Convert image to specific palette using PIL without dithering

Question

I am trying to convert an RGB image in PNG format to use a specific indexed palette using the Pillow library (Python Image Library, PIL). But I want to convert using the \"r

Answer 1

I took all these and made it faster, added notes for you to understand & converted to pillow instead of pil. Basically.

import sys
import PIL
from PIL import Image

def quantizetopalette(silf, palette, dither=False):
    """Convert an RGB or L mode image to use a given P image's palette."""

    silf.load()

    # use palette from reference image made below
    palette.load()
    im = silf.im.convert("P", 0, palette.im)
    # the 0 above means turn OFF dithering making solid colors
    return silf._new(im)

if __name__ == "__main__":
    import sys, os

for imgfn in sys.argv[1:]:
    palettedata = [ 0, 0, 0, 255, 0, 0, 255, 255, 0, 0, 255, 0, 255, 255, 255,85,255,85, 255,85,85, 255,255,85] 

#   palettedata = [ 0, 0, 0, 0,170,0, 170,0,0, 170,85,0,] # pallet 0 dark
#   palettedata = [ 0, 0, 0, 85,255,85, 255,85,85, 255,255,85]  # pallet 0 light

#   palettedata = [ 0, 0, 0, 85,255,255, 255,85,255, 255,255,255,]  #pallete 1 light
#   palettedata = [ 0, 0, 0, 0,170,170, 170,0,170, 170,170,170,] #pallete 1 dark
#   palettedata = [ 0,0,170, 0,170,170, 170,0,170, 170,170,170,] #pallete 1 dark sp

#   palettedata = [ 0, 0, 0, 0,170,170, 170,0,0, 170,170,170,] # pallet 3 dark
#   palettedata = [ 0, 0, 0, 85,255,255, 255,85,85, 255,255,255,] # pallet 3 light

#  grey  85,85,85) blue (85,85,255) green (85,255,85) cyan (85,255,255) lightred 255,85,85 magenta (255,85,255)  yellow (255,255,85) 
# black 0, 0, 0,  blue (0,0,170) darkred 170,0,0 green (0,170,0)  cyan (0,170,170)magenta (170,0,170) brown(170,85,0) light grey (170,170,170) 
#  
# below is the meat we make an image and assign it a palette
# after which it's used to quantize the input image, then that is saved 
    palimage = Image.new('P', (16, 16))
    palimage.putpalette(palettedata *32)
    oldimage = Image.open(sys.argv[1])
    oldimage = oldimage.convert("RGB")
    newimage = quantizetopalette(oldimage, palimage, dither=False)
    dirname, filename= os.path.split(imgfn)
    name, ext= os.path.splitext(filename)
    newpathname= os.path.join(dirname, "cga-%s.png" % name)
    newimage.save(newpathname)

#   palimage.putpalette(palettedata *64)  64 times 4 colors on the 256 index 4 times, == 256 colors, we made a 256 color pallet.

Answer 2

The parts of PIL implemented in C are in the PIL._imaging module, also available as Image.core after you from PIL import Image. Current versions of Pillow give every PIL.Image.Image instance a member named im which is an instance of ImagingCore, a class defined within PIL._imaging. You can list its methods with help(oldimage.im), but the methods themselves are undocumented from within Python.

The convert method of ImagingCore objects is implemented in _imaging.c. It takes one to three arguments and creates a new ImagingCore object (called Imaging_Type within _imaging.c).

• mode (required): mode string (e.g. "P")
• dither (optional, default 0): PIL passes 0 or 1
• paletteimage (optional): An ImagingCore with a palette

The problem I was facing is that quantize() in dist-packages/PIL/Image.py forces the dither argument to 1. So I pulled a copy of the quantize() method out and changed that. This may not work in future versions of Pillow, but if not, they're likely to implement the "extended quantizer interface in a later version" that a comment in quantize() promises.

#!/usr/bin/env python3
from PIL import Image

def quantizetopalette(silf, palette, dither=False):
    """Convert an RGB or L mode image to use a given P image's palette."""

    silf.load()

    # use palette from reference image
    palette.load()
    if palette.mode != "P":
        raise ValueError("bad mode for palette image")
    if silf.mode != "RGB" and silf.mode != "L":
        raise ValueError(
            "only RGB or L mode images can be quantized to a palette"
            )
    im = silf.im.convert("P", 1 if dither else 0, palette.im)
    # the 0 above means turn OFF dithering

    # Later versions of Pillow (4.x) rename _makeself to _new
    try:
        return silf._new(im)
    except AttributeError:
        return silf._makeself(im)

palettedata = [0, 0, 0, 102, 102, 102, 176, 176, 176, 255, 255, 255]
palimage = Image.new('P', (16, 16))
palimage.putpalette(palettedata * 64)
oldimage = Image.open("School_scrollable1.png")
newimage = quantizetopalette(oldimage, palimage, dither=False)
newimage.show()