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.

Is there any way to do this?

It is possible to create rectangular plots with color gradients easily enough (like Gradient facecolor matplotlib bar plot and this) but I can’t figure out how to do the same for circles/ellipses.

Answer 1

Here is function example using the idea from Alex’s post

import matplotlib.pyplot as plt,numpy as np

def gauplot(centers, radiuses, xr=None, yr=None):
        nx, ny = 1000.,1000.
        xgrid, ygrid = np.mgrid[xr[0]:xr[1]:(xr[1]-xr[0])/nx,yr[0]:yr[1]:(yr[1]-yr[0])/ny]
        im = xgrid*0 + np.nan
        xs = np.array([np.nan])
        ys = np.array([np.nan])
        fis = np.concatenate((np.linspace(-np.pi,np.pi,100), [np.nan]) )
        cmap = plt.cm.gray
        cmap.set_bad('white')
        thresh = 3
        for curcen,currad in zip(centers,radiuses):
                curim=(((xgrid-curcen[0])**2+(ygrid-curcen[1])**2)**.5)/currad*thresh
                im[curim<thresh]=np.exp(-.5*curim**2)[curim<thresh]
                xs = np.append(xs, curcen[0] + currad * np.cos(fis))
                ys = np.append(ys, curcen[1] + currad * np.sin(fis))
        plt.imshow(im.T, cmap=cmap, extent=xr+yr)
        plt.plot(xs, ys, 'r-')

And here is what you get when you run

    gauplot([(0,0), (2,3), (5,1), (6, 7), (6.1, 6.1)], [.3,. 4, .5, 1, .4], [-1,10], [-1,10])
             #           centers of circles           # radii of circles#