[Python] Avoid Turning Image Black When Removing Background Conditionally Using OpenCV

Problem Description:

I am working on a Python script to remove the background from images using OpenCV. The goal is to:

Check if background removal is needed (for images with simple or uniform backgrounds, it should skip the removal process). Apply background removal only when necessary, and save the resulting image with transparency. Currently, when I apply background removal on images that don't need it (e.g., with simple backgrounds), the image ends up turning black instead of retaining its original appearance.

What I Have Tried:

I check if background removal is needed by calculating the variance of pixel intensities. If the variance is low, I assume the background is uniform, and background removal is not required. For images with a complex background, I apply a thresholding mask to create transparency using OpenCV. Here is my current code:

import cv2
import numpy as np

# Function to check if background removal is needed
def is_background_removal_needed(image_path):
image = cv2.imread(image_path, cv2.IMREAD_GRAYSCALE)
if image is None:
raise FileNotFoundError(f"Image not found: {image_path}")

variance = np.var(image)
print(f"Image variance: {variance:.2f}")
return variance > 100 # Threshold for variance

# Function to remove background and save transparent image
def remove_background(image_path, output_path):
image = cv2.imread(image_path)
if image is None:
raise FileNotFoundError(f"Image not found: {image_path}")

gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
_, mask = cv2.threshold(gray, 200, 255, cv2.THRESH_BINARY_INV)

# Add transparency
result = cv2.cvtColor(image, cv2.COLOR_BGR2BGRA)
result[:, :, 3] = mask # Alpha channel

cv2.imwrite(output_path, result)
print(f"Background removed and saved to: {output_path}")

# Main process function
def process_image(image_path, output_path):
if is_background_removal_needed(image_path):
print("Background removal is needed. Processing...")
remove_background(image_path, output_path)
else:
print("Background removal is NOT needed. Saving the original image.")
image = cv2.imread(image_path)
cv2.imwrite(output_path, image)
print(f"Original image saved to: {output_path}")

# Input and output paths
input_image = "example.jpg" # Path to input image
output_image = "output_image.png"

# Run the process
process_image(input_image, output_image)


The Problem: When I run this script on an image with a simple or uniform background (e.g., an ID card), the output image sometimes turns black after applying the background removal process. However, I want the script to:

Detect that the image does not need background removal. Save the original image without modification if no processing is required. Questions:

Why does the image turn black in some cases when background removal is applied? How can I correctly skip background removal for simple images and retain the original image without altering it? Any insights or suggestions on improving this script would be greatly appreciated. Thank you!

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