Convert 24-bit PNG files to 8-bit color indexed images with pypng

Question

I'm trying to write a python script that takes in standard 24-bit pngs and converts them to 8-bit pngs for better compression. It looks like pypng can do this but I can't quite figure out how to use it. Image manipulation is a new area for me so this may seem silly. I have this currently:

r=png.Reader(<myfile>)
test = r.asRGBA8()

This gives me tuples in return (the layers of the image I believe). However I can't seem to write or save this back to an image. What am I missing? Here's a test image

Answer 1

Original Answer

I think this does what you ask:

from PIL import Image

# Load image
im = Image.open('logo.png')                                                                 

# Convert to palette mode and save
im.convert('P').save('result.png')

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Updated Answer

I can't find a way to get PIL to make a sensible palette image as a result, but can do it a couple of other ways...

Either with wand like this:

#!/usr/bin/env python3

from wand.image import Image

with Image(filename='logo.png') as img: 
    img.quantize(number_colors=256, colorspace_type='lab', treedepth=0, dither=False, measure_error=False)
    img.save(filename='result.png')

Or, by shelling out to ImageMagick at the command-line and doing:

magick logo.png -colors 255 png8:logo8.png      # use "convert" in place of "magick" if using v6

Newest Answer

Ok, I found a way to get PIL/Pillow to do a better job, and as expected, it makes use of libimagequant which is not normally built into Pillow (at least on macOS where I am). The code looks like this:

#!/usr/bin/env python3

from PIL import Image

# Load image
im = Image.open('logo.png')                                                                 

# Convert to palette mode and save. Method 3 is "libimagequant"
im.quantize(colors=256, method=3).save('result.png')

The steps, on macOS to build PIL/Pillow with libimagequant are as follows - they will differ on other platforms but you should be able to get the general idea and adapt:

pip uninstall pillow           # remove existing package
brew install libimagequant
brew install zlib
export PKG_CONFIG_PATH="/usr/local/opt/zlib/lib/pkgconfig"
pip install --upgrade Pillow --global-option="build_ext" --global-option="--enable-imagequant" --global-option="--enable-zlib"

Keywords : Python, image processing, PIL/Pillow, libimagequant, macOS, quantise, quantize.