[Image-SIG] going between numpy array and PIL.Image not behaving as expected
ned at nedbatchelder.com
Fri Nov 21 19:19:47 CET 2008
Yes, it definitely looks like a bug in fromarray. typestr is '<i4' in
my environment, and the check against typestr[1:] seems to recognize
this. The line
typestr = typestr[:2]
should change, perhaps to:
typestr = typestr[-2:]
Jim Vickroy wrote:
> 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 == '|' or typestr == _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'
> raise TypeError("Do not understand data.")
> I am relatively inexperienced with both PIL and numpy, but the
> * 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()?
>> 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:
>>> 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
>>> ''' % (extrema, image.getextrema())
>>> numpy image extrema (minimum,maximum): (0, 11)
>>> PIL image extrema (minimum,maximum): (0, 2)
>>> I would appreciate pointers on what I'm doing incorrectly.
>>> -- jv
>>> Image-SIG maillist - Image-SIG at python.org
>> Ned Batchelder, http://nedbatchelder.com
Ned Batchelder, http://nedbatchelder.com
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Image-SIG