Update biomat.md

math formula included in spherical extrusion description

Author: jiri-janacek

_pages/plugins/biomat.md

@@ -38,6 +38,15 @@ Example: [Synthetic contours](https://raw.githubusercontent.com/jiri-janacek/bio
 
  {% include img align="center" name="objects" src="https://raw.githubusercontent.com/jiri-janacek/biomat/d9ca3b6cfd867b04717b16a3debd45ef9157419e/media/objects_Spherical_extrusion.png" caption="Height of spherical extrusion" %}
 
+The binary image was extruded into 3D by union
+of balls which equatorial circles are inside the foreground $C$. The
+height of the extrusion $h$ as the function of 2D point $x$ in $A$
+is
+
+$h\left(x\right)=\max_{s}\sqrt{d\left(s,C^{c}\right)^{2}-d\left(x,s\right)^{2}}$
+
+where $d$ is Euclidean distance, $C^{c}$ is the background and $s$ is the centre of the equatorial circle. 
+
  {% include img align="center" name="objects" src="https://raw.githubusercontent.com/jiri-janacek/biomat/d9ca3b6cfd867b04717b16a3debd45ef9157419e/media/objects_Spherical_extrusion_res.png" caption="Area and mean height of spherical extrusion" %}
 
 Volume = 2 * Area * Mean.