How to check if an RGB image contains only one color?

Question

I'm using Python and PIL.

I have images in RGB and I would like to know those who contain only one color (say #FF0000 for example) or a few very close colors (#FF0000 and #FF0001).

I was thinking about using the histogram but it is very hard to figure out something with the 3 color bands, so I'm looking for a more clever algorithm.

Any ideas?

ImageStat module is THE answer! Thanks Aaron. I use ImageStat.var to get the variance and it works perfectly.

Here is my piece of code:

from PIL import Image, ImageStat

MONOCHROMATIC_MAX_VARIANCE = 0.005

def is_monochromatic_image(src):
    v = ImageStat.Stat(Image.open(src)).var
    return reduce(lambda x, y: x and y < MONOCHROMATIC_MAX_VARIANCE, v, True)

Answer 1

First, you should define a distance between two colors. Then you just have to verify for each pixel that it's distance to your color is small enough.

Answer 2

Try the ImageStat module. If the values returned by extrema are the same, you have only a single color in the image.

Answer 3

Here's a little snippet you could make use of :

import Image

im = Image.open("path_to_image")
width,height = im.size

for w in range(0,width):
  for h in range(0,height):
     # this will hold the value of all the channels
     color_tuple = im.getpixel((w,h))
     # do something with the colors here

Maybe use a hash and store the tuples as the key and it's number of appearances as value?