.PS
cct_init

include(liborth.m4)

# Usual defs...
qrt=dimen_/4;
hlf=dimen_/2;
dim=dimen_;


Org:Here
circle rad dim with .c at Org
line to Org
deg30=30/360*twopi_
line to (dim*cos(deg30), dim*sin(deg30))

arc <-> cw from Here to (dim,0) rad dim

"\ \ $x=r\theta $" at (dim, 0.3*dim) ljust_
"$r$" at (dim/2, 0) below_
"$\theta$" at 0.7*dim, 0.2*dim

"$\theta=\frac{x}{r}$ radians" at (0, -1.5*dim)

Org:Org+(3*dim,0)
move to Org
orthogaxes(2*dim)

# Colour first, black orthog lines after :-)
orthogmove(2*dim, pi_/4-pi_/40, pi_/3-pi_/30); 
A:line to Org;
orthogmove(2*dim, pi_/4+pi_/40, pi_/3-pi_/30); 
B:line to Org;
orthogmove(2*dim, pi_/4-pi_/40, pi_/3+pi_/30); 
C:line dotted to Org;
orthogmove(2*dim, pi_/4+pi_/40, pi_/3+pi_/30); 
D:line to Org;

line invis shaded "green" from A.start to B.start \
  then to D.start then to C.start then to A.start; \
  move to Org

# Now overlay the lines...
orthogphicirc(2*dim, pi_/4-pi_/40, pi_/3-pi_/15, pi_/3+pi_/15);
orthogphicirc(2*dim, pi_/4+pi_/40, pi_/3-pi_/15, pi_/3+pi_/15);

orthogthetacirc(2*dim, pi_/3-pi_/30, pi_/4-pi_/20, pi_/4+pi_/20);
orthogthetacirc(2*dim, pi_/3+pi_/30, pi_/4-pi_/20, pi_/4+pi_/20);


orthogmove(1*dim, pi_/4, pi_/3)
move left 0.5mm__ # clear the arrowhead.
spline <- up_ dim/3 right_ dim/3 then right_ dim
" $\Omega$" ljust_

orthogmove(2*dim, pi_/4, pi_/3); dot
spline <- up_ dim/3 right_ dim/3 then right_ dim/2
" Area=A" ljust_

"$\Omega = \frac{A}{r^2}$ steradians" at (4*dim,-1.5*dim)

"\tiny steradian" at (5*dim,-2*dim)
.PE