[Image-SIG] show a matrix as image

Elias Fotinis efotinis at yahoo.com
Tue Aug 11 18:06:34 CEST 2009

From: Milton Cezar Ribeiro
> I have an object that is a matrix (i.e. a list of lists)
> and I neet do plot it on screen as an image.
> My image have values 1 and 2, and I would
> like set different colors for each level.
> I tryed figure it out without success. In fact
> my images are 512x512, but I include a 10x10 sample below.
>    mymatrix=[[1,2,2,2,1,2,2,2,1,1],
>               [2,2,1,1,1,1,2,2,1,2],
>              ....

It's best to use a palettized image, since your data points are integers.
First, create one of the right size:

    from PIL import Image
    im = Image.new('P', (512,512))  # 'P' for palettized

Then, set the image data. You must use a flat list, so you need to flatten
your matrix:

    data = sum(matrix, [])  # flatten data

You also have to set the palette colors for the used indices (1 and 2). The
palette is a list of 768 ints (256*3), but it's somewhat easier to start
with a list of 3-tuples:

    pal = [(0,0,0) for i in range(256)]  # all black
    pal[1] = (255,0,0)  # juicy red
    pal[2] = (0,255,0)  # crazy green
    pal = sum(pal, ())  # flatten it

That's it. You should be able to test the result with this:


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