 123)
  glVertex2d(123.1, 200)
  glEnd()
  glFlush()
	
main()

but the program is still not displaying any lines.
please help

4

Thats because u have not setup the projection matrix and the viewport. Currently the projection matrix is set to identity matrix. Use glOrtho or gluOrtho2D and specify the projectoion volume as follows.


glViewport(0,0,640,480);
glMatrixMode(GL_PROJECTION)
glLoadIdentity()
gluOrtho2D(0,640,0,480);
5

thanks a lot. can you please explain me why we need these steps. in computer graphics using opengl by fs hill, these steps are not mentioned. btw what is the ‘red’ book of computer graphics?

6

in computer graphics using opengl by fs hill, these steps are not mentioned.

I highly doubt that in 920 pages, they never bothered to mention projection matrices. Maybe you just missed that part?