Error scaling BMP images in Plone 5.1.5

[ewohnlich]

When calling a scale on a .bmp image I get this error.

Could not scale "<plone.namedfile.file.NamedBlobImage object at 0x7f3dbe51c758>" of <bound method Image.absolute_url of <Image at /Plone/cobweb.bmp>>
Traceback (most recent call last):
  File "/sprj/st_zope_plone5/buildouts/eggs/plone.namedfile-4.2.6-py2.7.egg/plone/namedfile/scaling.py", line 253, in __call__
    **parameters
  File "/sprj/st_zope_plone5/buildouts/eggs/plone.namedfile-4.2.6-py2.7.egg/plone/namedfile/scaling.py", line 200, in create_scale
    **parameters
  File "/sprj/st_zope_plone5/buildouts/eggs/plone.scale-3.0.3-py2.7.egg/plone/scale/scale.py", line 74, in scaleImage
    if all(rgb[0] == rgb[1] == rgb[2] for c, rgb in colors):
  File "/sprj/st_zope_plone5/buildouts/eggs/plone.scale-3.0.3-py2.7.egg/plone/scale/scale.py", line 74, in <genexpr>
    if all(rgb[0] == rgb[1] == rgb[2] for c, rgb in colors):
IndexError: tuple index out of range

I dug into this a bit in a debugger.

>>> img=app.unrestrictedTraverse('/Plone/cobweb.bmp')
>>> portrait=img.portrait
>>> from plone.scale.scale import scaleImage
>>> scaleImage(portrait.data, 16, 16)

This gives the same error as above. the value of image.format is "BMP" which will cause the scale to assume converting into JPEG. This triggers a later condition where it tries to check the RGB values for equality to determine if it's gray.

>>> import PIL.Image
>>> from StringIO import StringIO
>>> image = PIL.Image.open(StringIO(portrait.data))
>>> colors = image.getcolors(maxcolors=256)
>>> colors
[(59, (255, 255)), (8, (253, 255)), (4, (251, 255)), (4, (249, 255)), (3, (247, 255)), (2, (245, 255)), (1, (241, 255)), (3, (237, 255)), (1, (235, 255)), (2, (227, 255)), (2, (225, 255)), (2, (223, 255)), (3, (217, 255)), (2, (211, 255)), (2, (209, 255)), (1, (205, 255)), (1, (201, 255)), (1, (199, 255)), (1, (183, 255)), (1, (181, 255)), (1, (177, 255)), (1, (167, 255)), (1, (163, 255)), (2, (159, 255)), (1, (157, 255)), (1, (151, 255)), (1, (149, 255)), (2, (147, 255)), (1, (143, 255)), (1, (139, 255)), (1, (135, 255)), (2, (125, 255)), (1, (121, 255)), (2, (113, 255)), (1, (109, 255)), (1, (101, 255)), (1, (99, 255)), (1, (97, 255)), (1, (95, 255)), (2, (93, 255)), (3, (87, 255)), (1, (79, 255)), (1, (77, 255)), (3, (73, 255)), (1, (69, 255)), (1, (65, 255)), (2, (61, 255)), (2, (59, 255)), (1, (53, 255)), (1, (51, 255)), (1, (47, 255)), (2, (37, 255)), (1, (31, 255)), (1, (29, 255)), (10, (25, 255)), (2, (21, 255)), (1, (19, 255)), (2, (17, 255)), (3, (15, 255)), (1, (13, 255)), (2, (9, 255)), (1, (7, 255)), (1, (5, 255)), (1, (1, 255)), (2, (254, 255)), (5, (252, 255)), (2, (250, 255)), (2, (248, 255)), (1, (246, 255)), (5, (244, 255)), (2, (242, 255)), (1, (228, 255)), (1, (226, 255)), (2, (224, 255)), (1, (218, 255)), (1, (214, 255)), (2, (212, 255)), (2, (210, 255)), (1, (206, 255)), (1, (200, 255)), (2, (196, 255)), (1, (194, 255)), (1, (188, 255)), (1, (186, 255)), (1, (168, 255)), (1, (164, 255)), (1, (138, 255)), (1, (136, 255)), (1, (134, 255)), (1, (132, 255)), (1, (130, 255)), (1, (128, 255)), (1, (124, 255)), (1, (122, 255)), (2, (118, 255)), (1, (110, 255)), (1, (108, 255)), (1, (106, 255)), (1, (94, 255)), (1, (88, 255)), (1, (86, 255)), (2, (84, 255)), (1, (78, 255)), (1, (72, 255)), (1, (70, 255)), (1, (56, 255)), (1, (54, 255)), (2, (50, 255)), (1, (38, 255)), (1, (30, 255)), (3, (28, 255)), (1, (26, 255)), (1, (24, 255)), (2, (22, 255)), (1, (18, 255)), (1, (16, 255)), (2, (6, 255)), (1, (2, 255)), (6, (0, 255))]

As you can see the rgb values only have two values so rgb[2] throws an IndexError