Inconsistent tonal values across different Photoshop functions

FS
Posted By
Frank_Spangenberg
Jul 16, 2008
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374
Replies
5
Status
Closed
Additionally there’s also a difference between the Windows and Mac version of Photoshop starting at 63%:
< http://spreadsheets.google.com/ccc?key=pdbV5FYYycngGI-oeBuGc Kw>
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GH
Gernot_Hoffmann
Jul 16, 2008
Frank,

starting with 70% in byte mode:
x = 70*2.55 = 178.5
Theoretically this should be rounded always to 179.
Practically it’s also correct to round this to 178.

Value for 70.2%:
x = 70.2*2.55 = 179.01 –> 179

Value for 69.8%:
x = 69.8*2.55 = 177.99 –> 178

The example suffers from being accurately at the
rounding threshold. Anything goes.

In a uniform area the Cyan value is either 179 or
178 (it isn’t coded by percentage). Doesn’t matter.
The appearance will be the same.

In a real image we don’t have uniform areas just
at the rounding threshold. There is enough variation
to distribute rounding errors randomly.

Best regards –Gernot Hoffmann
FS
Frank_Spangenberg
Jul 16, 2008
Gernot,
this problem does not affect “real pictures” from your digital camera, but “technical” images that you create in Photoshop and combine with e.g. vector objects in other applications. Also image files used for measurements are affected. In these fields it is very important to produce always the same tonal values, otherwise you’ll see/measure a difference. Try to imagine if this error appears in all 4 CMYK channels!

Btw. Photoshop maps 70% to an index of 78, 77 or 76 (0% = 255, 100% = 0). So this is not only an issue of rounding…
GH
Gernot_Hoffmann
Jul 17, 2008
Frank,

of course it’s inconsistent. Perhaps only the
indicated value in mode 32bpc is wrong.

About the CMYK coding, as tested by a hex editor
in a TIFF for K:
0% 00h (white)
50% 7Fh = 127
70% B2h = 178

A CMYK value 0..100% is mapped to 0..255.

I’m back on Sunday.

Best regards –Gernot Hoffmann
FS
Frank_Spangenberg
Jul 17, 2008
I also used some JavaScript for measuring (colorSamplers in CS3, channels.histogram in CS2) and they show the same results. So it’s not a problem of the info palette.

Photoshop uses the index 0 for 100% and 255 for 0% (histogram), even if it’s saved in the other way around…
FS
Frank_Spangenberg
Jul 18, 2008
Just to note, that there is a mistake in the first post:

Adobe Photoshop CS2
– Fill : 70,2%

< http://spreadsheets.google.com/ccc?key=pdbV5FYYycngGI-oeBuGc Kw>

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