Rob Sayre's Mozilla Blog

Update: Turns out <canvas> can do this (see bug 372196). The WHATWG spec has the details.

Sometimes it can be hard to spot the differences between two images generated from reftest. Here’s a little Python script to show the differences in white on a black background. It requires PIL.

from PIL import Image

testImage = Image.open("test1.png")
refImage = Image.open("test2.png")
diffImage = Image.new("1", testImage.size)

testData = testImage.getdata()
refData = refImage.getdata()

diffData = []
for i in range(len(testData)):
  if testData[i] == refData[i]:
    diffData.append(0)
  else:
    diffData.append(255)

diffImage.putdata(diffData)
diffImage.save("diff.png")