Normals to parametric surfaces.

Normals to parametric surfaces.

Post by Jason Heye » Tue, 17 Dec 2002 12:54:25



What is the best way to compute the normal to a parametric surface. The
normal to the parametric surface Q(s,t) is the cross product of the s
tangent vector with the t tangent vector. The s tangent vector is partial
d/ds(Q(s,t)) and the t tangent vector is partial d/dt(Q(s,t)). How should I
evaluate the cross product?
 
 
 

Normals to parametric surfaces.

Post by Hans-Bernhard Broeke » Tue, 17 Dec 2002 23:17:45



Quote:> What is the best way to compute the normal to a parametric surface. The
> normal to the parametric surface Q(s,t) is the cross product of the s
> tangent vector with the t tangent vector. The s tangent vector is partial
> d/ds(Q(s,t)) and the t tangent vector is partial d/dt(Q(s,t)). How should I
> evaluate the cross product?

What do you mean: "how should I evaluate"?  Are you trying to tell us
you don't know how the cross product is defined, or what?  Look it up
in your textbooks---there's nothing magic about it.
--

Even if all the snow were burnt, ashes would remain.

 
 
 

Normals to parametric surfaces.

Post by Gernot Hoffma » Wed, 18 Dec 2002 03:39:20



> What is the best way to compute the normal to a parametric surface. The
> normal to the parametric surface Q(s,t) is the cross product of the s
> tangent vector with the t tangent vector. The s tangent vector is partial
> d/ds(Q(s,t)) and the t tangent vector is partial d/dt(Q(s,t)). How should I
> evaluate the cross product?

Jason,

probably one doesnt want to use analytical partial derivatives,
because the surface description might be rather complex.

A quadriliteral in the object space (world coordinates) is given
by 4 points Pi(xi,yi,zi), i=1...4 , calculated by the mapping of
the parameter plane si,ti to xi,yi,zi.
Once you decide to use P1 as the reference point, you will find
two edge vectors E21=P2-P1 and E41=P4-P1.

Then calculate the cross product N = E21 x E41 , which is usually
done by a formalistical determinant operation:

        i    j    k
N =   E21x E21y E21z
      E41x E41y E41z

The normal vector should point outside.

      E21y*E41z-E21z*E41y
N = -(E21x*E41z-E21z*E41x)
      E21x*E41y-E21y*E41x

Then normalize N by dividing each component by the Euclidian norm of N.

Special attention is necessary for sphere coordinates near to the poles.

Best regards  --Gernot Hoffmann

 
 
 

Normals to parametric surfaces.

Post by Neil McLaughl » Wed, 18 Dec 2002 05:50:02


Hi,

To obtain the normal to a parametric surface (at a particular point
s,t) you can sample 3 points on the surface:

Point A = P(s,t)
Point B = P(s+x,t)
Point C = P(s,t+y)

(x and y are small offsets (0.0 < x,y < 1)).

Then use these three points to compute two vectors:

Vector AB
Vector AC

As you noted, the cross product of these two vectors is (a good
approximation of) the normal to the surface at point (s,t).

The smaller the x and y chosen, the more accurate the normal is, but
you may choose x and y proportional to the tesselation rate if you are
using polygons to render the surface.

Neil McLaughlin

On Mon, 16 Dec 2002 14:54:25 +1100, "Jason Heyes"


>What is the best way to compute the normal to a parametric surface. The
>normal to the parametric surface Q(s,t) is the cross product of the s
>tangent vector with the t tangent vector. The s tangent vector is partial
>d/ds(Q(s,t)) and the t tangent vector is partial d/dt(Q(s,t)). How should I
>evaluate the cross product?

 
 
 

1. Normals to parametric surfaces.

What is the best way to compute the normal to a parametric surface. The
normal to the parametric surface Q(s,t) is the cross product of the s
tangent vector with the t tangent vector. The s tangent vector is partial
d/ds(Q(s,t)) and the t tangent vector is partial d/dt(Q(s,t)). How should I
evaluate the cross product?

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