Brian
>>Is a value for the surface normal obtainable?
No.
>>Is the slope and curvature of a line obtainable?
No.
I used arraygem and arraycurve tools.
It would be great if Michael will make a getNormal method.

Frenchy Pilou
>>Do you say that I must select something before run the plugins?
Look at the first script description
Usage: Select vectors and surfaces. Run script. ;)

>>So you must put an alert message "Select something before run this plugin"!
:)) No. I don't want to add a pop-up alerts, or any other waste code to these scripts.

>>About the spray: Why lines are not perpendicular to the surfaces ?
If i take a cube only top & bottom surface have the lines on the good direction :)
They must not be perpendicular. Vectors always points up or down, depending of camera position.
I can implement orientation code, but I won't do it, because it significantly downgrades performance of the script.

>>And if I select a lateral face and the vertical lines result vOrient don't work!
vOrient don't work with faces. You can use surfaces or solids.