Detect if an image is negative

Question

I'm trying to clean some chest X-ray data to feed to a CNN. In my dataset are currently many images where the bones are shown in white (higher pixel value than the background), like so:

While others show the bones in a darker color than the background, like this:

Can you show me a way to label the two? I have no other external info about the image, though it can be assumed they are the same size (

Assuming they have the same size (about 1000x2000) and that the first row of pixels has more than 1 different values (i.e. is not a blank border), I've written this simple code to compare a middle-ish pixel to the top-left one (likely to be part of the background).

if img[0,0] > img[500, 500]: # if background lighter than center
        img = 255 - img # make the image negative

As you can see even from these samples I posted, this comparison is not always a good indicator (sometimes there is a halo round the background or pixel in [500,500] can be similar to background). Is there some more reliable other way to detect if an image of this kind is negative or not?

Consider that in the dataset are some images with very few details and shading, such as

Answer 1

Following the suggestion from Christoph Rackwitz, I get good result with this approach:

• Optionally normalize the image
• Extract corners from the image, more likely to be background (I considered just the top corners)
• Extract center from the image
• Compare the regions with a threshold (fixed or average greyscale value of image), obtaining list of truth values
• Compare amount of pixels above the threshold (in percentage) to decide if background is lighter or darker than the center of the image: hence decide to invert the values.

def invert_if_negative(img):
    img = my_contrast_stretch(img)
    # assuming image has fixed size of (1396, 1676)
    # corners
    top_left  = img[:200, :200].flatten()
    top_right = img[:200, 1250:].flatten()
    # more or less center
    center    = img[1000:1300, 500:800].flatten()
    threshold = 120 # or computed from average
    top_left  = top_left > threshold
    top_right = top_right > threshold
    center    = center > threshold
    perc_white_corners = (sum(top_left) + sum(top_right)) / (len(top_left) + len(top_right))
    perc_white_center = sum(center) / len(center)
    if perc_white_corners > perc_white_center:
        img = 255 - img
    return img

def my_contrast_stretch(img):
    if img.dtype == np.float64:
        img = (img * 255).astype(np.uint8)
    M=np.max(img)
    m=np.min(img)
    res = img - m
    res = res * (255 / (M - m))
    return res.astype(np.uint8)