matplotlib color gradient in patches?

Question

I\'d like to create ellipses in matplotlib with a fill color that has an alpha (opacity) value that depends on the radius;

e.g., a 2D Gaussian.

Answer 1

I don't think matplotlib currently supports gradient fills for patches - see this email.

john> Hello, I am trying to set a bar (a patched series of rectangles) with a fill pattern instead of just a solid color. Is there an easy way to do this in matplotlib?
john> I am thinking of something like Qt's QBrush which has cross, vertical, dense, etc. patterns.

There is no support for this currently -- it wouldn't be too hard to add for backends that support this kind of thing. Basically, we need to specify the API for it, and add support to backends. I have been wanting to add gradient fills for patches (eg polygons, rectangles) and it would be good to do both at once.

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Instead of using patches you could create a mesh, calculate the colours with a function then use imshow with interpolation:

# Taken from http://matplotlib.sourceforge.net/examples/pylab_examples/layer_images.html

def func3(x,y):
    return (1- x/2 + x**5 + y**3)*exp(-x**2-y**2)

# make these smaller to increase the resolution
dx, dy = 0.05, 0.05

x = arange(-3.0, 3.0, dx)
y = arange(-3.0, 3.0, dy)
X,Y = meshgrid(x, y)

xmin, xmax, ymin, ymax = amin(x), amax(x), amin(y), amax(y)
extent = xmin, xmax, ymin, ymax

fig = plt.figure(frameon=False)

Z2 = func3(X, Y)

im2 = imshow(Z2, cmap=cm.jet, alpha=.9, interpolation='bilinear', extent=extent)

show()

This will result in the following (ignore the chequered background):