A script that adds a watermark to the bottom right of each picture in a directory of pictures

watermark.py

from PIL import Image, ImageDraw
import os, math

#SETTINGS
WIDTH_RATIO = 3  #Ratio of image min width to watermark width
MIN_WIDTH = 200 #Minimum width of watermark. In my watermark, it is 1/3 of the normal width 
MIN_HEIGHT = 40 #Minimum height of watermark. In my watermark ,it is 1/3 of the normal height
WATERMARK_LOCATION = "watermark.png"

#Most helpful source: http://stackoverflow.com/questions/13662184/python-pil-lighten-transparent-image-and-paste-to-another-one 

def add_watermark(photo, wm_img):
    if photo == wm_img.split("\\")[-1]:
        print "%s is watermark" % photo
        return
    elif "_wm.png" in photo or "_wm.jpg" in photo:
        print "%s is already watermarked" % photo
        return
    print "Processing %s" % photo
    
    # Open the original image
    main = Image.open(photo)
    watermark = Image.open(wm_img)
    main_size = main.size
    wm_size = watermark.size
    
    resize_side = min(main.size[0], main.size[1])
    resize_width = (int)(resize_side/WIDTH_RATIO)
    resize_ratio = (float)(resize_width)/((float)(wm_size[0]))
    resize_height = (int)(wm_size[1]*resize_ratio)
    if resize_width <= MIN_WIDTH:
        resize_width = MIN_WIDTH
        resize_height = MIN_HEIGHT
    watermark = watermark.resize((resize_width, resize_height))

    x_offset = main_size[0]-resize_width
    y_offset = main_size[1]-resize_height
    
    #Estimate watermark area "chaoticness"
    pixel_count = 0
    shift_count = 0
    prev_pixel_up = None
    prev_pixel_left = None
    
    if main.mode == "P":
        main = main.convert("RGB")
    for x in range(x_offset, main_size[0]):
        for y in range(y_offset, main_size[1]):
            pixel = main.getpixel((x, y))
            
            truncated_pixel = trunc_pixel(pixel)
            if prev_pixel_up == None or y == y_offset:
                prev_pixel_up = truncated_pixel
            else:
                prev_pixel_up = trunc_pixel(main.getpixel((x, y-1)))

            if prev_pixel_left == None or x == x_offset:
                prev_pixel_left = truncated_pixel  
            else:
                prev_pixel_left = trunc_pixel(main.getpixel((x-1, y)))

            if truncated_pixel != prev_pixel_left:
                shift_count = shift_count+1

            if truncated_pixel != prev_pixel_up:
                shift_count = shift_count+1

            pixel_count = pixel_count+1

    shifts = (float)(shift_count/2)/(float)(pixel_count) #Approximation of number of shifts, i.e. how chaotic the picture is

    #The more jumbled the image under the watermark is, the more opaque the watermark will be.
    new_alpha = min(0.8, 0.4+(((int)(shifts*10))/20.0))
   
    wm_bands = list(watermark.split())
    if len(wm_bands) == 4: #assumes alpha is last band
        wm_bands[3] = wm_bands[3].point(lambda x: x*new_alpha) #Adjust opacity
    watermark.putalpha(wm_bands[3])
    
    # Paste the watermark (with alpha layer) at the bottom right and save it
    main.paste(watermark, (x_offset, y_offset), watermark)
    new_name = photo.replace(".", "_wm.")
    main.save(new_name, "PNG")

    print "Finished with %s" % photo

def trunc_pixel(pixel):
    #Dividing each value by 25 approximates its color, i.e 250, 50, 55  is equal to  255, 57, 59. Change the 25 to increase or decrease precision
    tpixel = ((math.floor(pixel[0]/25)), (math.floor(pixel[1]/25)), (math.floor(pixel[2]/25)))
    return tpixel

def loopdirs(paths, wm):
    for p in paths:
        for fn in os.listdir(p):
            fl = fn.lower()
            if fl[-4:] == ".png" or fl[-4:] == ".jpg":
                add_watermark(p+fn, wm)


if __name__ == '__main__':
    dirs = [] #Each dir you want to iterate through
    loopdirs(WATERMARK_LOCATION)