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Folks I am not sure whether this is a mathematical limitation or a babylon/javscript limitation (or a touslecoq limitation!). As part of the app I am developing I am comparing the normals of faces on 2 meshes in order to calculate the angle and axis of rotation required those faces. This works fine in most cases - however I found in certain cases that the rotation was seemingly random. Some investigation using the console and I determined that this occurs when the faces being compared are exactly 180 degrees apart. I have created a playground to recreate/illustrate the issue. This basically creates a mesh and rotates randomly in 3 dimensions; creates a second mesh with exactly the same rotation. calls a function to calculate the normals of a given face (0-12) for each mesh and calculate the angle between those normals and the axis of rotation (ie a vector that is 90 degrees to those normals). I call the function twice: comparing Mesh 1, Face 0 with Mesh 2, Face 2 - these faces are on opposite sides of the meshes. The angle (180 degrees) is correctly determined but the axis is calculated as (0,0,0). This I believe is the cause of the strange behaviour I have seen in my app. comparing Mesh 1, Face 0 with Mesh 2, Face 4 - these faces are on 2 perpendicular sides of the meshes and the angle (90 degrees) and axis are correctly determined See the console log for the outputs which include the vertices, normals of each selected face and the calculated angle and axis. Look out for axis = (0,0,0) for the second test. Re-run several times and it gets it wrong consistently. So ... million dollar (since pound is worthless now) question ... is there a workaround to calculate the normal to 2 vectors that are 180 degrees apart? http://www.babylonjs-playground.com/#VFKAZ#12 Cheers Rich