[Image-SIG] going between numpy array and PIL.Image not behaving as expected

[Ned Batchelder]

I'm not sure exactly what you are trying to do here, but the issue has 
to do with the mapping of numpy array elements into pixels.  Your code 
uses 32-bit ints, and fromarray defaults to "L" mode, which is 8-bit 
grayscale pixels.  fromarray uses the shape of the array to create the 
shape of the image, but then just reads bytes until the image has all 
the data it needs.  In your case, it only needs to read 3 32-bit ints to 
get enough bytes to fill the 3x4 "L" mode image.  In the first three 
ints, the min byte is zero and the max byte is 2, which your image 
extrema verifies.

If you change your code to use this:

    source  = numpy.arange(0,12,dtype=numpy.int8)

then everything will match up: your array has byte elements, and your 
image will have byte pixels.

--Ned.

Jim Vickroy wrote:
> Hello all,
>
> I am having no success getting numpy and PIL to behave as expected 
> when starting with a numpy array (see the attached script).
>
> Here is the output on my computer:
>
> <output>
> Python version: 2.5.2 (r252:60911, Feb 21 2008, 13:11:45) [MSC v.1310 
> 32 bit (Intel)]
> numpy version:  1.2.1
> PIL version:    1.1.6
> numpy source array:
> [[ 0  1  2  3]
> [ 4  5  6  7]
> [ 8  9 10 11]]
> numpy source array shape: (3, 4)
> PIL image size: (4, 3)
> Traceback (most recent call last):
>  File "C:\Documents and Settings\jim.vickroy\My 
> Documents\Projects\GOES\SXI\__trials__\numpy-PIL.py", line 30, in 
> <module>
>    ''' % (extrema, image.getextrema())
> AssertionError:
>   numpy image extrema (minimum,maximum): (0, 11)
>   PIL   image extrema (minimum,maximum): (0, 2)
> </output>
>
>
> I would appreciate pointers on what I'm doing incorrectly.
>
> Thanks,
> -- jv
> ------------------------------------------------------------------------
>
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>   

-- 
Ned Batchelder, http://nedbatchelder.com

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