Creating a PNG file in Python

Question

I have an application where I would like to be able to generate PNG images from data in Python. I've done some searching and found "PIL" which looked pretty outdated. Is there some other library that would be better for this?

Thanks,

Answer 1

Simple PNG files can be generated quite easily from pure Python code - all you need is the standard zlib module and some bytes-encoding to write the chunks. Here is a complete example that the casual reader may use as a starter for their own png generator:

#! /usr/bin/python
""" Converts a list of list into gray-scale PNG image. """
__copyright__ = "Copyright (C) 2014 Guido Draheim"
__licence__ = "Public Domain"

import zlib
import struct

def makeGrayPNG(data, height = None, width = None):
    def I1(value):
        return struct.pack("!B", value & (2**8-1))
    def I4(value):
        return struct.pack("!I", value & (2**32-1))
    # compute width&height from data if not explicit
    if height is None:
        height = len(data) # rows
    if width is None:
        width = 0
        for row in data:
            if width < len(row):
                width = len(row)
    # generate these chunks depending on image type
    makeIHDR = True
    makeIDAT = True
    makeIEND = True
    png = b"\x89" + "PNG\r\n\x1A\n".encode('ascii')
    if makeIHDR:
        colortype = 0 # true gray image (no palette)
        bitdepth = 8 # with one byte per pixel (0..255)
        compression = 0 # zlib (no choice here)
        filtertype = 0 # adaptive (each scanline seperately)
        interlaced = 0 # no
        IHDR = I4(width) + I4(height) + I1(bitdepth)
        IHDR += I1(colortype) + I1(compression)
        IHDR += I1(filtertype) + I1(interlaced)
        block = "IHDR".encode('ascii') + IHDR
        png += I4(len(IHDR)) + block + I4(zlib.crc32(block))
    if makeIDAT:
        raw = b""
        for y in xrange(height):
            raw += b"\0" # no filter for this scanline
            for x in xrange(width):
                c = b"\0" # default black pixel
                if y < len(data) and x < len(data[y]):
                    c = I1(data[y][x])
                raw += c
        compressor = zlib.compressobj()
        compressed = compressor.compress(raw)
        compressed += compressor.flush() #!!
        block = "IDAT".encode('ascii') + compressed
        png += I4(len(compressed)) + block + I4(zlib.crc32(block))
    if makeIEND:
        block = "IEND".encode('ascii')
        png += I4(0) + block + I4(zlib.crc32(block))
    return png

def _example():
    with open("cross3x3.png","wb") as f:
        f.write(makeGrayPNG([[0,255,0],[255,255,255],[0,255,0]]))

Answer 2

Here's a Python3 example:

import png

width = 255
height = 255
img = []
for y in range(height):
    row = ()
    for x in range(width):
        row = row + (x, max(0, 255 - x - y), y)
    img.append(row)
with open('gradient.png', 'wb') as f:
    w = png.Writer(width, height, greyscale=False)
    w.write(f, img)