FAQ

[Image-SIG] PIL Image array interface has the wrong size for YCbCr

David Coles
Oct 8, 2010 at 4:51 am
PIL's Image class has incorrect dimension specified for YCbCr images.
This causes issues when converting to or from NumPy arrays.

According to http://www.pythonware.com/library/pil/handbook/concepts.htm
YCbCr should be "3x8-bit pixels, colour video format". Instead it
appears to be converted to a 4x8-bit format.

The incorrect definition is at line 206 of
http://svn.effbot.python-hosting.com/pil/PIL/Image.py.

Example: Load up any image and convert to YCbCr.
import numpy
import Image as im
image = im.open('bush640x360.png')
ycbcr = image.convert('YCbCr')
Using the Array interface produces a HxWx4 array, which is the wrong
dimensions for YCbCr. Thus when selecting a single channel it displays
incorrectly:
A = numpy.asarray(ycbcr)
print A.shape
(360, 640, 4)
im.fromarray(A[:,:,0], "L").show()
Here's an example decoding the image byte string ourselves gives the
correct result:
B = numpy.ndarray((image.size[1], image.size[0], 3), 'u1',
ycbcr.tostring())
print B.shape
(360, 640, 3)
im.fromarray(B[:,:,0], "L").show()
Attached is a patch against the 1.1.7-2 (python-imaging) package in
Ubuntu as I can't find the development repository for 1.1.7. See
https://bugs.edge.launchpad.net/ubuntu/+source/python-imaging/+bug/656666 for details.

Cheers,
David
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