Functions to convert RGB values to HSV and vice versa, and a code sample to get them working on an image.

hsv_to_rgb.py

def hsv_to_rgb(pixel):
    """ converts a pixel expressed in RGB to an HSV expression
        see https://en.wikipedia.org/wiki/HSL_and_HSV#HSV_to_RGB
        Args:
            pixel<(h, s, v)>:
                h <float[0-360]>: hue
                s <float[0-1]>: saturation
                v <float[0-1]>: "value" (brightness)
        Return:
            r <int[0-255]>: red value
            g <int[0-255]>: green value
            b <int[0-255]>: blue value
    """
    h, s, v = pixel

    chroma = v * s
    h /= 60
    x = chroma * (1 - abs((h % 2) - 1))

    if 0 <= h <= 1:
        r1, g1, b1 = chroma, x, 0
    elif 1 < h <= 2:
        r1, g1, b1 = x, chroma, 0
    elif 2 < h <= 3:
        r1, g1, b1 = 0, chroma, x
    elif 3 < h <= 4:
        r1, g1, b1 = 0, x, chroma
    elif 4 < h <= 5:
        r1, g1, b1 = x, 0, chroma
    elif 5 < h < 6:
        r1, g1, b1 = chroma, 0, x
    else:
        r1, g1, b1 = 0, 0, 0

    m = v - chroma

    r = int(round((r1 + m) * 255))
    g = int(round((g1 + m) * 255))
    b = int(round((b1 + m) * 255))

    return r, g, b
  

main.py

from rgb_to_hsv import rgb_to_hsv
from hsv_to_rgb import hsv_to_rgb
import png  # example in __main__ uses pypng
import sys

from itertools import chain

def read_img(f):
    """ Reads the image. This returns a tuple of the form
        (height, width, raw_image_data, other_metadata_dict)
    """
    img = png.Reader(file=f).read()
    raw_img = img[2]
    step = 4 if img[3]['alpha'] else 3

    processed_img = []
    for row in raw_img:
        new_row = []
        for i in range(0, len(row), step):
            r, g, b = row[i], row[i + 1], row[i + 2]
            new_row.append((r, g, b))
        processed_img.append(new_row)
    return processed_img

def convert_image(img, fun, args=()):
    """ Applies a pixel-wise function to an entire image.
        Args:
            img <list<list<tuple>>>: an image represented as a list of rows
                with each row a list of pixels with each pixel a tuple
            fun <func>: the function to apply
            args <tuple>: arguments to pass on to the pixel-wise function
        Return:
            new_img: a new image (same representation as argument) after
                the pixel-wise function has been applied
    """
    new_img = []
    for row in img:
        new_row = []
        for px in row:
            new_row.append(fun(px, *args))
        new_img.append(new_row)
    return new_img
 

if __name__ == '__main__':
    # getting filename
    fname = sys.argv[1]
    
    with open(fname, 'rb') as f:
        img = read_img(f)
    
    # converting image from RGB to HSV
    img = convert_image(img, rgb_to_hsv)
    
    # do stuff with your HSV image
    
    # converting modified image back to RGB from HSV
    img = convert_image(img, hsv_to_rgb)
    
    # overwriting image file
    w = png.Writer(len(img[0]), len(img), greyscale=False)
    img = [chain(*row) for row in img]  # only necessary wih pypng 0.0.20+
    with open(fname, 'wb') as f:
        w.write(f, img)
    

rgb_to_hsv.py

def rgb_to_hsv(pixel):
    """ converts a pixel expressed in RGB to an HSV expression
        see https://en.wikipedia.org/wiki/HSL_and_HSV#From_RGB
        Args:
            pixel<(r, g, b)>:
                r <int[0-255]>: red value
                g <int[0-255]>: green value
                b <int[0-255]>: blue value
        Return:
            h <float[0-360]>: hue
            s <float[0-1]>: saturation
            v <float[0-1]>: "value" (brightness)
    """
    r, g, b = pixel

    r /= 255
    g /= 255
    b /= 255

    MAX = max(r, g, b)
    MIN = min(r, g, b)

    if MAX == MIN:
        h = 0
    elif MAX == r:
        h = 60 * (((g - b) / (MAX - MIN)) + 0)
    elif MAX == g:
        h = 60 * (((b - r) / (MAX - MIN)) + 2)
    else:
        h = 60 * (((r - g) / (MAX - MIN)) + 4)
    if h < 0:
        h += 360

    if MAX == 0:
        s = 0
    else:
        s = (MAX - MIN) / MAX

    v = MAX
    return h, s, v

This code is released under the .