Detect if image is color, grayscale or black and white with Python/PIL

Question

I extract pages images from a PDF file in jpeg format and I need to determine if each image is much more grayscale, color ou black and white (with a tolerance factor).

Answer 1

I have found a way to guess this with the PIL ImageStat module. Thanx to this post for the monochromatic determination of an image.

from PIL import Image, ImageStat

MONOCHROMATIC_MAX_VARIANCE = 0.005
COLOR = 1000
MAYBE_COLOR = 100

def detect_color_image(file):
    v = ImageStat.Stat(Image.open(file)).var
    is_monochromatic = reduce(lambda x, y: x and y < MONOCHROMATIC_MAX_VARIANCE, v, True)
    print file, '-->\t',
    if is_monochromatic:
        print "Monochromatic image",
    else:
        if len(v)==3:
            maxmin = abs(max(v) - min(v))
            if maxmin > COLOR:
                print "Color\t\t\t",
            elif maxmin > MAYBE_COLOR:
                print "Maybe color\t",
            else:
                print "grayscale\t\t",
            print "(",maxmin,")"
        elif len(v)==1:
            print "Black and white"
        else:
            print "Don't know..."

The COLOR and MAYBE_COLOR constant are quick switches to find the differences between color and grayscale images but it is not safe. As an exemple, I have several JPEG images that are view as color but in real are grayscale with some color artefacts due to a scan process. That's why I have another level to note really shure color image from the others.

If someone has a better approch, let me know.