.TH SDERIVE
 6 "IRIT Version 6.0" 
.SH NAME
SDERIVE



 SurfaceType SDERIVE( SurfaceType Srf, NumericType Dir )

Returns a vector field surface representing the differentiated surface
in the given direction (ROW or COL). Evaluation of the returned surface at
a given parameter value will return a vector tangent to Srf in
Dir at that parameter value.

 DuSrf = SDERIVE( Srf, ROW );
 DvSrf = SDERIVE( Srf, COL );
 Normal = coerce( seval( DuSrf, 0.5, 0.5 ), VECTOR_TYPE ) ^
          coerce( seval( DvSrf, 0.5, 0.5 ), VECTOR_TYPE );

computes the two partial derivatives of the surface Srf and computes
its normal as their cross product, at the parametric location (0.5, 0.5).