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# Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
# GTR_POLYCLIP -- Clip a convex polygon to a box. If the polygon is entirely
# outside the box 0 is returned; if the polygon is entirely within the box 1
# is returned, otherwise the polygon is clipped and a value other than 0 or 1
# is returned. This is based on code by Paul Heckbert from Graphics Gems,
# 1985/1989.
int procedure gtr_polyclip (pv, npts, x1, x2, y1, y2)
short pv[ARB] #U polygon to be clipped
int npts #U number of points in polygon
int x1,x2,y1,y2 #I clipping box
pointer sp, p1, p2, pt
int x1out, x2out, y1out, y2out, i
int gtr_cliptoplane()
define nopts_ 91
begin
x1out = 0; x2out = 0
y1out = 0; y2out = 0
# Count vertices which are outside with respect to each of the
# four planes.
do i = 1, npts*2, 2 {
if (pv[i+0] < x1) x1out = x1out + 1
if (pv[i+0] > x2) x2out = x2out + 1
if (pv[i+1] < y1) y1out = y1out + 1
if (pv[i+1] > y2) y2out = y2out + 1
}
# Is the polygon entirely inside the clipping box?
if (x1out + x2out + y1out + y2out == 0)
return (1)
# Is the polygon entirely outside the clipping box?
if (x1out == npts || x2out == npts || y1out == npts || y2out == npts)
return (0)
# If we get here the polygon partially intersects the clipping box.
# Clip against each of the planes that might cut the polygon, clipping
# the previously clipped polygon in each step. This is done in
# floating point to minimize accumulation of error when interpolating
# to the clipping plane to compute a new polygon vertex when the plane
# is crossed.
call smark (sp)
call salloc (p1, npts * 4, TY_REAL)
p2 = p1 + npts * 2
call achtsr (pv, Memr[p1], npts * 2)
if (x1out > 0)
if (gtr_cliptoplane (p1, p2, npts, 0, -1.0, real(x1)) == 0)
goto nopts_
else {
pt = p1; p1 = p2; p2 = pt
}
if (x2out > 0)
if (gtr_cliptoplane (p1, p2, npts, 0, 1.0, real(x2)) == 0)
goto nopts_
else {
pt = p1; p1 = p2; p2 = pt
}
if (y1out > 0)
if (gtr_cliptoplane (p1, p2, npts, 1, -1.0, real(y1)) == 0)
goto nopts_
else {
pt = p1; p1 = p2; p2 = pt
}
if (y2out > 0)
if (gtr_cliptoplane (p1, p2, npts, 1, 1.0, real(y2)) == 0)
goto nopts_
else {
pt = p1; p1 = p2; p2 = pt
}
call achtrs (Memr[p1], pv, npts * 2)
call sfree (sp)
return (npts)
nopts_
call sfree (sp)
return (0)
end
# GTR_CLIPTOPLANE -- Clip the convex polygon P1 against a plane, copying
# the inbounds portion to the output polygon P2.
int procedure gtr_cliptoplane (p1, p2, npts, index, s, ref)
pointer p1 #I pointer to input polygon
pointer p2 #I pointer to output polygon
int npts #U number of polygon points or vertices
int index #I index of coordinate to be tested
real s #I sign for comparison
real ref #I value to compare against
int nout, i
pointer op, u, v
real tu, tv, t
begin
nout = 0
op = p2
u = p1 + (npts - 1) * 2
tu = s * Memr[u+index] - ref
v = p1
do i = 1, npts {
# On old polygon P1, U is previous vertex, V is current vertex,
# TV is negative if vertex V is in.
tv = s * Memr[v+index] - ref
if (! ((tu <= 0 && tv <= 0) || (tu > 0 && tv > 0))) {
# Edge crosses plane; add intersection point to P2.
t = tu / (tu - tv)
Memr[op+0] = Memr[u+0] + t * (Memr[v+0] - Memr[u+0])
Memr[op+1] = Memr[u+1] + t * (Memr[v+1] - Memr[u+1])
nout = nout + 1
op = op + 2
}
if (tv <= 0) {
# Vertex V is in, copy it out.
Memr[op+0] = Memr[v+0]
Memr[op+1] = Memr[v+1]
nout = nout + 1
op = op + 2
}
u = v
tu = tv
v = v + 2
}
npts = nout
return (nout)
end
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