Testbed `look_at()` method doesn't seem to center the camera at the location given

[MJohnson459]

Testbed `look_at()` method doesn't seem to center the camera at the location given

[sebcrozet]

Hi!

Perhaps explaining how the projection is setup will help understand how look_at works.

If you have a zoom factor equal to 1, the coordinate system of the 2D projection matrix is such that:

• The point (0, 0) is at the center of the window.
• The point (W/2, H/2) is at the top-right corner of the window. In other words, you will see objects in the physics world with their position equal to (W/2, H/2) at the top-right corner of the screen.
• The point (-W/2, -H/2) is at the bottom-left of corner the window. In other words, you will see objects in the physics world with their position equal to (-W/2, -H/2) at the bottom-left corner of the screen.

where W and H are the width and height of the window.
So if you have a zoom factor equal to 20 as in your example, simply divide those values by 20, e.g., the point (W/40, H/40) will be at the top-right corner of the window.

In any case, no matter the zoom factor, if you want to position the camera such that objects at the coordinate (0, 0) lie in the center of the window, then you should simply look_at the point (0, 0).

[MJohnson459]

Thanks for the explanation, I wasn't really sure what the zoom factor was. I think my problem is still a potential bug though as in the example I set the look_at position to be positive (30, 20) which I would expect to make the window center on that position instead of (0,0). However it seems to be doing something different (perhaps centering on (-30, -20)?).

The image isn't very clear but that line in the top half is a visible x-axis and there are balls spawned at (30,20) which should be visible.

    let center = Point2::new(WIDTH / 2.0, HEIGHT / 2.0);
    println!("Center = {}", center);
    testbed.look_at(center, 20.0);

[MJohnson459]

This should be fixed in nphysics as of 0.14.0 as it updates kiss3d to 0.23.0 which contains the fix.