# Synopsis

PDX_MSGT gdx_EvalOffsetSurfDerivs(long option, T_GDX_OFFSET_SURF *gdx_offset_surf, int max_derivs, double uval, double vval, double derivs[][GDX_MAX_DEG + 1][3])

# Purpose

Evaluate derivatives on a GDX offset surface entity

# Description

At a given U and V parameter value evaluate derivatives on a GDX offset surface entity. A max_derivs of 0 is equivalent to evaluating a point on the GDX surface entity. The resulting derivatives are mixed partial derivatives in U and V. The GDX surface entity must be a valid GDX definition. The derivatives can be evaluated from the left or right. A max_derivs > 2 returns zero derivative values. The first derivs index are the V derivatives with respect to U (ie derivs[1][0] is the V tangent derivative).

# Input

## option

### GDX_UNNORM

Unnormalize the parameter values

### GDX_FROMLEFT

Evaluate derivatives from the left

### GDX_FROMLOW

Evaluate derivatives from the low parameter value

## gdx_offset_surf

The GDX offset 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
          Low level error return
          .fi