Add description for ST_PointOnSurface and comparison example between ST_PointOnSurface and ST_Centroid

_includes/centroid-pointonsurface-cf.html

@@ -0,0 +1,14 @@
+{% highlight mysql %}
+CREATE TABLE input(geom GEOMETRY);
+INSERT INTO input VALUES(
+    'POLYGON((0 0, 5 0, 5 1, 1 1, 1 4, 5 4, 5 5, 0 5, 0 0))');
+SELECT ST_PointOnSurface(geom) POINT,
+       ST_Centroid(geom) CENTROID FROM input;
+-- Answer: 
+-- |      POINT      |            CENTROID            |
+-- |-----------------|--------------------------------|
+-- | POINT (0.5 2.5) | POINT (2.0384615384615383 2.5) |
+
+{% endhighlight %}
+
+<img class="displayed" src="../ST_PointOnSurface.png"/>

docs/dev/ST_Centroid.md

@@ -67,7 +67,11 @@ SELECT ST_Centroid('GEOMETRYCOLLECTION(
 
 <img class="displayed" src="../ST_Centroid_4.png"/>
 
+##### Comparison with [`ST_PointOnSurface`](../ST_PointOnSurface)
+
+{% include centroid-pointonsurface-cf.html %}
 
 ##### See also
 
+* [`ST_PointOnSurface`](../ST_PointOnSurface)
 * <a href="https://github.com/irstv/H2GIS/blob/master/h2spatial/src/main/java/org/h2gis/h2spatial/internal/function/spatial/properties/ST_Centroid.java" target="_blank">Source code</a>

docs/dev/ST_PointOnSurface.md

@@ -2,28 +2,59 @@
 layout: docs
 title: ST_PointOnSurface
 category: h2spatial/properties
-description: 
+description: Return a <code>POINT</code> that lie on the surface of a Geometry
 prev_section: ST_PointN
 next_section: ST_SRID
 permalink: /docs/dev/ST_PointOnSurface/
 ---
 
-### Signatures
+### Signature
 
 {% highlight mysql %}
+POINT ST_InteriorPoint(GEOMETRY geom);
 {% endhighlight %}
 
 ### Description
 
-
+Returns a `POINT` that lie on the surface of a Geometry. If it's impossible to calculate the surface of a Geometry, the point may lie on the boundary of the Geometry. 
+The returned point is always the same for the same Geometry.
 
 {% include sfs-1-2-1.html %}
 
 ### Examples
 
 {% highlight mysql %}
+SELECT ST_PointOnSurface('POINT(1 5)');
+-- Answer: POINT(1 5)
+
+SELECT ST_PointOnSurface('MULTIPOINT((4 4), (1 1), (1 0), 
+                                     (0 3)))');
+-- Answer: POINT(1 1)
+
+SELECT ST_PointOnSurface('LINESTRING(-1 5, 0 10)');
+-- Answer: POINT(0 10)
+
+SELECT ST_PointOnSurface('POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))');
+-- Answer: POINT(2.5 2.5)
 {% endhighlight %}
 
+<img class="displayed" src="../ST_PointOnSurface_1.png"/>
+
+{% highlight mysql %}
+SELECT ST_PointOnSurface('GEOMETRYCOLLECTION(
+                              POLYGON((1 2, 4 2, 4 6, 1 6, 1 2)), 
+                              LINESTRING(2 6, 6 2), 
+                              MULTIPOINT((4 4), (1 1), (0 3)))');
+-- Answer: POINT(2.5 4)
+{% endhighlight %}
+
+<img class="displayed" src="../ST_PointOnSurface_2.png"/>
+
+##### Comparison with [`ST_Centroid`](../ST_Centroid)
+
+{% include centroid-pointonsurface-cf.html %}
+
 ##### See also
 
+* [`ST_Centroid`](../ST_Centroid)
 * <a href="https://github.com/irstv/H2GIS/blob/master/h2spatial/src/main/java/org/h2gis/h2spatial/internal/function/spatial/properties/ST_PointOnSurface.java" target="_blank">Source code</a>