# paste this code at the end of VectorFieldPlot 2.3
doc = FieldplotDocument('VFPt_capacitor-round-plate_uniform',
    width=800, height=600, commons=True)
# capacitor dimensions
l = 4.5
d = 1.5

# To model the real, non-uniform charge distribution on the capacitor plate,
# we we cut the plate into several finite rings and equalize their potential.
nrings = 10 # caution, increases computing effort a lot.
q_list = sc.ones(nrings)
# higher segment density towards the edges, where charge density varies more
r_list = l/2. * (1.0 - sc.linspace(1, 0, nrings + 1)[1:]**2)

for i_iter in range(50):
    discs = []
    for iring in range(nrings):
        r = r_list[iring]
        Q = r**2 * pi * (q_list[iring])
        if iring < nrings - 1:
            Q -= r**2 * pi * q_list[iring+1]
        discs.append({'x0':-r, 'y0':d/2., 'x1':r, 'y1':d/2., 'Q':Q})
        discs.append({'x0':-r, 'y0':-d/2., 'x1':r, 'y1':-d/2., 'Q':-Q})
    field = Field([ ['charged_disc', p] for p in discs])
    
    V_list = [field.V([(r_list[0]) / 2., d/2.])]
    for i in range(1, nrings):
        V_list.append(field.V([(r_list[i-1] + r_list[i]) / 2., d/2.]))
    
    # We want the potential to be 1 everywhere on the plate,
    # so iteratively adapt the charges
    q_list = q_list / V_list

print 'ring charge densities', q_list
print 'ring potentials', V_list

Q_list = [q_list[0] * r_list[0]] + [q_list[i] * (r_list[i] - r_list[i-1]) for i in range(1, nrings)]
charge_sums = sc.cumsum([0.] + Q_list[::-1] + Q_list)
relative_charge_position = ip.interp1d(charge_sums / charge_sums[-1],
    list(-r_list[::-1]) + [0.] + list(r_list))

def startpath(t):
    # take an oval with stright lines and half-cirles around one plate
    tt = (t%1) * (2 * l + pi * d)
    if tt <= l*0.5:
        return sc.array([tt, d])
    elif tt <= l*0.5 + pi/2.*d:
        phi = (tt - l*0.5) / (d/2.)
        return sc.array([l*0.5 + d*0.5*sin(phi), d*0.5 + d*0.5*cos(phi)])
    elif tt <= l*1.5 + pi/2.*d:
        return sc.array([l - (tt - pi/2.*d), 0.])
    elif tt <= l*1.5 + pi*d:
        phi = (tt - l*1.5) / (d/2.)
        return sc.array([-l*0.5 + d*0.5*sin(phi), d*0.5 + d*0.5*cos(phi)])
    else:
        return sc.array([tt - (l*2. + pi*d), d])

nlines = 22
startpoints = Startpath(field, startpath).npoints(nlines)

# plot field lines
for p0 in startpoints:
    line = FieldLine(field, p0, directions='both')
    doc.draw_line(line, linewidth=2.4, arrows_style={'dist':2, 'min_arrows':1})

# plot round plates
D = 0.055
lw = 0.01
nsign = nlines
plus = 'M 0,-0.02 v 0.04 M -0.02,0 h 0.04'
minus = 'M -0.02,0 h 0.04'
defs = doc.draw_object('g', {})
grad = doc.draw_object('linearGradient', {'id':'grad',
    'x1':str(l/2.), 'x2':str(-l/2.), 'y1':'0', 'y2':'0',
    'gradientUnits':'userSpaceOnUse'}, defs)
for o, c, a in ((0, '#000', 0.3), (0.3, '#999', 0.2),
                (0.8, '#fff', 0.25), (1, '#fff', 0.65)):
    doc.draw_object('stop', {'id':'grad',
         'offset':str(o), 'stop-color':c, 'stop-opacity':str(a)}, grad)

for iplate in range(2):
    yplate = d / 2. * {0:-1., 1:1.}[iplate]
    M = sc.array([0., yplate])
    R = sc.array([l/2., 0.])
    a = atan2(R[1], R[0])
    if iplate == 1:
        col = '#e22'
        sign = plus
    else:
        col = '#45e'
        sign = minus
    transform = 'translate({:.6g},{:.6g})'.format(M[0], M[1])
    transform += ' rotate({:.6g})'.format(degrees(a))
    doc.draw_object('rect', {'x':-vabs(R)-D/2., 'width':2*vabs(R)+D,
        'y':-D, 'height':2*D, 'transform':transform,
        'style':'fill:{:s}; stroke:none'.format(col)})
    doc.draw_object('rect', {'x':-vabs(R)-D/2., 'width':2*vabs(R)+D,
        'y':-D, 'height':2*D, 'transform':transform,
        'style':'fill:url(#grad); stroke:#000; stroke-width:{:.6g}'.format(lw)})
    for i in range(nsign):
        pos = [relative_charge_position((i + 0.5) / nsign), yplate]
        doc.draw_object('path', {'d':sign,
        'transform':'translate({:.6g},{:.6g})'.format(*pos),
        'style':'fill:none; stroke:#000; stroke-width:{:.6g}; '.format(2*lw) +
        'stroke-linecap:square'})

doc.write()