PS: A lot of the reaction of course also depend on how you handle the image
data once loaded – and obviously what your expected unit test results are.
I’ve tried to implement a full “image handling” library a while ago (a layer up
from libpng, tiff, …) , when I found that most higher level open source library
don’t do a very good job with color management in my opinion, at least not the
ones I’ve tested. Unfortunately I didn’t have the time yet to finish it or get
it to a point where it is really useful for others, especially the “build
system” is more than clunky (I wanted to create cmake environment, so that it
is easy to use it cross platform … ) and the unit tests are more manual test
entry points than real unit tests at this time. Again haven’t got a lot of time
to work on that as it’s a private project and I’m currently loaded with
job-work, but if I remember correctly, the PNG READ handling should be OK to
some extent. If I also remember correctly, the code to write PNG files is not
checked in as I haven’t validated it’s correctness for all color formats and
bit depths.
The library Itself doesn’t do a lot “upstream” with the image files, although I
think the (more manual …) test code had some color space conversion examples,
especially like “convert everything to sRGB” if I remember correctly.
https://github.com/freudi74/mfimage
Hope that helps more than adding more confusion…
Marco
Von: Marco Freudenberger
Gesendet: Dienstag, 21. Februar 2017 14:51
An: 'Aaron Boxer'
Cc: Noel Carboni; lcms-user@lists.sourceforge.net
Betreff: AW: [Lcms-user] Apply profile with PCS = cmsSigXYZData, Color Space =
cmsSigGrayData
Well, if you set BOTH gammas to 2.2, it should work.
What test cases do you have and what are the expected results?
Von: Aaron Boxer [mailto:boxe...@gmail.com]
Gesendet: Dienstag, 21. Februar 2017 12:58
An: Marco Freudenberger
Cc: Noel Carboni; lcms-user@lists.sourceforge.net
Betreff: Re: [Lcms-user] Apply profile with PCS = cmsSigXYZData, Color Space =
cmsSigGrayData
On Tue, Feb 21, 2017 at 1:15 PM, Marco Freudenberger
<marco.freudenber...@entrustdatacard.com<mailto:marco.freudenber...@entrustdatacard.com>>
wrote:
No. I’m actually talking about images that do not have an ICC profile embedded.
Well – maybe all, actually.
Lines 187-191:
if( !png_get_gAMA(png, info, &gamma))
gamma = 1.0;
/* we're not displaying but converting, screen gamma == 1.0 */
png_set_gamma(png, 1.0, gamma);
- You try to read the file gamma. If none found, you set it to 1.0.
That I think is the first mistake…
- Then you set the screen gamma to 1.0. This is PROBABLY the next
mistake, as the typical screen gamma is ~ 2.2. Linear gamma is unusual. The two
“mistakes” might eliminate each other for images with NO gAMA chunk (because no
correction is done for the screen).
- I think png_set_gamma actually makes sure that the RGB (or gray …)
values you are getting from the file are adjusted by (file gamma) / (screen
gamma) – so you are not reading the values originally encoded in the file... I
might be wrong, I didn’t check the documentation.
So …
- If the file has an embedded gamma of 2.2, you would modify the
values because the “screen gamma” is set to 1.0
Thanks. I tried using 2.2 and it broke a lot of my unit tests. I guess
allowing the user to set the gamma would be best - can blame the user if
something goes wrong :)
Another big problem is, that this seems to be applied EVEN THOUGH an ICC
profile might be embedded in the file, which already does any file color space
-> screen color space for you. So if you have a file gamma AND an ICC profile,
you might change values (especially “light intensity” of colors) while actually
the ICC profile does all of that.
IF you want to use the file gamma to correct images when displaying them (or
converting to another ICC profile), you should rather generate an “on the fly”
icc profile based on the file gamma (and given (cHRM chunk) or assumed
chromatic primaries rather than having libpng modifying the values you read for
you. This is often an area of confusion with libpng ... I hope I expressed that
all in a way that it can be understood… I’m not a English is not my mothers
tongue and I tend to make technical descriptions extremely complicated …
Thanks, your writing is crystal clear - just a lot of detail to think over. I
have changed my code so that gamma is only applied if there is no ICC.
Best,
Aaron
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