Want to save image using OpenCV but Matplotlib insists I save a figure instead

Question

OpenCV 4.7, Matplotlib 3.7.1, Spinnaker-Python 3.0.0.118, Python 3.10, Win 10 x64

I'm acquiring images from a FLIR thermal camera via their Spinnaker API.

I am displaying the images using OpenCV & concurrently displaying a histogram of the images using Matplotlib. The relevant code snippet, (that sits in a function which also contains camera set up & error handling)

cam.BeginAcquisition()
print('Acquiring images...')
print('Press enter to close the program..')

fig = plt.figure(1)
fig.canvas.mpl_connect('close_event', handle_close)
acq_count = 0 # number of saved images
        
# Retrieve and display images
while(continue_recording):
    try:
                
        image_result = cam.GetNextImage(1000)

        #  Ensure image completion
        if image_result.IsIncomplete():
            print('Image incomplete with image status %d ...' % image_result.GetImageStatus())

        else:                    

            # Getting the image data as a numpy array
            image_data = image_result.GetNDArray()
                      
            image_data = cv2.rotate(image_data, cv2.ROTATE_90_COUNTERCLOCKWISE)
            image_data = cv2.flip(image_data, 1)
            image_copy = image_data
                
            count, bins = np.histogram(image_data.flatten())
                        
            cv2.imshow("FLIR", image_data)
                    
            plt.stairs(count, bins)
            plt.pause(0.001)
            plt.clf()
            
            # if 's' pressed then save image data           
            if cv2.waitKey(100) & 0xFF == ord('s'):
                        
                f_name = "test_" + str(acq_count) + '.png'
                cv2.imwrite(f_name, image_data)
                acq_count += 1
                        
            # If user presses enter, close the program
            if keyboard.is_pressed('ENTER'):
                        
                print('Program is closing...')
                cv2.destroyAllWindows()
                plt.close('all')             
                input('Done! Press Enter to exit...')
                continue_recording=False                        

            image_result.Release() # clears camera buffer

        except PySpin.SpinnakerException as ex:
            print('Error: %s' % ex)
            return False

cam.EndAcquisition()

When s is pressed though a blocking popup appears asking me where I want to save figure 1 and then proceeds to save the histogram but not the image!

EDIT :: I should also mention that it all works fine as long as there is no Matplotlib figure.

Which window has focus when you press the key? My guess is that it's the matplotlib one, not the OpenCV one. — Dan Mašek, Apr 13 '23 at 15:36
@DanMašek yeah you might be onto something there as the histogram is plotted after the output to the `cv2` window. Wierd though as I'm writing out the contents of `image_data` which is a `numpy` array & not the contents of any of the open windows. Will check when I'm back at work next week. — DrBwts, Apr 13 '23 at 18:47
Right, my point is, the window with focus is the one that will get the keypress event. If it's the matplotlib window, then its own message loop with handle the `s`, which happens to trigger the matplotlib "save plot" feature. OpenCV's `waitKey` (which runs the message loop for the OpenCV window) never gets the keypress, so the `imwrite` never happens in that case. — Dan Mašek, Apr 13 '23 at 18:54
@DanMašek thanks, I disabled `matplotlib`'s callback with `plt.rcParams['keymap.save'].remove('s')` & then used `keyboard`'s event handler instead of OpenCV's keyboard handler & now it works. Do you want to write up an answer/solution? if not I'll do it later — DrBwts, Apr 17 '23 at 08:05

score 0 · Answer 1 · answered Apr 19 '23 at 15:27

As per the discussion in the commets to the OP.

As OpenCV & Matplotlib both have a window open when the keyboard is scanned and an s is pressed Matplotlib's save callback automatically kicks in.

To disable this,

plt.rcParams['keymap.save'].remove('s')

I also got rid of the OpenCV keyboard manager & just used the keyboard libraries keyboard manager.

Want to save image using OpenCV but Matplotlib insists I save a figure instead

1 Answers1