Saturday, August 13, 2011

Me And The Mysterious Spyder

[Please note that this post is part of a series describing my first wobbly steps into the world of ICC color profiles. The aim is to build an ICC color profile editor from scratch. Seeing how I started this project with no knowledgde whatsoever on the subject, it's wise to realise that some of my findings described in these posts are incorrect. The incorrect conclusions are usually corrected in later posts. For the whole series in one go, click here.]

So, how far am I with my color quest? Well...

I have part of it under control in my fledgeling application... I can select a profile and see the consequences in a loaded picture, JPG or otherwise. I can also show color transitions between two profiles...

But here's the deal: it didn't make a lot of sense so far.

I think I know why...

The profile the Spyder made, shows horrible grays when I load it into Windows.

But, when I select the profile to convert a photo (under my regular color profile with normal grays loaded into Windows), the grays of the converted photo (converted from regular to the spyder profile) stay gray. The colors change - I can see the adaptation - but the yellow cast isn't there, and the colors don't seem outrageous at all.

That didn't make sense to me.

I expected yellow grays to show up in the photo, since yellow grays is what I see when I load the profile into Windows 7.

Now, I had written a long story about this, with possible causes, but then I stumbled upon a website which claimed something simple I wasn't fully aware of: loading the profile into Windows 7 does change the monitor settings (called 'calibration'), but NOT the colors (called 'profiling') - of course, the colors look changed due to the monitor calibration being different, but it's not due to any color conversion going on - there's no conversion between an sRGB standard profile and the Spyder's one.

That totally made sense.

A color managed application has to explicitly convert the photos it shows, using the profile set by windows with a possible starting point being an embedded profile in the photo itself (or standard sRGB if no profile is embedded). And the desktop and any 'color unmanaged' application is not following the color part of the profile: it's only following the calibration part (it can't escape that, because calibration is done through the video card).

The yellow cast is caused by something in the video card's LUT (LookUp Table), not by the color conversion in the profile.

Mind you - to add to my confusion - a color profile can be set to four different rendering intents (the way a photo is 'painted' to the screen). And the Spyder profile does produce yellow cast in a converted photo when I set it to render Absolute in stead of Perceptual. That might (or not) be an indication what it is in the LUT causing the yellow cast, since the renderings differ for instance in how they treat the white point. It could be that the loaded profile uses an absolute setting where the LUT is concerned... but I'm speculating here.

This whole business was confirmed by testing with a completely different profile selected in a color managed application - Photoshop - with that weird Spyder profile loaded into Windows... the yellow grays stayed, no matter which profile I selected in the proofing part of Photoshop...

The yellow cast is like a completely different property that's skipped by the color management functions. I couldn't get a black and white photo to look black and white on my screen, no matter which profile I selected in the proofing module.

Imagine: my color transitions work fine, but it's as if someone (the video card) draped a yellowish filter over the screen.

I'm not sure how to deal with this problem, nor what's causing this specifically.

It's not the color transformation math. So does the LUT contain a seperate entry for grays? Is it the gamma? Has it to do with the white point?

In the case of 'Me And The Mysterious Spyder' it seems I'll have to tweak the video card portion of the profile, not so much the color part... but at the moment the knowledge is lacking on how the two interact...

I'm still a bit unclear on the interaction, because I can't imagine the profile designed specifically for my monitor by the Spyder doesn't somehow mix the two. If I tweak the LUT part, is the color part still correct or is the color transformation actually counting on a lack of one of the color channels?

Well, let's find out...

So it's back to the documentation, I guess you now understand why, seeing my confusion about the subject :-)

In the meantime I'll work further on the proofing part, because I have an explanation for the lacking yellow in converted photos, so the application is actually working properly so far...

I contemplated showing you the examples, but your browser might be color managed, in which case you probably would see something different than what I am seeing. And apart from the philosophical questions one can also ask (are you seeing what I'm seeing and how to confirm that?), this subject is unclear enough as it is...

No comments:

Post a Comment