Why Does OpenCV Alter Image Colors When Loading and Saving?

The behavior you're observing is most likely due to two factors:

1. Default Channel Handling:
   By default, OpenCV’s imread function loads images in color (BGR) and discards any alpha channel. This means that if your original image has an alpha channel (e.g., PNG with transparency), that information is lost when you load it with the default flag. Later, when you save the image with imwrite, it only contains the BGR channels, so the overall appearance (including how colors blend with the background) might differ from the original.

2. Color Channel Order:
   OpenCV uses BGR order by default, while many image formats and viewers assume an RGB or even RGBA order. This switch might also contribute to the slight color differences you see between the original and the processed image.

How to Preserve the Alpha Channel

To ensure that the alpha channel is retained when you load the image, you should use the flag IMREAD_UNCHANGED when calling imread. This flag tells OpenCV to load the image as-is (including the alpha channel). For example:

PYTHONCopied
import cv2

# Load the image including the alpha channel
image = cv2.imread('path_to_image.png', cv2.IMREAD_UNCHANGED)

# Check if the image has an alpha channel
if image is not None and image.shape[2] == 4:
    print("Image loaded with alpha channel.")
else:
    print("Alpha channel not present; loaded image has ", image.shape if image is not None else "no data.")

# Save the image as-is
cv2.imwrite('output_image.png', image)

Additional Considerations

• Color Conversion:
  If you need to display the image using libraries that expect an RGB image (e.g., Matplotlib), you might need to convert from BGR (or BGRA) to RGB (or RGBA):

  PYTHONCopied
  if image.shape[2] == 4:  # For BGRA to RGBA
      image = cv2.cvtColor(image, cv2.COLOR_BGRA2RGBA)
  else:  # For BGR to RGB
      image = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)

• Avoiding Unnecessary Conversion:
  Using IMREAD_UNCHANGED ensures no data is lost during the loading phase. Just be mindful when processing or displaying the image, as some functions or viewers might assume a specific channel order.

By loading the image with cv2.IMREAD_UNCHANGED, you can retain the exact image data of the original file, including its alpha channel, and thereby minimize differences when saving it back to disk with imwrite.