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

[Jim Vickroy]

Ned Batchelder wrote:
> 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.  
Thanks for your clear and detailed reply and sorry for the vagueness of 
my posting; you did correctly read my mind!
> 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.
Thanks, this does indeed work and your explanation made me wonder why 
specifying dtype=int (as in my posted script) did not work. 

Here is the signature of PIL.Image.fromarray in my installation (v 1.1.6):

    * fromarray(obj, mode=None)

so when mode is not specified, the procedure determines it from the 
attributes of "obj" as follows:
    if mode is None:
        typestr = arr['typestr']
        if not (typestr[0] == '|' or typestr[0] == _ENDIAN or
                typestr[1:] not in ['u1', 'b1', 'i4', 'f4']):
            raise TypeError("cannot handle data-type")
        typestr = typestr[:2] ##### why isn't this: typestr = 
typestr[1:] or typestr = typestr[1:3] ? ##################
        if typestr == 'i4':
            mode = 'I'
        elif typestr == 'f4':
            mode = 'F'
        elif typestr == 'b1':
            mode = '1'
        elif ndim == 2:
            mode = 'L'
        elif ndim == 3:
            mode = 'RGB'
        elif ndim == 4:
            mode = 'RGBA'
        else:
            raise TypeError("Do not understand data.")

I  am  relatively inexperienced with both PIL and numpy, but the  statement:

    * typestr = typestr[:2]

seems to be incorrect; after it is executed, I do not see how typestr 
can ever be any of ('i4' , 'f4', 'b1').

If I make the indicated change to

    * typestr = typestr[1:]

then "mode" is correctly inferred from "obj" and I do not have to 
explicitly specify it when applying the fromarray() procedure.

Is this a logic error in fromarray()?
>

>
> --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
>> ------------------------------------------------------------------------
>>
>> _______________________________________________
>> Image-SIG maillist  -  Image-SIG at python.org
>> http://mail.python.org/mailman/listinfo/image-sig
>>   
>
> -- 
> Ned Batchelder, http://nedbatchelder.com
>   

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