Image Processing: Merging images with PIL.paste

Question

i have a 2 list of png images, list _c and list _v. I want to paste _v on _c using a code like:

from PIL import Image

background = [Image.open(path, 'r') for path in glob.glob(list_c_path)]
foreground = [Image.open(path, 'r') for path in glob.glob(list_v_path)]

for im in range(len(background)):
    pasted = background[im].paste(foreground[im], (0, 0), foreground[im])

This code won't work but it will give you and idea of what i want. I also need to have the images read in grayscale format before they are pasted.

Here's a sample of a background image:

Here's a sample of a foreground image:

And this is the desired result:

I pasted this images using this code:

background = Image.open('1000_c.png')
foreground = Image.open('1000_v.png')


background.paste(foreground, (0, 0), foreground)
background.save('example.png')

How can i achieve this??

Thanks in advance

Answer 1

Mmmm... your result images are identical to your foreground images because although the foreground images have an alpha/transparency layer, they are fully opaque and completely conceal your backgrounds. You need to have a rethink!

You can use ImageMagick in the Terminal to inspect your images. So, let's look at your foreground image:

identify -verbose fg.png

Sample Output

Image: fg.png
  Format: PNG (Portable Network Graphics)
  Mime type: image/png
  Class: DirectClass
  Geometry: 118x128+0+0
  Units: Undefined
  Colorspace: sRGB
  Type: PaletteAlpha             <--- Image does have alpha/transparency layer
  Base type: Undefined
  Endianess: Undefined
  Depth: 8-bit
  Channel depth:
    Red: 8-bit
    Green: 8-bit
    Blue: 8-bit
    Alpha: 1-bit
  Channel statistics:
    Pixels: 15104
    Red:
      min: 30  (0.117647)
      ...
      ...
    Alpha:
      min: 255  (1)              <--- ... but alpha layer is fully opaque
      max: 255 (1)
      mean: 255 (1)
      standard deviation: 0 (0)
      kurtosis: 8.192e+51
      skewness: 1e+36
      entropy: 0

So there is no point pasting a fully opaque image over a background as it will fully conceal it.

If we punch a transparent hole in your foreground image with ImageMagick:

convert fg.png -region 100x100+9+14 -alpha transparent fg.png

It now looks like this:

And if we then run your code:

#!/usr/local/bin/python3

from PIL import Image
background = Image.open('bg.png')
foreground = Image.open('fg.png')

background.paste(foreground, (0, 0), foreground)
background.save('result.png')

It works:

So the moral of the story is that your foreground image either needs some transparency to allow the background to show through, or you need to use some blending mode to choose one or the other of the foreground and background images at each location, or to choose some combination - e.g. the average of the two, or the brighter of the two.

---

If you want to average the two images, or in fact, do any other blending mode, you could consider using Pillow's ImageChops module - documentation here. So, an average would look like this:

#!/usr/local/bin/python3

from PIL import Image, ImageChops
bg = Image.open('bg.png')
fg = Image.open('fg.png')

# Average the two images, i.e. add and divide by 2
result = ImageChops.add(bg, fg, scale=2.0)
result.save('result.png')