Question

I want to adjust the colour levels of an image in python. I can use any python library that can easily be installed on my Ubuntu desktop. I want to do the same as ImageMagick’s -level ( http://www.imagemagick.org/www/command-line-options.html#level ). PIL (Python Image Library) doesn’t seem to have it. I have been calling convert on the image and then reading in the file back again, but that seems wasteful. Is there a better / faster way?

Answer 1

If I understood correctly the -level option of ImageMagick, then the level_image function I provide should do what you want.

Two things to note:

• the speed definitely can be improved
• it currently only works with RGB images
• the algorithm goes through the HSV colorspace, and affects only the V (brightness) component

The code:

import colorsys

class Level(object):

    def __init__(self, minv, maxv, gamma):
        self.minv= minv/255.0
        self.maxv= maxv/255.0
        self._interval= self.maxv - self.minv
        self._invgamma= 1.0/gamma

    def new_level(self, value):
        if value <= self.minv: return 0.0
        if value >= self.maxv: return 1.0
        return ((value - self.minv)/self._interval)**self._invgamma

    def convert_and_level(self, band_values):
        h, s, v= colorsys.rgb_to_hsv(*(i/255.0 for i in band_values))
        new_v= self.new_level(v)
        return tuple(int(255*i)
                for i
                in colorsys.hsv_to_rgb(h, s, new_v))

def level_image(image, minv=0, maxv=255, gamma=1.0):
    """Level the brightness of image (a PIL.Image instance)
    All values ≤ minv will become 0
    All values ≥ maxv will become 255
    gamma controls the curve for all values between minv and maxv"""

    if image.mode != "RGB":
        raise ValueError("this works with RGB images only")

    new_image= image.copy()

    leveller= Level(minv, maxv, gamma)
    levelled_data= [
        leveller.convert_and_level(data)
        for data in image.getdata()]
    new_image.putdata(levelled_data)
    return new_image

If there is some way to do the RGB→HSV conversion (and vice versa) using PIL, then one can split into the H, S, V bands, use the .point method of the V band and convert back to RGB, speeding up the process by a lot; however, I haven’t found such a way.