bf_control(3G)

NAME

bf_control − activate or deactivate attributes for backfacing polygons.

SYNOPSIS

C Syntax:

void bf_control(fildes,rev_normals,attr);
int fildes,rev_normals,attr;

FORTRAN77 Syntax:

subroutine bf_control(fildes,rev_normals,attr)
integer∗4 fildes,rev_normals,attr

Pascal Syntax:

procedure bf_control(fildes,rev_normals,attr:integer);

DESCRIPTION

Input Parameters

fildes Integer file descriptor returned by gopen when an I/O path to a graphics device is opened.

rev_normals If TRUE (1), any normals associated with polygons can point in either direction when light source equations are applied. The normal pointing more towards the light is used for computation.

attr If TRUE (1), backfacing polygons are rendered with the backface polygon attributes.

Discussion

The rev_normals flag is useful in the case where normals are being calculated on polygons whose vertices are not arranged in a consistently clockwise or counter-clockwise order (see vertex_format procedure). It is also useful for polygons that are to be viewed as two-sided surfaces.

If attr is TRUE, polygons that are backfacing with respect to the viewpoint are rendered by the light source equations using polygon or vertex normals pointing in the opposite direction and a new set of attributes. The attributes include the fill color specified by bf_fill_color (3G), perimeter color specified by bf_perimeter_color (3G), interior style specified by bf_interior_style (3G), and the shading model specified by bf_surface_model (3G) and bf_surface_coefficients (3G). If backface culling is enabled by the hidden_surface(3G) cull parameter, any subsequent call to bf_control with attr TRUE has no effect because backface culling remains active. See "Exceptions to Standard Starbase Support" in the HP98721 Device Driver section of the Starbase Device Drivers Library Manual.

This routine replaces backface_control which, although still supported, is not recommended because it modifies only the fill color of the back facing polygons.

DEFAULTS

rev_normals = FALSE (0).
attr = FALSE (0).

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