16 Replies Latest reply on Sep 18, 2013 2:34 PM by TurgayOrhan

    From ACR to PS in Lab mode

    TurgayOrhan Level 1

      The new version of ACR can open images directly in Lab in PS ...

       

      I would like to learn advantages/disadvantages of using Lab as a workflow color space in ACR over the ProPhoto RGB ...

       

      Thanks a lot.

        • 1. Re: From ACR to PS in Lab mode
          thedigitaldog MVP & Adobe Community Professional

          ProPhoto primaries are used as the internal processing color space in Adobe raw converters. So converting to Lab really buys you nothing but another color space conversion. Depending on who you talk to, Lab is either the most amazing and special color model to edit in, or it's a flavor of the month for editing spaces. There are no capture or output devices that produce Lab. It's an intermediate color space at best. It's not the most intuitive set of values to deal with if you're not familiar with it. Just about anything you can do in Lab, you can do in an RGB working space depending on where you apply these edits. For example, one area Lab proponents like to point out is you can affect tone (luminance) without color. But in Photoshop you can do this by using the Blend mode called Luminance. It's not 100% identical to the same edit in Lab but when rubber meets the road, the separation of tone and color moves applies. And again, in LR or ACR, you're working in an RGB engine, so if you're editing there, you're editing in RGB. If you've produced the desired color and tone edits you wish before further pixel retouching in Photoshop, you don't need Lab one bit.

          • 2. Re: From ACR to PS in Lab mode
            TurgayOrhan Level 1

            Thanks a lot,

            There are two points I want to ask ...

             

            Lab is a profile connection space ... so, we already use it when we want to output our files from ProPhoto to any output color space. So, I thought that converting to Lab at the beginning reduce our profile conversion steps in PS if we output our files for web or paper. Not correct?

             

            And, secondly, can we continue to work in the linear workflow in PS with the Lab?

            • 3. Re: From ACR to PS in Lab mode
              thedigitaldog MVP & Adobe Community Professional

              Lab is a profile connection space ... so, we already use it when we want to output our files from ProPhoto to any output color space. So, I thought that converting to Lab at the beginning reduce our profile conversion steps in PS if we output our files for web or paper. Not correct?

              Nope. The output conversion is indeed necessary and it indeed uses Lab. And that's going to happen but isn't necessary until that point. If you're printing yourself, it's going to take place when you hit the Print button, this doesn't reduce a conversion.

               

              You could work in a Linear RGB space in Photoshop if you wanted to. There's no reason why this is necessary.

              • 4. Re: From ACR to PS in Lab mode
                G.Hoffmann Level 3

                Orhan,

                 

                this book explains clearly the power of CIELab (Photoshop Lab):

                 

                http://www.amazon.com/Photoshop-LAB-Color-Adventures-Colorspace/dp/0321356780

                 

                Data in an RGB space can be linear or nonlinear (gamma-encoded) with respect

                to the physical color space CIEXYZ.

                Lab is always nonlinear with respect to CIEXYZ. There is no alternative - on purpose.

                 

                Lab is the profile connection space between RGB-spaces and CMYK-spaces and

                between RGB-spaces and large Monitor profiles. Small Monitor profiles don't contain

                tables but only mathematical functions. Here we have CIEXYZ as profile connection

                space between working space RGB and monitor RGB.

                 

                Best regards  --Gernot Hoffmann

                • 5. Re: From ACR to PS in Lab mode
                  TurgayOrhan Level 1

                  I've heard the good reputation of this book.

                  I think ... it is a good time now to open a place in my reading programme for it.

                   

                  Thank you so much ...

                  • 6. Re: From ACR to PS in Lab mode
                    TurgayOrhan Level 1

                    How would it be if ACR and LR use Lab as a working space?

                    Is it necessary to use a space from the RGB color model for raw conversion?

                    • 7. Re: From ACR to PS in Lab mode
                      thedigitaldog MVP & Adobe Community Professional

                      TurgayOrhan wrote:

                       

                      How would it be if ACR and LR use Lab as a working space?

