nmode(3G)

NAME
     nmode - specify renormalization of normals

FORTRAN SPECIFICATION
     subroutine nmode(mode)
     integer*4 mode

PARAMETERS
     mode   expects a symbolic constant.  There are two defined constants for
            this parameter:

            NAUTO causes normals to be renormalized only if the current
            ModelView matrix is not orthonormal. (default)

            NNORMALIZE causes normals to always be renormalized, regardless of
            the current ModelView matrix.

DESCRIPTION
     IRIS systems transform vertex normals from object-coordinates to eye-
     coordinates before doing lighting calculations.  While the matrix mode is
     MVIEWIN, a separate Normal matrix is maintained to support this
     transformation.  The Normal matrix is the inverse transpose of the
     upper-left 3 x 3 portion of the ModelView matrix.

     Transformed normals must be unit length if the lighting calculations are
     to be meaningful.  Transformed normals will be unit length if 1) they
     were unit length in object-coordinates, and 2) the current Normal matrix
     is orthonormal (see notes).  If one or both of these conditions are not
     met, the normal must be normalized (corrected to have unit length) after
     it is transformed.  nmode helps the system determine when normalization
     is required.

     Each time the ModelView matrix is changed, the IRIS determines whether
     the resulting (inverse-transpose) Normal matrix is orthonormal or not,
     and saves the result of the test as a flag.  After each normal is
     transformed, both this flag and the nmode flag are tested.  If nmode is
     NAUTO, the normal is normalized if and only if the flag is set (i.e. the
     ModelView matrix is not orthonormal).  NAUTO mode is appropriate when the
     model normals are known to be unit length.  If nmode is NNORMA, the
     normal is normalized unconditionally.  NNORMA mode is appropriate when
     the model normals may not be unit length.

     NAUTO is the default nmode.

     Because normalization involves division by a computed square root, it can
     adversely affect system performance.

SEE ALSO
     mmode, loadma, multma, rot, scale, transl, lmbind
NOTES
     IRIS-4D G, GT, and GTX models, and the Personal Iris, do not support
     nmode.

     nmode cannot be used while draw mode is MSINGL.

     On Impact and Infinite Reality NAUTO is  not supported.  Normals are
     always normalized.

     For our purposes a matrix is orthogonal if it transforms normals to the
     same length regardless of their direction, and it is orthonormal if this
     length is the same as the untransformed length.  Rotation matrixes are
     always orthonormal. Scale matrixes are orthogonal but not orthonormal if
     the three scale values are identical, neither orthogonal nor orthonormal
     otherwise.  Uniform scale ModelView matrices can be normalized to the
     identity matrix, and are therefore ignored by the Normal matrix.
     Translations do not affect the upper-left 3x3 ModelView matrix, and are
     therefore also ignored by the Normal matrix.

     The length of a normal is the square root of its dot product with itself.