Shadertoy: Build a cross intersection - 001.

Based on this tutorial created by Inigo Quilez I created a new cross intersection shader:

The shader is very simple.
I created a black box and then colored all the areas in the corner of this box with black.
I left a white area to see the cross.
You can see better if you change this line of source code:

...into this:

p_m = 1.0-p_m/(p_m+0.1);

or this:

p_m = 1.0-p_m*10.0/(p_m*10.0+0.1);

The shader can be ru and testd here: