watermark.py

from PIL import Image

# 0.2 = take max 20% of the canvas size
thumbnail_factor = 0.2
watermark_padding = 0.05

# (w, h)
POS_TOPLEFT = (0, 0)
POS_TOPRIGHT = (1, 0)
POS_BOTTOMLEFT = (0, 1)
POS_BOTTOMRIGHT = (1, 1)


def watermark_dimensions(canvas_size: tuple, watermark_size: tuple):
    thumbnail_max_side = thumbnail_factor * min(canvas_size[0], canvas_size[1])
    scale = min(thumbnail_max_side / watermark_size[0], thumbnail_max_side / watermark_size[1])
    w = scale * watermark_size[0]
    h = scale * watermark_size[1]
    return (int(w), int(h))


def corner_coordinates(canvas_size: tuple, watermark_size: tuple, corner: tuple):
    padding_pixels = min(canvas_size[0], canvas_size[1]) * watermark_padding

    # side type 0 is width, 1 is height
    def calc_side(side_type):
        side = 0
        if not corner[side_type]:
            side += padding_pixels
        else:
            offset = watermark_size[side_type] + padding_pixels
            side = canvas_size[side_type] - offset
        return side
    w = calc_side(0)
    h = calc_side(1)
    return (int(w), int(h))


background = Image.open('bg.jpg')
watermark = Image.open('watermark.png')
# calculate the watermark dimentions in relation with
# the backgroud/canvas size
wm_size = watermark_dimensions(background.size, watermark.size)
# resize the watermark
watermark = watermark.resize(wm_size)
# find the coordinates for corners
# the way I programmed the corners is 'adoptable'
# with slight adjustments to the math
# an alignment logic can also be added to make things nicer
corner_wh = corner_coordinates(background.size, watermark.size, POS_BOTTOMLEFT)
# actually, this isn't the overlay. This is the actual image being stamped on
# the overlay is the watermark that masks this solid color layer
# replace 'color_overlay' with 'watermark' to get the actual image as watermark
color_overlay = Image.new('RGB', wm_size, (130, 130, 130))
background.paste(color_overlay, corner_wh, mask=watermark)
background.show()