python image library making dotty gifs

Alex Hunsley lard at tardis.ed.ac.molar.uk
Fri Jun 25 13:33:02 CEST 2004


I'm using python image library 1.1.4 (http://www.pythonware.com/products/pil/) 
to plot images.
However, when I save an image to a gif file, it comes out very dotty/ithered 
looking, even if it's a picture consisting only of one colour!

Here's some example code that produces a dotty gif:

#!/usr/bin/python

# pymand joy!

import Image
import ImageDraw


imageWidth=300
imageHeight=300

im = Image.new("RGB", (imageWidth, imageHeight))


draw = ImageDraw.Draw(im)
for y in range (0,imageHeight):
     for x in range (0, imageWidth):
         draw.point((x, y), (128,128,128))

# saving as a gif comes out dotty/dithered looking!
im.save("plotImage.gif", "GIF")


you can see the output gif here:

http://ohmslaw.ogr.uk/plotImage.gif

what causes the dottyness, anyone know?

alex




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