Pillow png compressing - django

Question

Im making a simple app that able to compress images with jpeg and png format using Pillow library, python3 and Django. Made a simple view that able to identify formats, save compress images and give some statistics of compressing. With images in jpeg format it works really fine, i got compressicons close to 70-80% of original size, and it works really fast, but if i upload png i works much worse. Compression takes a long time, and it only 3-5% of original size. Trying to find some ways to upgrade compress script, and stuck on it.
Right now ive got this script in my compress django view:
from django.shortcuts import render, redirect, get_object_or_404, reverse
from django.contrib.auth import login, authenticate, logout
from django.contrib.auth.models import User
from django.http import HttpResponse, HttpResponseRedirect
from django.http import JsonResponse
from django.contrib import auth
from .forms import InputForm, SignUpForm, LoginForm, FTPForm
import os
import sys
from PIL import Image
from .models import image, imagenew, FTPinput
from django.views import View
import datetime
from django.utils import timezone
import piexif
class BasicUploadView(View):
def get(self, request):
return render(self.request, 'main/index.html', {})
def post(self, request):
form = InputForm(self.request.POST, self.request.FILES)
if form.is_valid():
photo = form.save(commit=False)
photo.name = photo.image.name
photo.delete_time = timezone.now() + datetime.timedelta(hours=1)
photo.user = request.user
photo.size = photo.image.size
photo = form.save()
name = (photo.name).replace(' ', '_')
picture = Image.open(photo.image)
if picture.mode in ('RGB'):
piexif.remove('/home/andrey/sjimalka' + photo.image.url)
picture.save('media/new/'+name,"JPEG",optimize=True,quality=75)
newpic = 'new/'+name
new = imagenew.objects.create(
name = name,
image = newpic,
delete_time = timezone.now() + datetime.timedelta(hours=1),
user = request.user,
)
if new.image.size < photo.image.size:
diff = round((new.image.size-photo.image.size)/float(photo.image.size)*100, 2)
else:
diff = str(round((new.image.size-photo.image.size)/float(photo.image.size)*100, 2))+' Не удалось сжать файл'
oldsize = round(photo.image.size/1000000, 2)
newsize = round(new.image.size/1000000, 2)
id = new.pk
imagenew.objects.filter(pk=id).update(size=new.image.size)
elif picture.mode != ('RGB'):
picture.save('media/new/'+name,"PNG", optimize=True, quality=75)
newpic = 'new/'+name
new = imagenew.objects.create(
name = name,
image = newpic,
delete_time = timezone.now() + datetime.timedelta(hours=1),
user = request.user,
)
if new.image.size < photo.image.size:
diff = round((new.image.size-photo.image.size)/float(photo.image.size)*100, 2)
else:
diff = str(round((new.image.size-photo.image.size)/float(photo.image.size)*100, 2))+' Не удалось сжать файл'
oldsize = round(photo.image.size/1000000, 2)
newsize = round(new.image.size/1000000, 2)
id = new.pk
imagenew.objects.filter(pk=id).update(size=new.image.size)
data = {'is_valid': True, 'name': new.image.name, 'url': new.image.url, 'diff': diff,
'oldsize':oldsize, 'newsize':newsize,}
else:
alert = 'Данный формат не поддерживается. Пожалуйста загрузите картинки форматов png или jpg(jpeg)'
data = {'is_valid': False, 'name': alert,}
return JsonResponse(data)
The question: is there any ways to make script with png upload work faster, and (that much more important) make png size compressions closer to jpeg? Maybe i should use another python library?

Answer 1

how tinypng works then? They compressing same png files with 50-60%
They probably reduce the colour palette from 24-bit to 8-bit. Here's a detailed answer about that - https://stackoverflow.com/a/12146901/1925257
Basic method
You can try that in Pillow like this:
picture_8bit = picture.convert(
mode='P', # use mode='PA' for transparency
palette=Image.ADAPTIVE
)
picture_8bit.save(...) # do as usual
This should work similar to what tinypng does.
If you don't want transparency, it's better to first convert RGBA to RGB and then to P mode:
picture_rgb = picture.convert(mode='RGB') # convert RGBA to RGB
picture_8bit = picture_rgb.convert(mode='P', ...)
Getting better results
Calling convert() as shown above will actually call quantize() in the background and Median Cut algorithm will be used by default for reducing the colour palette.
In some cases, you'll get better results with other algorithms such as MAXCOVERAGE. To use a different algorithm, you can just call the quantize() method directly:
picture_rgb = picture.convert(mode='RGB') # convert RGBA to RGB
picture_8bit = picture.quantize(colors=256, method=Image.MAXCOVERAGE)
You have to understand that downsizing the colour palette means that if the image has lots of colours, you will be losing most of them because 8-bit can only contain 256 colours.

Answer 2

The document of Pillow Image.quatize displays a more convenient way to compress png files. In a personal experiment, the following code could make png about 70% compression of the original size, which is also close to the result created by ImageMagick.
# Image.quantize(colors=256, method=None, kmeans=0, palette=None)
# method: 0 = median cut; 1 = maximum coverage; 2 = fast octree
img = img.quantize(method=2)