8-bit image acts like a 16!

MH
Posted By
Maggie_H
Feb 20, 2008
Views
353
Replies
7
Status
Closed
Hi all, Photoshop newbie here. We create images with a color linescan camera and save to a TIF file. They are all 8-bit images, no colorspace assigned.

I took 2 images within 6 minutes of each other. Photoshop (CS2) identifies both of them as 8-bit images. BUT, it treats one file like it’s a 16-bit image with missing filters etc but is fine with the other image. The only difference I can see is the file size. The file that works is about 110MB, while the problem file is about 161MB.

Has anyone seen anything similar, or knows how Photoshop decides when an image is "filter-worthy" and when it’s not? Thanks in advance!
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DM
dave_milbut
Feb 20, 2008
look in image> mode. if it’s 8 or 16 bit it should say there.
MH
Maggie_H
Feb 20, 2008
Yes, Photoshop definitely identifies the mode on both files as 8-bit. What I can’t figure out is why it would treat them differently with respect to the number of available filters.
DM
dave_milbut
Feb 20, 2008
not sure if one is in cmyk or lab mode rather than rgb if that would affect it…
MH
Maggie_H
Feb 20, 2008
What’s interesting is that a simple resize of the image fixes the behavior of the filter menu. So going from say ~520 ppi to 400 ppi, all of a sudden you have access to the greyed-out filters. But you only have to do that on the larger of the 2 files. They both have the same native resolution (we scan at 2048 pixels/line at 200 lines/cm).

It’s also cross-platform, same problem for Macs and PC’s running the same version of Photoshop. Strange…
BD
Brett_Dalton
Feb 22, 2008
are you ACTUALLY changing the pixel dimensions or just changing the ppi setting? if its the former then some filters will not work with files over a certain pixel dimensions. If it’s just telling the file it’s 400ppi then thats just plain weird.

BRETT
MH
Maggie_H
Feb 22, 2008
No, all we do is change the PPI. Whatever Photoshop does when it resizes seems to fix or refresh its memory that the file is in fact 8-bit.
BD
Brett_Dalton
Feb 24, 2008
It would appear to be the case that some filters are using fractional pixles in calculations which may or may not be influenced by the PPI setting and not just the actual number of pixels… interesting.

Photoshop isn’t doing any rescaling if the pixel dimensions are the same but the PPI changes.

BRETT

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