Opening and saving a .bmp file changes bit depth

Question

I have a file in bitmap format test.bmp. When looking at its properties, the bit depth of the file is 32. This is correct, as the file format is supposed to be RGBA.

For test purposes, I open this file using cv2 and then save it:

img_path = os.path.join(path_to_folder, "test.bmp")
img = np.array(cv2.imread(img_path, flags=cv2.IMREAD_UNCHANGED))

When I print the shape, I lose information on the alpha channel.

img.shape - (1200, 1920, 3)

I save the image again to see if the bit depth is conserved:

out_f_name = os.path.join(save_to_test, "test_save.bmp")
cv2.imwrite(out_f_name, img)

When looking at the bit depth of the saved image, I get 24.

This is a problem as I need an output depth of 32 for further use.

I use version 4.7.0 of cv2

Answer 1

I have tried what you describe, but I'm not able to reproduce the error you are having, for me everithing works well. The original BMP with 4 channels is loaded correctly by OpenCV and when I write the image, the resulting one is with 32 bits depth.

here is my code:

import cv2 as cv
import numpy as np

img_path = "C:\\Users\\me\\Downloads\\snail4.bmp"
im = cv.imread(img_path, flags=cv.IMREAD_UNCHANGED)
print(im.shape)
(height, width, channels) = im.shape
cv.imshow('original', im)
for i in range(channels):
  s = im[:, :, i]
  cv.imshow(f'channel {i}',s)

cv.waitKey(0)
cv.destroyAllWindows()

cv.imwrite(img_path[:-4] + '_imwritten.bmp', im)

I have python 3.10.4, numpy==1.24.1 and opencv-python==4.7.0.68

hope that helps