Inspiring

Question

Photoshop LAB Color Space

Forum|Forum|8 years ago
March 2, 2017
4 replies
5699 views

Hello,

I have sRGB input image.

It seems that Adobe Photoshop's LAB Color Space conversion is different from anything I saw on other sources.

Anyone knows why is it different?

What Photoshop do differently?

Thank You.

This topic has been closed for replies.

T

Test Screen Name

Legend

I read an interesting article here sRGB profiles on ICC's web site. It talks about different profiles (V2,V4); indicates the V4 will be better (hence, different); makes it plain that they do contain different mappings for different rendering intents; and talks about variations from still other profiles related to D50/D65 mapping. This should make it unambiguous that there is no single, correct XYZ <-> sRGB mapping; it will depend on the profile and rendering, if not black point (which it says is set to zero in the V4 profile).

So the first site mentioned, no matter how good its formula, is only describing one of the many possible and valid mappings between these two colour spaces.

D Fosse

Community Expert

The version 4 specification for sRGB has been out for a while, and it does look promising on the face of it. Support for perceptual rendering intent should be interesting.

The problem is that no one so far uses it or supports it. The icc itself says that v4 may not always work well with v2, and recommends a full version 4 path. But there is no version 4 Adobe RGB or ProPhoto yet.

The only ones who have started to implement v4 profiles are the makers of calibration software. Version 4 is increasingly becoming the default option for monitor profiles - and people get into trouble over it, and have to change back to v2.

In any case, a revision of the sRGB spec is overdue. It's still widely used as a generic monitor profile, but no longer as appropriate for that purpose as it once was. The primaries aren't quite the same in LCD vs CRT, tone response curves aren't identical, and the flat "toe" can go away and be replaced with standard black point compensation.

Still, sRGB is underrated and the bad reputation is wholly undeserved. It was at the time a monumental achievement, and it is the foundation that modern color management was built on. Even today, sRGB is what allows people to work without color management and still see a roughly right representation on screen. It does what it is supposed to do remarkably well, even today.

T

Test Screen Name

Legend

Is sRGB not a device profile? I don't recall a technical difference. Why would you bypass BPC for it?

T

Test Screen Name

Legend

Just reviewed Easyrgb.com. It has some colour science but only partially. It is worthless and damaging in its view of CMYK conversion, and its RGB-XYZ is worthless because it does not define which RGB is its reference. Any site which gives "RGB to anything" is worthless and damaging if it does not define which RGB, except for certain derived spaces like HSB.

Royi AAuthor

Inspiring

I mostly followed Bruce Lindbloom site:

Welcome to Bruce Lindbloom's Web Site

tmyusuf74

Inspiring

This will be the many reason but I am told you original point, the original point is Photoshop have the RGB or Hex values be entirely different, RGB/HEX VALUES is the main different here.

Royi AAuthor

Inspiring

tmyusuf74, Sorry I didn't get you.

What do you mean RGB HEX Values are the main different here?

Thank You.

D Fosse

Community Expert

Just disregard that, Royi. Hexadecimal notation is just missing the point and obviously irrelevant.

I'm getting the impression you want to dissect this down to the mathematical basis. I can't help you there, but as Chris Cox stated in that other (old) thread you posted to, Photoshop follows the specifications from the icc strictly. As it has to of course, since the whole business of color management rests on icc specs.

In any case, that's where I'd start.

D Fosse

Community Expert

It can't be.

Lab is a standard and standardized color space, used for instance as one of two Profile Connection Spaces in color management operations (the other is CIE XYZ). If it wasn't consistent, the whole thing would fall apart.

Lab is supposed to be a reference.

Rounding errors can happen, particularly in 8 bit operations. Or, what you thought was sRGB is in fact untagged, or the values read in a non-color managed environment.

I think you need to give examples.