Finding the pixel coordinates in a font for RGB matrix

I have a 64*32 LED-matrix on which I want to create something like a maze in which you move one LED around using a joystick.
Now if there's 'collision' with a LED that's not black, the controllable LED should go back to the beginning i.e. coordinates (0,0).
Every time a LED is switched on, I call a function which stores the coordinates, and if the coordinates to which the controllable LED should go, match with one of the stored coordinates, it goes back to (0,0).

Now I want to use some letters on the screen.
I use Adafruit_GFX library and this allows use of fonts on the matrix.
But now I need to store the pixels that are switched on by these letters. But how do I do that?