Method for converting PNGs to premultiplied alpha [closed]

Question

Looking for some kind of simple tool or process for Windows that will let me convert one or more standard PNGs to premultiplied alpha.

Command line tools are ideal; I have easy access to PIL (Python Imaging Library) and Imagemagick, but will install another tool if it makes life easier.

Thanks!

Answer 1

A more complete version of the cssndrx answer, using slicing in numpy to improve speed:

import Image
import numpy

im = Image.open('myimage.png').convert('RGBA')
a = numpy.fromstring(im.tostring(), dtype=numpy.uint8)
alphaLayer = a[3::4] / 255.0
a[::4]  *= alphaLayer
a[1::4] *= alphaLayer
a[2::4] *= alphaLayer

im = Image.fromstring("RGBA", im.size, a.tostring())

Et voila !