EDIT: I changed the bottom left to -1,-1 and the top right to 1,1, and while I can draw objects within this range, I cannot map the mouse x,y to the world space.

I have been trying to learn OpenGL and my current goal is to move a circle on mouse drag. I have created the circle, but I cannot seem to be able to map my mouse coordinates to the world coordinates using glOrtho. I also set the initial window position to 100, 100, I don't know if that is relevant to the issue though.

For reference this is my init code:

def init():
    # select clearing color
    glClearColor(0.0, 0.0, 0.0, 0.0)

    # initialize viewing values
    glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0)

I know that the glOrtho takes the left, right, bottom, top, near and far values, but when I try to convert to world space coordinates from mouse position(since I only need the x and y so I can calculate distance from the circle's origin) I end up with coordinates in the range 0.0 - 1.25, which makes no sense to me at all, because it should be 0.0 - 1.0.

This is my glutMotionFunc:

def click(x, y):
    global current_circle

    nx = float(x * SCREEN_SIZE / SCREEN_SIZE)
    ny = float(y * SCREEN_SIZE / SCREEN_SIZE)

    print("Mouse click", nx, ny)

    for circle in circles:
        print("Circle center", circle.cx, circle.cy)
        if (np.square(nx - circle.cx) + np.square(ny - circle.cy)) <= np.square(circle.r * (SCREEN_SIZE / 10)):
            current_circle = circle
            current_circle.set_position(nx, ny)


Also, why is my window 300x300 when it should be 250x250? I don't set the window size anywhere but the above init function.

  • \$\begingroup\$ to put screen space coordinates to world space multiply screen space coordinates by inverse view-projection matrix (iirc for perspective projection matrix you would also need to divide result by their w coordinate, not sure for orthogonal projection, probably you don't need to do that). You would also need to make sure your screen space coordinates are normalized to [-1; 1] \$\endgroup\$
    – Ocelot
    Commented Oct 7, 2020 at 12:21
  • \$\begingroup\$ Alright, so I managed to get my objects within -1 to 1 and I can't seem to figure out how to convert the mouse coordinates, so I can check if a drawn object is within a certain radius of the mouse for dragging. \$\endgroup\$ Commented Oct 8, 2020 at 6:44

1 Answer 1


I figured out my issues:

  1. First off, I need to set the window size before I create it.
  2. I was not using normalized device coordinates.
  3. I found the simple equation to convert my mouse position to device coordinates: new_x = glut_mouse_x / (SCREEN_SIZE/2) - 1.0 new_y = -1 * (glut_mouse_y / (SCREEN_SIZE/2) - 1.0)

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