I can't make working keyCallback function in OpenGL and glfw

Question

import glfw
from OpenGL.GL import *
import numpy as np

currentMode = GL_LINE_LOOP

def render(currentMode):


def key_callback(window, key, scancode, action, mods):
    if key==glfw.KEY_1:
        if action==glfw.PRESS:
            currentMode = GL_POINTS
    elif key==glfw.KEY_2:
        if action==glfw.PRESS:
            currentMode = GL_LINES
    elif key==glfw.KEY_3:
        if action==glfw.PRESS:
            currentMode = GL_LINE_STRIP

while not glfw.window_should_close(window):
    glfw.poll_events()
    render(currentMode)
    glfw.swap_buffers(window)
    print(currentMode)
glfw.terminate()

I try to change primitive type to use render function's parameter. But, It doesn't work. What should I do?

Answer 1

You missed the global statement in key_callback:

def key_callback(window, key, scancode, action, mods):
    global currentMode # <--- THIS IS MISSING
   
    if key==glfw.KEY_1:
        if action==glfw.PRESS:
            currentMode = GL_POINTS
    # [...]

The variable currentMode exists twice. It is a variable in global namespace, and local variable in the scope of the function key_callback. Use the global statement to interpreted currentMode as global and to write to the global variable currentMode with key_callback.

Of course the same applies to the main function, too:

def main():
    global currentMode
    # [...]

Furthermore you have to clear the framebuffer in render:

def render(currentMode):
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)

See the example:

import glfw
from OpenGL.GL import *
import numpy as np

def render(currentMode):
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
    glLoadIdentity()
    
    print(currentMode)
    glBegin(currentMode)
    #Draw Something
    glVertex2f(-0.5, -0.5)
    glVertex2f(0.5, 0.5)
    glVertex2f(-0.5, 0.5)
    glEnd()

def key_callback(window, key, scancode, action, mods):
    global currentMode
    if key==glfw.KEY_1:
        if action==glfw.PRESS:
            currentMode = GL_POINTS
    elif key==glfw.KEY_2:
        if action==glfw.PRESS:
            currentMode = GL_LINES
    elif key==glfw.KEY_3:
        if action==glfw.PRESS:
            currentMode = GL_LINE_STRIP
    
def main():
    global currentMode

    if not glfw.init():
        return
    window = glfw.create_window(480, 480, "Test", None, None)
    
    if not window:
        glfw.terminate()
        return
    glfw.make_context_current(window)
    glfw.set_key_callback(window, key_callback)
    glfw.swap_interval(1)    

    while not glfw.window_should_close(window):
        glfw.poll_events()
        render(currentMode)
        glfw.swap_buffers(window)
    glfw.terminate()
        
if __name__ == "__main__":
    currentMode = GL_LINE_LOOP
    main()