BACKFACE_CONTROL(3G)

NAME

backface_control − define aspects of backfacing polygons.

SYNOPSIS

C Syntax:

void backface_control(fildes,rev_normals,color,red,green,blue);
int fildes,rev_normals,color;
float red,green,blue;

FORTRAN77 Syntax:

subroutine backface_control(fildes,rev_normals,color,red,green,blue)
integer*4 fildes,rev_normals,color
real red,green,blue

Pascal Syntax:

procedure backface_control(fildes,rev_normals,color:integer;
red,green,blue:real);

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 point in either direction when light source equations are applied.

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

red, green, blue Values (in the range 0.0 to 1.0) that define the backface polygon color.

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 color is on, polygons that are back-facing 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 surface color defined by red,green,blue. Polygons can be thought of as two-sided surfaces where front-side surface characteristics are defined using fill_color and surface_model procedures and the back-side surface characteristics are defined using backface_control. If backface culling is enabled by the hidden_surface(3G) cull parameter, any subsequent call to backface_control with color 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 .

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