backface_control(3G)

NAME

backface_control − (superceded) define aspects of back-facing polygons.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), back-facing polygons are rendered with the back-face polygon color.

red, green, blue Values (in the range 0.0 to 1.0) that define the back-face 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 TRUE, 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-face surface characteristics are defined using fill_color and surface_model procedures and the back-face surface characteristics are defined using backface_control. Note, back-facing polygons derive their vertex colors from the back-face polygon fill color, not from colors supplied with each vertex. If back-face cull is enabled by the hidden_surface(3G) cull parameter, any subsequent call to backface_control with color TRUE has no effect because back-face cull remains active.

This routine, although still supported, has been replaced by bf_control. A separate set of attributes for back-facing polygons can be specified by calls to bf_control and related routines listed under the SEE ALSO heading.

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