help里的建模例子，怎么实现的呢，给解释一下，谢谢。
这个程序是怎么实现的呢？new
def parm
rad=10.0 ; radius of spherical cavity
len=20.0 ; length of outer box edge
in_size=6 ; number of zones along outer cube edge
rad_size=10 ; number of zones in radial direction
end
parm
gen zone radbrick edge len size in_size in_size in_size rad_size &
rat 1.0 1.0 1.0 1.2 dim rad rad rad
pl set rot 340 0 30
pl surf wh
def make_sphere
; Loop over all GPs and remap their coordinates:
;assume len>rad
p_gp=gp_head
i = 0
loop while p_gp#null
; Get gp coordinate: P=(px,py,pz)
px=gp_xpos(p_gp)
py=gp_ypos(p_gp)
pz=gp_zpos(p_gp)
; Compute A=(ax,ay,az)=point on sphere radially "below" P.
dist=sqrt(px*px+py*py+pz*pz)
if dist>0 then ;为什么要dist>0呢？
k=rad/dist
ax=px*k
ay=py*k
az=pz*k
; Compute B=(bx,by,bz)=point on outer box boundary radially "above" P.
maxp=max(px,max(py,pz))
k=len/maxp
bx=px*k
by=py*k
bz=pz*k
; Linear interpolation: P=A+u*(B-A)
u=(maxp-rad)/(len-rad)
gp_xpos(p_gp)=ax+u*(bx-ax)
gp_ypos(p_gp)=ay+u*(by-ay)
gp_zpos(p_gp)=az+u*(bz-az)
i = i + 1
end_if
p_gp=gp_next(p_gp)
end_loop
end
make_sphere
pl surf yel 自己先顶一个，希望高手帮忙，谢谢@！ 版主在不》？给解释一下？
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