---
name: "NormalIndex"
section: "3"
platform: "linux"
package: "libvolpack1-dev"
version: "1.0b3-10"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/NormalIndex"
---

# NormalIndex(3)

encode/decode a surface normal vector

## NAME

vpNormalIndex, vpNormal - encode/decode a surface normal vector

## SYNOPSIS

\#include \<volpack.h\>

int

**vpNormalIndex(***nx, ny, nz***)**

-

  double *nx, ny, nz;*

vpResult

**vpNormal(***n, nx\_ptr, ny\_ptr, nz\_ptr***)**

-

  int *n;*

-

  double \**nx\_ptr,* \**ny\_ptr,* \**nz\_ptr;*

## ARGUMENTS

- *nx*

  X component of surface normal vector.

- *ny*

  Y component of surface normal vector.

- *nz*

  Z component of surface normal vector.

- *n*

  Encoded surface normal vector.

- *nx\_ptr*

  Pointer to storage for returning X component of surface normal vector.

- *ny\_ptr*

  Pointer to storage for returning Y component of surface normal vector.

- *nz\_ptr*

  Pointer to storage for returning Z component of surface normal vector.

## DESCRIPTION

These two routines are used to convert surface normal vectors between encoded and unencoded forms.  An unencoded vector is described by its three components (stored as double-precision floating point numbers) in an orthogonal coordinate system.  An encoded vector is described by a single integer that is suitable for use as an index in a shading lookup table.  VolPack's built-in shading routines require the use of surface normal vectors encoded using **vpNormalIndex** (or one of the higher-level normal vector computation routines, **vpVolumeNormals** or **vpScanlineNormals**).  User-defined shading routines or lookup tables are not required to use encoded normal vectors.

**vpNormalIndex** computes an encoded vector from the components of an unencoded vector.  The vector must be normalized (*nx\*nx + ny\*ny* *+ nz\*nz = 1*).  The maximum possible value of an encoded normal is given by the constant VP\_NORM\_MAX.

**vpNormal** computes the components of a vector from the encoded integer form.  The encoded normal has less resolution than the unencoded form, so the vector computed by **vpNormal** may not be equal to the original vector passed to **vpNormalIndex**.

## ERRORS

**vpNormalIndex** always returns a valid encoded normal vector.

**vpNormal** normally returns the value VP\_OK.  The following return value is possible:

- VPERROR\_BAD\_VALUE

  The encoded normal vector is invalid.

## SEE ALSO

[VolPack(3)](/VolPack/3.md), vpVolumeNormals(3), vpScanlineNormals(3)