pythonopencvimage-processing

Add colored overlay on a gray image

I am trying to create an image similar to the attached image.

required

Here is my code:

mri_image = cv2.imread('mri_image.png', cv2.IMREAD_GRAYSCALE) 
mask_image = cv2.imread('mask_image.png', cv2.IMREAD_GRAYSCALE) # Read mask image (contains black and white)

mri_image_rgb = cv2.cvtColor(mri_image, cv2.COLOR_GRAY2RGB)


colored_mask = np.zeros_like(mri_image_rgb)
colored_mask[mask_image > 0] = [255, 0, 0]

overlayed_image = cv2.addWeighted(mri_image_rgb, 0.7, colored_mask, 0.3, 0)

cv2.imwrite('overlayed_image.png', overlayed_image)

The resultant image has problems; the tissue part appears all black and not grayscale values and the red mask is not transparent.

Solution

  • addWeighted() is the wrong thing to do.

    To "colorize" parts of an image, you need to multiply.

    overlay = cv.cvtColor(mask_image, cv.COLOR_GRAY2BGR) / 255

overlay_color = np.array([0, 0, 255]) / 255 # BGR order
overlay = overlay * overlay_color

overlay_strength = 0.3
overlay = 1 - ((1 - overlay) * overlay_strength)

composite = (mri_image_color * overlay).astype(np.uint8)