I have looked into Povray's code source, and especially torus.cpp. I have
understood the torus_inersection() function, but the torus_normal()
function is strange.

If I'm not right, in order to obtain the normal vector at a given point on a
surface, I have to get the partial derivative of the surface equation (the
gradient).

Vector N(df/dx, df/dy, df/dz)

So for the torus, I obtain:





In torus_normal() function, Povray doesn't proceed like this... I tried to
inject my normal formula, but it doesn't work.

So, why it doesn't work ? And how Povray proceed to compute normal vector ?

Thanks,

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