void xf_clear( void )
extern XF_SPEC *xf_context
extern char **xf_argend
The xf_clear function may be used to clear the transform stack and free any associated memory, but this is usually not necessary since xf begin and end entities are normally balanced in the input.
The global xf_context variable points to the current transformation context, which is of the type XF_SPEC, described in "parser.h":
typedef struct xf_spec {
long xid; /* unique transform id */
short xac; /* context argument count */
short rev; /* boolean true if vertices reversed */
XF xf; /* cumulative transformation */
struct xf_array *xarr; /* transformation array pointer */
struct xf_spec *prev; /* previous transformation context */
} XF_SPEC; /* followed by argument buffer */
The
xid
member is a identifier associated with this transformation,
which should be the same for identical transformations, as an aid to
vertex sharing.
(See also the
c_hvertex
page.)
The
xac
member indicates the total number of transform arguments, and is
used to indicate the position of the first argument relative to the
last one pointed to by the global
xf_argend
variable.
The first transform argument starts at xf_argv, which is a macro defined in "parser.h" as:
#define xf_argv (xf_argend - xf_context -> xac)Note that accessing this macro will result in a segmentation violation if the current context is NULL, so one should first test the second macro xf_argc against zero. This macro is defined as:
#define xf_argc (xf_context==NULL ? 0 : xf_context -> xac)
Normally, neither of these macros will be used, since there are routines for transforming points, vectors and scalars directly based on the current transformation context. (See the xf_xfmpoint page for details.)
The rev member of the XF_SPEC structure indicates whether or not this transform reverses the order of polygon vertices. This member will be 1 if the transformation mirrors about an odd number of coordinate axes, thus inverting faces. The usual thing to do in this circumstance is to interpret the vertex arguments in the reverse order, so as to bring the face back to its original orientation in the new position.
The xf member contains the transformation scalefactor (in xf.sca) and 4x4 homogeneous matrix (in xf.xfm), but these will usually not be accessed directly. Likewise, the xarr and prev members point to data that should not be needed by the calling program.