Reports

It sounds like your Raspberry Pi is hanging or freezing right when cv2.imshow() is called — the window shows up black, small, and the system becomes unresponsive. That's definitely not normal OpenCV behavior, especially for a simple image display.

Here’s a breakdown of what could be going wrong and how to fix or debug it:

Likely Cause: OpenCV GUI backend and Raspberry Pi GUI conflict

OpenCV’s imshow() relies on a GUI environment (like X11 on Linux). If your Raspberry Pi is:

Running headless (no monitor or desktop GUI)
Not running in a graphical session (like LXDE/X11)
Or your script is being launched in a virtual terminal or via SSH

Then imshow() has no display to show the window, and trying to do so can cause serious issues (including freezes).

Solutions

1. Check if you're running in GUI mode

Make sure you're running your script from a desktop environment on the Pi:

NOT via SSH unless X11 forwarding is set up.
If headless, consider using matplotlib or saving the image to a file instead.

2. Test a minimal OpenCV window

Before running your full function, test this:

import cv2
import numpy as np

img = np.zeros((300, 600, 3), dtype=np.uint8)
cv2.imshow("Test", img)
cv2.waitKey(0)
cv2.destroyAllWindows()

If even this causes the Pi to hang or show a black window → the issue is with the OpenCV GUI backend setup.

3. Use `cv2.namedWindow()` explicitly

Try specifying the window type before imshow():

cv2.namedWindow("Confirm Images (Front & Back)", cv2.WINDOW_NORMAL)
cv2.imshow("Confirm Images (Front & Back)", combined_img)

This gives more control over how OpenCV creates the window and can prevent weird size or render bugs.

4. Don't move the window manually

The cv2.moveWindow() call may cause issues on limited environments. Try commenting this out:

# cv2.moveWindow("Confirm Images (Front & Back)", 1, 1)

It’s not needed unless you know the Pi's screen resolution and want precise placement.

5. Avoid `waitKey(0)` in non-interactive environments

Instead of waiting forever for a key press, try a timeout:

key = cv2.waitKey(5000)  # wait 5 seconds
if key == ord('y'):
    print("Images confirmed.")
elif key == ord('n'):
    print("Retaking images...")
else:
    print("No key pressed. Defaulting to confirm.")

6. Print debug info before GUI

Before calling imshow(), print out image shapes and check for None:

print(f"Front image shape: {front_img.shape}")
print(f"Back image shape: {back_img.shape}")

This confirms the images loaded and resized properly.

Alternative: Save the image and preview in another way

If OpenCV GUI keeps failing, save and preview:

cv2.imwrite("combined.jpg", combined_img)
print("Saved combined image. Please open 'combined.jpg' manually to confirm.")

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