                      Is it necessary to use a space from the RGB color model for raw conversion?

                      In Adobe raw processors, it's alway an RGB path and the internal RGB color space is a variant of ProPhoto RGB (same gamut/primaries, linear encoding). Just save out ProPhoto RGB and be done with it IMHO. Lab brings nothing to the party but another conversion. Unless you want to be in the Lab flavor of the month club. About the only  benefit of Lab is it is truly a device independent color space and self defining (unless you start mixing up it's white point from the assumed D50). But ProPhoto is tagged out of LR/ACR so unlesss someone does something stupid, it's definition is known.

                       

                      There's no such thing as a prefect working space or we'd all be using just one. There are a number of problems with Lab and we could talk about some of the issue with ProPhoto RGB (mostly 'colors' that fall outside human vision). To some degree, Lab's been oversold and we should keep in mind it's original design intentions which took place well before Photoshop was invented, let alone ACR/LR.

                       

                       

                      CIELAB was 'recommended' by the CIE in 1976, to address a specific problem, namely, while identical XYZ values could tell you when two stimuli would be experienced as the same 'color' by most observers, it did not tell you how 'close' two colors were if they were not exactly the same XYZ value. Where Lab is useful is for predicting the degree to which two sets of tristimulus values will match under defined conditions thus it is not anywhere close to being an adequate model of human color perception. It works well as a reference space for colorimetrically defining device spaces, but as a space for image editing, it has many problems. There are a slew of other perceptual effects that Lab ignores. Lab assumes that hue and chroma can be treated separately, but numerous experimental results indicate that our perception of hue varies with the purity of color. Mixing white light with a monochromatic light does not produce a constant hue, but Lab assumes it does! This is seen in Lab modelling of blues. It's the cause of the dreaded blue-magenta color issues or shifts. Lab is no better, and in many cases can be worse than a colorimetrically defined color space based on real or imaginary primaries.

                      • 8. Re: From ACR to PS in Lab mode
                        TurgayOrhan Level 1

                        Mixing white light with a monochromatic light does not produce a constant hue, but Lab assumes it does.

                         

                        Can you explain this a little, if you have time ...

                        Does this mean that our famous Lab sharpening is not so perfect in fact? ...

                        Or, it means ... Lab sharpening results are not realistic?

                         

                         

                        This is seen in Lab modelling of blues. It's the cause of the dreaded blue-magenta color issues or shifts.

                         

                        Today, I've made some readings on this blue-magenta shift ... from the Bruce Lindbloom site:

                         

                        "The problem starts with the observation that a typical full blue on a monitor simply cannot be reproduced on a printer. Since an accurate reproduction is physically impossible, a substitute color must be used in its place. It would be nice if this substitute color was the same apparent hue, although perhaps less saturated. A printer profile will typically choose the substitute color by selecting a color of similar hue angle, as measured in the CIE Lab color system. Unfortunately, the Lab color system is not perfect in its ability to match its hue angle with perceptual hue."

                         

                        He also states that there is a similar problem for reds as well ... they are shifting to oranges.

                         

                        As far as I can understand (I'm not a color expert, just a learning student) shifting problem is attributed to the converting step between Lab and printer profiles.

                         

                        At this point, there are two questions I would like to ask ...

                         

                        1. We're already using Lab for converting from the working space to the printer profiles ... for example, if we use ProPhoto RGB as a working space, our path is ProPhoto RGB > Lab > Printer Profile ... so I think that these shifting problems can still be realized, even if we don't use the Lab as working space. Am I wrong?

                         

                        2. These shifting problems are attributed to the converting step to the printer profile, as I indicated above. Is it also possible to see them, if we use Lab as a working space ... as an editing space?

                        • 9. Re: From ACR to PS in Lab mode
                          G.Hoffmann Level 3

                          Orhan,

                           

                          a remark about this statement:

                          Mixing white light with a monochromatic light does not produce a constant hue, but Lab assumes it does.

                           

                          White light plus monochromatic light creates light with constant hue in the fundamental CIE (1931) color model,

                          which is used as well for CIELAB. These models don't contain any special color appearance feature.

