Origin: Project(0,0,0)
# Components of view vector W
w1 = view3D1
w2 = view3D2
w3 = view3D3
# Shape factor of the ellipse on the xy plane
q = Cos(40)
# cost function
h = 0.5
c = 1
# The projected ellipse is (x/q)^2 + y^2 = c.
# The cost is v = c+h
define(`vs',`(`$2')*q*cos(`$1'),(`$2')*sin(`$1')')
define(`vp',`vs(`$1',`$2'),0')
define(`vx',`sum3D(vp(`$1',`$2'),0,0,h+(`$2')^2)')
# The gradient of v is (2x/q, 2y, -1) and the line
# separating front and back is W^T * grad(v) = 0
# This line intersects the projected ellipse at
# x1,y1 and x2,y2
ap = w2^2*q^2/w1^2+1
bp = -w2*w3*q^2/w1^2
cp = w3^2*q^2/4/w1^2-c
m = sqrt(bp^2-4*ap*cp)
y1 = (-bp+m)/ap/2 ; x1 = (w3-2*y1*w2)*q/2/w1
y2 = (-bp-m)/ap/2 ; x2 = (w3-2*y2*w2)*q/2/w1
t1 = atan2(y1,x1)
t2 = atan2(y2,x2)
theta1 = min(t1,t2)
theta2 = max(t1,t2)
# tangent curve
nT = 11
for i = 0 to nT do {
y = y1 + (y2-y1)/nT*i
theta = atan2(y,(w3-2*y*w2)*q/2/w1)
r = y/sin(theta)
T[i]: Project(vx(theta,r))
}
# front and back parts of the top curve
n = 12
for i = 0 to n do {
theta = theta1 + (theta2-theta1)/n*i
F[i]: Project(vx(theta,c))
Fp[i]: Project(vp(theta,c))
}
for i = 0 to n do {
theta = theta2 + (theta1+twopi_-theta2)/n*i
B[i]: Project(vx(theta,c))
Bp[i]: Project(vp(theta,c))
}
# First draw the inside back
# B is the back curve
# T is the outline
ifpstricks(`
\psset{gradbegin=lightgray,gradend=darkgray,gradlines=1000}
\pscustom[fillstyle=gradient,gradmidpoint=0.7]{
fitcurve(B,n)
for i = 0 to nT do {TT[i]: T[nT-i] }
fitcurve(TT,nT)
\relax} ',
` fitcurve(B,n)
for i = 0 to nT do {TT[i]: T[nT-i] }
fitcurve(TT,nT) ')
# Centre axis
thinlines_
line from Origin to Project(0,0,h)
# F[0] is the leftmost point of the front curve
line from F[0] to Fp[0]
# F[n] is the rightmost point of the front curve
line from F[n] to Fp[n]
thicklines_
# Now draw the outside front
ifpstricks(`
\newgray{gray1}{0.9}%
\newgray{gray2}{0.4}%
\psset{gradbegin=gray1,gradend=gray2,gradlines=1000}
\pscustom[linewidth=0pt,fillstyle=gradient,gradmidpoint=0.99]{
fitcurve(F,n)
fitcurve(T,nT)
\relax} ',
` shade(1,fitcurve(F,n)
fitcurve(T,nT)) ')
# T is the limit curve of visibility
fitcurve(T,nT)
# F is the top front
fitcurve(F,n)
# Front and back projections of the top on xy
fitcurve(Fp,n)
fitcurve(Bp,n)
# The trajectory in pieces, to allow dashed parts
fitcurve(X1,nx)
fitcurve(X2,nx,dotted 0.025)
fitcurve(X3,nx)
fitcurve(X4,3,dotted 0.015)
arca(from X4[4] to X4[3],ccw,0.3,<-)
# Projected trajectory
np = np-2
fitcurve(Xp,np-1)
arca(from Xp[np] to Xp[np-2],ccw,0.18,<-)
"$X(t)$" at Xp[np]-(2bp__,0) ljust
# Axes and vertical lines
thinlines_
line from X1[0] to Xp[0]
line from X4[4] to Xp[np]
arrow from Origin to Project(1.5,0,0)
"$x_1$" rjust below
arrow from Origin to Project(0,1.5,0)
"$x_2$" ljust
line dashed from Project(0,0,h) to F[n/2] chop 0 chop arrowht/4
arrow from F[n/2] to Project(0,0,2)
"$v(X)$" ljust
"`${0}$'" at Origin+(0,1 pt__) below
"$\Omega$" at Project(0,0.9*c,0)+(0,3bp__) above
"`$v(X) = c$'" at (Project(vp(100*dtor_,c)))+(2bp__,0) above ljust
