Iterating a function over the RGB values of individual pixels

Question

I've already opened my image and can access the individual pixels' RGB values, but what I'm trying to do now is apply a function to the RGB values of each of the pixels individually. That is, I don't want to apply it the same way across all the pixels in the entire image; I want to apply it differently depending on whether, for each individual pixel, the blue value is > red > green (rather than green > red > blue, etc, etc).

So my question is, how do I access the individual RGB elements within each pixel (as opposed to accessing all of the red, green, and blue values across the entire image at once)? Eventually my question will be "what's the fastest way to do this?" since it's obviously going to take a while to apply a function across each pixel individually but for now I'd be happy just to have any solution at all.

Thanks for any suggestions.

EDIT for clarity/more specificity:

I'm actually trying to apply a different set of instructions depending the ordering of 127.5 - abs(127.5 - red/green/blue)), not simply on the order of red>green>blue (as stated initially above, bc I was trying to simplify). Once that ordering is determined for a given pixel, then the appropriate set of instructions is applied. Again, this is pixel-by-pixel -- I'm not ordering things based on ALL red values across the image, just the rgbs of the individual pixels. So what I'm trying to do would look something like this (here I'm playing out just one of the six possible orders; I've omitted the five other possibilities for brevity):

def rgb_manip(red,green,blue):
    r_max = int(127.5 - abs(127.5 - red))
    g_max = int(127.5 - abs(127.5 - green))
    b_max = int(127.5 - abs(127.5 - blue))
    if r_max >= g_max >= b_max:
        if r_max >= g_max  +  b_max:
            new_red = red + g_max + b_max
            new_green = green - g_max
            new_blue = blue - b_max
        else:
            new_red = red + r_max
            new_green = green - r_max + b_max
            new_blue = blue - b_max
    # elif... And so on, with a different set of instructions for each of the 6 possibilities depending on the order of the r_max, g_max, b_max values (e.g., r_max >= b_max >= g_max or g_max >= r_max >= b_max, etc, etc)

Answer 1

If you convert your image into an array, you can access the RGB values for one pixel, or one of the R, G, or B values for all pixels:

from __future__ import division
import numpy as np
from PIL import Image

im = Image.open(imfile)
arr = np.asarray(im)

arr[..., 0]  # All Red values
arr[..., 1]  # All Green values
arr[..., 2]  # All Blue values
arr[0, 0]    # RGB for first corner pixel
arr[m, n]    # RGB for pixel at [m, n]
arr[m, n, 0] # R value for pixel [m, n]
arr[m, n, c] # value for color c at pixel [m, n]

You can get a ranking for each pixel using argsort, as in:

def transform(a, c=255/2):
    return c - np.abs(c - a)

ranking = transform(arr).argsort(axis=-1)

which ranks the criterion values from smallest to largest value along the last (color) axis. So this gives a new array where each 'color' array instead of being the RGB values are the sorting of the transformed R, B, and G values (call them "R', B', G'"), so if the corner pixel had G' > B' > R', then ranking[0, 0] would be [0, 2, 1] because R' (0) is smallest, then next is B' (2), finally the largest is G' (1).

The advantage of doing the above is that you have an array that says which method to use on which pixel. It can have at most six orderings of the transformed channels. I suggest defining a separate function like so for each of orderings. Then, only one decision must be made within the function (the second nested if/else in your example), and it can be done with np.where which applies one thing to parts of an array where a condition is met, and another thing to the rest. This only works for two options, but if there are multiple options (if/elif/else), other techniques can work equally well.

def bgr(a):
    """ for when B' < G' < R'
    """
    t = transform(a)
    red, green, blue = a.transpose([2,0,1])
    # same as: red, green, blue = a[..., 0], a[..., 1], a[..., 2] 
    r_max, g_max, b_max = t.transpose([2,0,1])
    assert np.all((b_max <= g_max) & (g_max <= r_max)), "doesn't match rank"
    condition = r_max >= g_max + b_max
    new_red = np.where(condition, red + g_max + b_max, red + r_max)
    new_green = np.where(condition, green - g_max, green - r_max + b_max)
    new_blue = blue - b_max
    return np.dstack([new_red, new_green, new_blue])

this function only works for the first if in yours. I would make a new function for each of those six things, and fill them into a dict like so:

functions = {
        (0, 1, 2) : rgb, # for R'<G'<B'
        (0, 2, 1) : rbg, # for R'<B'<G'
        #etc...
        }

If your output has RGB values too:

out = np.empty_like(arr)

Then loop through all six rankings/functions:

for rank, func in functions.items():
    mask = np.all(transform(arr).argsort(-1) == rank, -1)
    out[mask] = func(arr[mask])