# opengl – Help to find coordinates for the left of the screen in this frustum

I have a frustum with `left = -2`, `right = 2`, `close = 0.5`, `far = 10`.

I've put the eye of the camera in `(0, 0, -2.5)`looking directly at `(0, 0, 0)` with up `(0, 1, 0)`.

Suppose I want to place an object centered on `z = -1`. I want to find the `X` coordinate so that under this trunk appears exactly centered on the left edge of the screen.

I set up the following diagram to help me:

``````                                                                                                + z
|
| /
-X<_______________|______/__________>X
__ x__ | -one /
| /
| /
l_ | __ /
a | / | close to dist = n = 0.5
| / |
| -2.5
|
-z
``````

To find `X` I can use trig trigonometry simple. As `tan a = l / n` Y `tan a = x / 1.5` then the formula for `X` is `x = l / n * 1.5 = 2 / 0.5 * 1.5 = 6`.

But when I use this value of x to draw an object to the left of the center, it does not appear centered at the edge of the screen. What am I doing wrong?