Tag 262 (PhotometricInterpretation) is changed when saving as a TIFF

[eliu-alltrust]

What did you do?

save an uncompressed tiff file to group 4 compressed tiff file

What did you expect to happen?

I expected to get a group 4 compressed tiff file and kept all other tags the same as original file

What actually happened?

I got a group 4 compressed tiff file, but Tag 262 (PhotometricInterpretation) was changed from 0 to 1

What are your OS, Python and Pillow versions?

• OS: Window 10
• Python: Python 3.6.8
• Pillow: 7.2.0

from PIL import Image

img = Image.open('uncompressed.tiff')
print(img.tag)
img.save('group4.tiff', compression="group4", tiffinfo=img.tag)
img = Image.open('group4.tiff')
print(img.tag)

pillowIssue.zip

[radarhere]

As an explanation of what is happening, the image you have provided has mode 1. This is how we are setting the photometric interpretation when saving.

Pillow/src/PIL/TiffImagePlugin.py

Line 1446 in 5c9faf0

rawmode, prefix, photo, format, bits, extra = SAVE_INFO[im.mode]

Pillow/src/PIL/TiffImagePlugin.py

Lines 1418 to 1421 in 5c9faf0

	SAVE_INFO = {
	# mode => rawmode, byteorder, photometrics,
	# sampleformat, bitspersample, extra
	"1": ("1", II, 1, 1, (1,), None),

So depending on the mode, it will not always be 1. But I agree that we're not using the same value that was set.

It controls "the color space of the image data". This is not a random tag that we've just missed - if we were to just flip a switch and set it to 0, it would invert the colors for this case. We could invert the pixels correspondingly, but the value of doing so is not clear - is there any reason you would like it to be the same?

[TebanoE]

I am facing the same problem as eliu-alltrust and I think it is a bug in pillow.

The reason for the existence of tag 262 is that there is no general agreement on how values in 1-bit images are interpreted. "1" can be seen as "high intensity" = white or as "set a point" = black. Tag 262 specifies which of those two interpretations should be used when reading a file, not whether pixels are to be inverted when writing a file.

However, the later is what happens in pillow: I changed the hard coded photometrics value in TiffImagePlugin.py line 1421 to 0 and pillow inverted my image data when saving. It should, however, leave the pixels unchanged regardless of photometrics and write tag 262 with the value specified by the user.

[radarhere]

So there is a specific need for tag 262 to be set to zero for 1-bit images? Ok then, I have created PR #5655 to resolve this.