Issue with control surface code

[joergbrech]

Hi Martin,

I just reproduced parts of this code for some OpenGL example for students.

I thin this code will not work if the p1 and p2 share the same y coordinate. Is this checked somewhere else?

I guess It is better to check for this case before constructing eyV, so if exV.y == 0 construct a different vector for eyV. For example:

gp_Vec eyV(2, 1 , -(2*exV.x + exV.y)/exV.z);

And then instead of creating the third axis "manually" we can simply calculate the cross product between exV and eyV which should then result in a local coordinate system that is always valid.

We would also have to negate the alpha rotation then, If I am correct...

taken from @Mk-arc's comment here: 4cc8a90#r100084385

[Mk-arc]

See #949 for reference.