Matplotlib: Convert plot to numpy array without borders

Question

I use scatter() to produce this plot:

Then I convert the plot to a numpy array for further processing and get this:

How can I get rid of the border?

Here is my code:

import matplotlib.pyplot as plt
import numpy as np

n = 500
domain_size = 1000

x = np.random.randint(0,domain_size,(n,2))

fig, ax = plt.subplots(frameon=False)
fig.set_size_inches((5,5))
ax.scatter(x[:,0], x[:,1], c="black", s=200, marker="*")
ax.set_xlim(0,domain_size)
ax.set_ylim(0,domain_size)
fig.add_axes(ax)

fig.canvas.draw()

X = np.array(fig.canvas.renderer._renderer)
X = 0.2989*X[:,:,1] + 0.5870*X[:,:,2] + 0.1140*X[:,:,3]

plt.show()
plt.close()

plt.imshow(X, interpolation="none", cmap="gray")
plt.show()

Crop away the borders using a slice, or set xlim and ylim on the imshow plot... — Nils Werner
– Nils Werner, Commented Jun 27, 2018 at 10:31
@NilsWerner For further processing I need a numpy array without white borders. Cropping away the borders is not a clean solution, since the size of the borders depends on fig.set_size_inches(()). Setting xlim or ylim has no effects on X and therefore does not solve my problem. Any other suggestion? — Gilfoyle
– Gilfoyle, Commented Jun 27, 2018 at 17:49

swatchai · Accepted Answer · 2018-06-27 11:28:29Z

2

You should turn off the axis each time before rendering the plots. Here is the modified code that does so.

import matplotlib.pyplot as plt
import numpy as np

n = 500
domain_size = 100
x = np.random.randint(0,domain_size,(n,2))

fig, ax = plt.subplots()
fig.set_size_inches((5,5))
ax.scatter(x[:,0], x[:,1], c="black", s=200, marker="*")
ax.set_xlim(0,domain_size)
ax.set_ylim(0,domain_size)
ax.axis('off')

fig.add_axes(ax)
fig.canvas.draw()

# this rasterized the figure
X = np.array(fig.canvas.renderer._renderer)
X = 0.2989*X[:,:,1] + 0.5870*X[:,:,2] + 0.1140*X[:,:,3]

plt.show()
plt.close()

# plot the image array X
fig2, ax2 = plt.subplots()
plt.imshow(X, interpolation="none", cmap="gray")

ax2.axis('off')

plt.show()

The resulting plot:

answered Jun 27, 2018 at 11:28

swatchai

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3 Comments

Gilfoyle Over a year ago

The border is still there when I use your code. Any other suggestion?

swatchai Over a year ago

Do you mean white border?

Gilfoyle Over a year ago

Yes, when I use your code, I still get a white border. Any suggestion how to remove it without using a slice?

Gilfoyle · Accepted Answer · 2019-03-26 08:16:22Z

2

I figured out how to get rid of the borders. Just replace

fig, ax = plt.subplots(frameon=False)

with

fig = plt.figure()
ax = fig.add_axes([0.,0.,1.,1.])

and it works just fine.

edited Mar 26, 2019 at 8:16

answered Jul 1, 2018 at 15:36

Gilfoyle

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