def getCaptcha(n=5,fontName='Courier',fontSize=14,text=None,fillColor=None):
'''return n random chars in a string and in a byte string structured
as a GIF image'''
from reportlab.graphics.shapes import Drawing, Group, String, \
rotate, skewX, skewY, mmult, translate, Rect
if not text:
from random import randint, uniform
text=''.join([_allowed[randint(0,_mx)] for i in range(n)])
else:
uniform = lambda l,h: 0.5*(l+h)
n = len(text)
baseline = 0
x = 0
G0 = Group()
for c in text:
x += 1
A = translate(x,uniform(baseline-5,baseline+5))
A = mmult(A,rotate(uniform(-15,15)))
A = mmult(A,skewX(uniform(-8,8)))
A = mmult(A,skewY(uniform(-8,8)))
G = Group(transform=A)
G.add(String(0,0,c,fontname=fontName,fontSize=fontSize))
G0.add(G)
x0,y0,x1,y1 = G0.getBounds()
x = 1+x1
G0.transform=translate(2-x0,2-y0)
W = 4+x1-x0
H = 4+y1-y0
D = Drawing(W,H)
if fillColor:
from reportlab.lib.colors import toColor
D.add(Rect(0,0,W,H, fillColor = toColor(fillColor),strokeColor=None))
D.add(G0)
return text, D.asString('gif')
