Matplotlib figure not updating on data change

Question

I'm implementing an image viewer using matplotlib . The idea is that changes being made to the image (such as filter application) will update automatically.

I create a Figure to show the inital image and have added a button using pyQt to update the data. The data does change, I have checked, but the Figure does not. However , if after I've pressed the filter application button, I move the image using matplotlib 's standard tool bar, the image is then updated.

I assume I'm doing something wrong when updating the image, but since the fact of moving it actually forces the update, it then shows the data change. I would like for this to happen when I press the button, though.

Below is some of the code. This is the initial figure initialization, which shows the original image:

    self.observableFig = Figure((4.0, 4.0), dpi=100)
    self.canvas = FigureCanvas(self.observableFig)
    self.canvas.setParent(self.observableWindow)
    self.canvas.setFocusPolicy(Qt.StrongFocus)
    self.canvas.setFocus()

    self.canvas.mpl_connect('button_press_event', self.on_click)

    # Showing initial data on Window
    self.observableFig.clear()
    self.observableAxes = self.observableFig.add_subplot(1, 1, 1)
    min, max = self.min, self.max
    self.observableAxes.imshow(self.data, vmin=min, vmax=max, origin='lower') 

And this is the event for when the button that changes the data is pressed:

    self.observableAxes.imshow(self.data/2, origin='lower')
    #    plt.clf()
    #    plt.draw()
    #    plt.show()

I have tried draw() , show() , basically anything I've found on pyplot about this. I have also tried both with and without plt.ion() at the beginning, but it hasn't made a difference in this.

Thanks in advance.

Answer 1

The reason that nothing is updating is that you're trying to use pyplot methods for a figure that's not a part of the pyplot state machine. plt.draw() won't draw this figure, as plt doesn't know the figure exists.

Use fig.canvas.draw() instead.

Regardless, it's better to use fig.canvas.draw() that plt.draw() , as it's clear which figure you're drawing (the former draws one, the latter draws all, but only if they're tracked by pyplot ).

Try something along these lines:

import numpy as np
import matplotlib.pyplot as plt

data = np.random.random((10,10))

# To make a standalone example, I'm skipping initializing the 
# `Figure` and `FigureCanvas` and using `plt.figure()` instead...
# `plt.draw()` would work for this figure, but the rest is identical.
fig, ax = plt.subplots()
ax.set(title='Click to update the data')
im = ax.imshow(data)

def update(event):
    im.set_data(np.random.random((10,10)))
    fig.canvas.draw()

fig.canvas.mpl_connect('button_press_event', update)
plt.show()