                           

                          There is nowhere in Photoshop any color model which is based on anything else but the classic CIE approach,

                          somewhat improved by Bradford adaptation, concerning the white point (in PhS Lab, D65 – D50 for example).

                          http://docs-hoffmann.de/cielab03022003.pdf

                           

                          Handling of colors by RGB models is nowhere better than handling by Lab. Lab is just another mathematical

                          description for the same color space (unique, if the RGB space has been defined).

                           

                          HLS can be considered as a simple decomposition of an RGB space by Hue, Lightness and Saturation.

                          Lines of constant (calculated) hue are lines H=const. Not even here the calculated hue reflects exactly the

                          perceived hue (see below in the mentioned doc).

                           

                          Nobody did ever say,  that in Lab in the plane a,b  straight lines from the origin to any  point (a,b) are

                          lines of constant perceptual hue.

                           

                          Here I would like to refer to p.60:

                          (1)

                          http://docs-hoffmann.de/cielab03022003.pdf

                           

                          Where does it matter?

                           

                          Coming from any RGB space and going to a CMYK printer space, one has to apply a gamut compression.

                          (2)

                          http://docs-hoffmann.de/gamshow15052009.pdf

                           

                          Traditionally this has been applied by a linear interpolation in Lab, but it should be applied along a curve, the

                          curve of equal perceptual hue in Lab. Bruce Lindbloom had emphasized this, my doc (1) shows the same issue.

                           

                          Lab is the profile connection space for going from RGB to CMYK. Not Lab is wrong but the common method of

                          interpolation.

                           

                          There is nowhere a cheap alternative – like avoiding Lab and go directly from RGB to CMYK.

                           

                          Basics of CIE color systems are explained here:

                          (3)

                          http://docs-hoffmann.de/ciexyz29082000.pdf

                           

                          Of course there are many other docs besides my owns.

                           

                          We can easily divide the discussion in to two parts:

                          a) about color spaces

                          b) which space is optimal for editing (this issue is actually not discussed here)

                           

                          Best regards --Gernot Hoffmann

                          • 10. Re: From ACR to PS in Lab mode
                            thedigitaldog MVP & Adobe Community Professional

                            TurgayOrhan wrote:

                            Does this mean that our famous Lab sharpening is not so perfect in fact? ...

                            Perfect? Probably unnecessary in terms of my understanding why one would sharpen in that space. Namely to keep a color shift from showing up, which can be accomplished in RGB by running Fade Luminosity after USM in RGB.

                             

                            Keep in mind that CIELab was just an attempt to create a perceptually uniform color space where equal steps correlated to equal color closeness based on the perception of a viewer. The CIE didn't claim it was prefect (because its not). Most color scientists will point out that Lab exaggerates the distance in yellows and consequently underestimate the distances in blues. Lab assumes that hue and chroma can be treated separately. There's an issue where hue lines bend with increase in saturation perceived by viewers as an increase in both saturation and a change in hue when that's really not supposed to be happening. Further, according to Karl Lang, there is a bug in the definition of the Lab color space. If you are dealing with a very saturated blue that's outside the gamut of say a printer, when one uses a perceptual rendering intent, the CMM preserves the hue angle and reduces the saturation in an attempt to make a less saturated blue within this gamut. The result is mathematically the same hue as the original, but the results end up appearing purple to the viewer. This is unfortunately accentuated with blues, causing a shift towards magenta. Keep in mind that the Lab color model was invented way back in 1976, long before anyone had thoughts about digital color management.

                             

                            In terms of conversions with Lab from color space to color space,the actual conversions to and from Lab (as you'd do in Mode Change) are too slow and unnecessary. What Photoshop does is it builds a conversion table and to do so. It uses LAB to find the equivalents from source to destination in cases where it needs to translate such color spaces, using 20-bit precision so you get less quantization errors than you would actually converting the pixels to LAB. I assume ACR/LR does the same process. In LR, if you ask for Lab, it is the equivalent of a Mode change in Photoshop in that you are converting TO Lab from the internal RGB color space.

