Normalize histogram (brightness and contrast) of a set of images using Python Image Library (PIL)

Question

I have a script which uses Google Maps API to download a sequence of equal-sized square satellite images and generates a PDF. The images need to be rotated beforehand, and I already do so using PIL.

I noticed that, due to different light and terrain conditions, some images are too bright, others are too dark, and the resulting pdf ends up a bit ugly, with less-than-ideal reading conditions "in the field" (which is backcountry mountain biking, where I want to have a printed thumbnail of specific crossroads).

(EDIT) The goal then is to make all images end up with similar apparent brightness and contrast. So, the images that are too bright would have to be darkened, and the dark ones would have to be lightened. (by the way, I once used imagemagick autocontrast, or auto-gamma, or equalize, or autolevel, or something like that, with interesting results in medical images, but don't know how to do any of these in PIL).

I already used some image corrections after converting to grayscale (had a grayscale printer a time ago), but the results weren't good, either. Here is my grayscale code:

#!/usr/bin/python

def myEqualize(im)
    im=im.convert('L')
    contr = ImageEnhance.Contrast(im)
    im = contr.enhance(0.3)
    bright = ImageEnhance.Brightness(im)
    im = bright.enhance(2)
    #im.show()
    return im

This code works independently for each image. I wonder if it would be better to analyze all images first and then "normalize" their visual properties (contrast, brightness, gamma, etc).

Also, I think it would be necessary to perform some analysis in the image (histogram?), so as to apply a custom correction depending on each image, and not an equal correction for all of them (although any "enhance" function implicitly considers initial contitions).

Does anybody had such problem and/or know a good alternative to do this with the colored images (no grayscale)?

Any help will be appreciated, thanks for reading!

Answer 1

What you are probably looking for is a utility that performs "histogram stretching". Here is one implementation. I am sure there are others. I think you want to preserve the original hue and apply this function uniformly across all color bands.

Of course there is a good chance that some of the tiles will have a noticeable discontinuity in level where they join. Avoiding this, however, would involve spatial interpolation of the "stretch" parameters and is a much more involved solution. (...but would be a good exercise if there is that need.)

Edit:

Here is a tweak that preserves image hue:

import operator

def equalize(im):
    h = im.convert("L").histogram()
    lut = []
    for b in range(0, len(h), 256):
        # step size
        step = reduce(operator.add, h[b:b+256]) / 255
        # create equalization lookup table
        n = 0
        for i in range(256):
            lut.append(n / step)
            n = n + h[i+b]
    # map image through lookup table
    return im.point(lut*im.layers)