# Problem with gfm.math.matrix (some gamedevs out there ?)

Thomas earthwormjimmy at gmx.at
Thu Sep 3 12:36:35 UTC 2020

```Hi!

I am following some examples in C++ about how OpenGL is working.
There are great tutorials out there and most of it works also
with D.
For my code I am using gfm.math.matrix (Version 8.0.3) to be able
to calculate the mouse position in a 3d world.
Now I have some problems with the projection matrix by inverting
it.

My example code:
---------------------
import std.stdio;

int main()
{

import gfm.math.matrix;

const int width = 800;
const int height = 600;

auto projectionMatrix = mat4!(float).identity();
auto ratio = cast(float)width / cast(float)height;

projectionMatrix = mat4!(float).perspective( 45.0f, ratio,
0.0f, 100.0f );

writeln("projectionMatrix: ", projectionMatrix );

auto inversedMatrix = mat4!(float).identity();
inversedMatrix = projectionMatrix.inverse();  // <-- why this
does not work ?
writeln("inversedMatrix: ", inversedMatrix );

return 0;
}

The projection matrix will be calculated correctly with
[1.34444, 0, 0, 0, 0, 1.79259, 0, 0, 0, 0, -1, -0, 0, 0, -1, 0]
assuming that the screen size is 800x600.

But using the .inverse() function in gfm returns only a matrix
with following values:
[-nan, -nan, -nan, -nan, -nan, -nan, -nan, -nan, -nan, -nan,
-nan, -nan, -nan, -nan, inf, -inf]

I don't know what I am doing wrong here:
- do I call the function wrong way ? (but there is no other way)
- is there a bug in the function ? (I do not believe that
because the library is battle proved)

So I looked a couple of hours for an answer, but did not find
anything useful.

Is somebody out there who could help me out ? Maybe one developer
of the d gfm library ?

Thank you for you time!

Thomas
```