Merge pull request #3488 from Calinou/mention-normal-map-coordinates

Mention the normal map coordinate system expected by Godot

getting_started/workflow/assets/importing_images.rst

@@ -128,6 +128,17 @@ reimported automatically.
 
 Note that RGTC compression affects the resulting normal map image. You will have to adjust custom shaders that use the normal map to take this into account.
 
+.. note::
+
+  Godot requires the normal map to use the X+, Y- and Z+ coordinates. In other
+  words, if you've imported a material made to be used with another engine, you
+  may have to convert the normal map so its Y axis is flipped. Otherwise, the
+  normal map direction may appear to be inverted on the Y axis.
+
+  More information about normal maps (including a coordinate order table for
+  popular engines) can be found
+  `here <http://wiki.polycount.com/wiki/Normal_Map_Technical_Details>`__. 
+
 Flags
 -----
 

tutorials/3d/spatial_material.rst

@@ -354,15 +354,27 @@ lighting in the scene.
 
 .. image:: img/spatial_material15.png
 
-Normalmap
-~~~~~~~~~
+Normal map
+~~~~~~~~~~
 
 Normal mapping allows you to set a texture that represents finer shape detail.
 This does not modify geometry, only the incident angle for light. In Godot,
-only the red and green channels of normal maps are used for wider compatibility.
+only the red and green channels of normal maps are used for better compression
+and wider compatibility.
 
 .. image:: img/spatial_material16.png
 
+.. note::
+
+  Godot requires the normal map to use the X+, Y- and Z+ coordinates. In other
+  words, if you've imported a material made to be used with another engine, you
+  may have to convert the normal map so its Y axis is flipped. Otherwise, the
+  normal map direction may appear to be inverted on the Y axis.
+
+  More information about normal maps (including a coordinate order table for
+  popular engines) can be found
+  `here <http://wiki.polycount.com/wiki/Normal_Map_Technical_Details>`__. 
+
 Rim
 ~~~