# Synopsis

PDX_MSGT gdx_EvalRevSurfDerivs(long option, T_GDX_REV_SURF *gdx_rev_surf, int max_derivs, double uval, double vval, double derivs[][GDX_MAX_DEG + 1][3])

# Purpose

Evaluate derivatives on a GDX revolve surface

# Description

At a given U and V parameter value, evaluate derivatives on the GDX revolve surface entity. A max_derivs of 0 is equivalent to evaluating a point on the GDX revolve surface entity. The resulting derivatives are mixed partial derivatives in U and V. The GDX revolve surface entity must be a valid GDX definition. The derivatives can be evaluated from the left or right. A max_derivs > 3 returns zero derivative values. The first derivs index are the V derivatives with respect to U (ie derivs[1][0] is the first derivative of V with respect to U - the tangent in the V direction ).

# Input

## option

### GDX_UNNORM

Unnormalize the parameter values

### GDX_FROMLEFT

Evaluate derivatives from the left

## gdx_rev_surf

The GDX revolve surface entity structure pointer

## max_derivs

The maximium derivative required

## uval

The surface U parameter value to evaluate

## vval

The surface V parameter value to evaluate

# Output

## derivs

The 3D mixed partial derivatives

# Return

          GDX_NOERR
          GDX_PARMOUTOFRANGE
          GDX_NOMEMORY
          Low level error return
          .fi