                            • 11. Re: From ACR to PS in Lab mode
                              TurgayOrhan Level 1

                              Thanks to both of you for your valuable inputs ...

                               

                               

                               

                               

                              b) which space is optimal for editing (this issue is actually not discussed here)

                               

                               

                              I would really like to hear your replies to this question ... and, if possible, with the reasons numbered and listed one under the other.

                              • 12. Re: From ACR to PS in Lab mode
                                thedigitaldog MVP & Adobe Community Professional

                                TurgayOrhan wrote:

                                 

                                Thanks to both of you for your valuable inputs ...

                                 

                                 

                                 

                                 

                                G.Hoffmann wrote:

                                 

                                b) which space is optimal for editing (this issue is actually not discussed here)

                                 

                                 

                                I would really like to hear your replies to this question ... and, if possible, with the reasons numbered and listed one under the other.

                                For me, it's ProPhoto RGB. That's what is happening under the hood in the ACR engine. Adobe RGB (1998) is too small a color gamut for my output needs. Here's why:

                                 

                                Everything you thought you wanted to know about color gamut

                                A pretty exhaustive 37 minute video examining the color gamut of RGB working spaces, images and output color spaces. All plotted in 2D and 3D to illustrate color gamut.

                                High resolution: http://digitaldog.net/files/ColorGamut.mov

                                Low Res (YouTube): http://www.youtube.com/watch?v=n0bxSD-Xx-Q

                                • 13. Re: From ACR to PS in Lab mode
                                  TurgayOrhan Level 1

                                  Thank you Andrew ... also for the video.

                                   

                                  I would like to ask ...

                                  In the Lab coordinates in 2D view, there is a grid that contains a and b axis ... does this grid match the horseshoe in the Yxy?

                                  • 14. Re: From ACR to PS in Lab mode
                                    thedigitaldog MVP & Adobe Community Professional

                                    TurgayOrhan wrote:

                                    In the Lab coordinates in 2D view, there is a grid that contains a and b axis ... does this grid match the horseshoe in the Yxy?

                                    I'm not sure <g> I have to look at the video at the spot you're referring to. That said, if it's a 2D plot in ColorThink, then it is set for Yxy.

                                    • 15. Re: From ACR to PS in Lab mode
                                      G.Hoffmann Level 3

                                      Orhan,

                                       

                                      this docs shows as well the mapping of Lab to Yxy:

                                      http://docs-hoffmann.de/cielab03022003.pdf

                                       

                                      On page 56 we see

                                      a) lines of constant 'hue' or angle in a,b for varying L in Yxy

                                      b) lines of constant 'chroma' or radius in a.b for L=50 in Yxy

                                       

                                      By the way: page 58 shows that Lab with a,b = -128 to +127

                                      (standard) is sufficient for all visible colors (and that it contains

                                      like ProPhoto a range of not existing 'colors').

                                      Lines a=0 and b=0 are marked. 

                                       

                                      Best regards -- Gernot Hoffmann

                                      • 16. Re: From ACR to PS in Lab mode
                                        TurgayOrhan Level 1

                                        Andrew, Gernot ... Thanks a lot.

                                         

                                         

                                        G.Hoffmann wrote:

                                         

                                        Orhan,

                                         

                                        this docs shows as well the mapping of Lab to Yxy:

                                        http://docs-hoffmann.de/cielab03022003.pdf

                                         

                                        On page 56 we see

                                        a) lines of constant 'hue' or angle in a,b for varying L in Yxy

                                        b) lines of constant 'chroma' or radius in a.b for L=50 in Yxy

                                         

                                        By the way: page 58 shows that Lab with a,b = -128 to +127

                                        (standard) is sufficient for all visible colors (and that it contains

                                        like ProPhoto a range of not existing 'colors').

                                        Lines a=0 and b=0 are marked. 

                                         

                                        Best regards -- Gernot Hoffmann

                                         

                                        Exactly what I'm looking for ... thank you again.