1999.11.26 14:21 "Re: CCITT compressed TIFF files.", by John Aldridge
I've been trying to work out whether some sample TIFF files that I have are correct or not. They are CCITT compressed (G3, but I'm interested in checking the others as well), and a mixture of WhiteIsZero and BlackIsZero. I need to know whether the image data is correctly set or whether I have it negated.
So far I've been using PaintShopPro and PhotoShop to view these images, but the two disagree on whether the images are negated or not, and making changes to the tags appears to lead to unexpected results.
Here's a copy of a bug report I sent to JASC:
>Paint Shop Pro
>Build date Sep 24 1997 @ 11:13:09
>Windows NT 4.0 SP3
>Paint Shop Pro seems to deal erroneously with the PhotometricInterpretation
>tag when dealing with CCITT Group 3 encoded TIFF files.
>When displaying existing TIFF files, it ignores the tag; and treats all
>such images as if they had WhiteIsZero specified.
>When performing a "Save As..." TIFF, CCITT Group 3, the data in the file
>uses zero bits to represent white, but it writes a BlackIsZero tag.
>Thus, although Paint Shop Pro apparently displays its own images properly
>(because it ignores the erroneous BlackIsZero tag it has written), other
>packages such as xv display such images black-white reversed.
To which they replied:
>Thank you for your information, we are aware of this occurrence and will
>be addressing this issue in future versions of the program.