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reference white - Adobe Photoshop 7, CS, CS2 & CS3

Hello all of you, I undersatnd PS is workngn in the D50 reference white space. I was wondering if there is a facility to change the white reference in a picture (so that it is lighting in a different environment, D65, A, B, C or other color temperature). This could be implemented by using for instance the Bradford transform. I can't find this in the help files. Can someone help me? All the best, Victor...

  1. #1

    Default reference white

    Hello all of you,

    I undersatnd PS is workngn in the D50 reference white space. I was wondering if there is a facility to change the white reference in a picture (so that it is lighting in a different environment, D65, A, B, C or other color temperature).
    This could be implemented by using for instance the Bradford transform.

    I can't find this in the help files. Can someone help me?

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  2. #2

    Default Re: reference white

    Victor,

    working space and monitor space are defined for white point D65.
    Prints are optimized for D50 in a way that the appearance is VERY SIMILAR to the monitor.

    I donīt think that you can directly shift the color temperature of neutral gray in an image by PhS,
    but I am not a PhS expert.

    The shift to D50 (red-ish look) or to D93 (blue-ish look) is done by a matrix multiplication
    RGB-new=M*RGB-old.
    The accurate algorithm is available, but for simple tests this will be be sufficient:
    (1) for D65--D50: multiply R by 1.1765, G by 0.9765 , B by 0.7218
    (2) for D65--D93: multiply R by 0.8986, G by 0.9938 , B by 1.3602
    By the way, this will cause clipping problems for Red (1) or Blue (2) .
    Green is not much modified.
    Perhaps it helps to adjust an image towards the one direction or the other.

    Best regards --Gernot Hoffmann
    Gernot_Hoffmann@adobeforums.com Guest

  3. #3

    Default Re: reference white

    Victor,

    forgot to add that this has to be done in the linear RGB working space.
    R=Rī^gamma. Apply modification. Rī= R^(1/gamma).

    G.H.
    Gernot_Hoffmann@adobeforums.com Guest

  4. #4

    Default Re: reference white

    Oke understand it Gernot,

    One thing though; I understood that PhotoShop was using sRGB with D50, instead of the standard definition of sRGB at D65. You are now saying it is sRGB at D65 for PhotoShop, are you sure?
    See also:
    <http://www.brucelindbloom.com/WorkingSpaceInfo.html>
    Section on 'Adapted primaries'

    It is all quite confusing to me (not being an expert);-)
    I am trying to do some color optimization for a standard scale made by IFRAO see: <http://www.iol.ie/~geniet/eng/IFRAOcoloropt.htm>
    and for this I try to get a good idea what happens. Any input is VERY welcome.

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  5. #5

    Default Re: reference white

    No, Photoshop uses sRGB exactly as it is defined in the IEC standard.
    Chris_Cox@adobeforums.com Guest

  6. #6

    Default Re: reference white

    You have many mistakes in your web page.

    1) sRGB always uses a D65 white point (as given in the IEC standard), and Photoshop uses sRGB exactly as given by the standard.

    2) You can do gamma adjustments in Photoshop (there is no need for ImageReady in your steps)

    3) Photoshop does not contain a capital S, ever

    4) You don't need Brightness/Contrast at all -- just Levels

    5) Photoshop 4 didn't have ANY color calibration, so you couldn't reproduce sRGB in it.

    6) The TRC/gamma functions used in sRGB and most other standard color spaces are not pure power functions - they include some slope limiting to improve matching to human vision and provide invertibility for the TRC function (otherwise the inifinite slope of the power function messes things up).

    You really should compare the accuracy of your method to other calibration methods (typically using many more color samples, such as an IT8.7/2 chart or a MacBetch ColorChecker DC).
    Chris_Cox@adobeforums.com Guest

  7. #7

    Default Re: reference white



    Photoshop does not contain a capital S, ever




    Except when you type PHOTOSHOP. Sorry, couldn't resist.
    YrbkMgr@adobeforums.com Guest

  8. #8

    Default Re: reference white

    Victor,

    formulas and a diagram for the sRGB Tone Reproduction Curve are e.g. here:
    <http://www.fho-emden.de/~hoffmann/gamuts08072002.pdf> (150kB)

    My suggestion for WP shifts didnīt contain anything about adaptation.
    This test is valid:
    Working space sRGB (with D65 according to standard) .
    Make background in image neutral gray: R=G=B=118 (or so).
    Measure color temperature by instrument: nearly 6500K.
    Convert image to D50.
    Measure color temperature by instrument: nearly 5000K.
    And so on. The test requires an accurately calibrated monitor.
    A set of test images D50/D65/D93 is here:
    <http://www.fho-emden.de/~hoffmann/caltutor270900.pdf> (550kB)

    By the way: Iīm not trying to automate image processing. The only exception
    are WP corrections if dominating neutral gray areas are available. This is very
    helpful for digital photos.

    Best regards --Gernot Hoffmann
    Gernot_Hoffmann@adobeforums.com Guest

  9. #9

    Default Re: reference white

    Hello Chris,

    Thanks for your feedback. This is very valuable to me!
    Can you be sure about the D65 (e.g. I know Paint Shop Pro is using indeed D65)? To be honest I had a big discussion about this before I changed it to D50 for Photoshop (see also the earlier URL given, according to this source Photoshop's sRGB uses the same definition as ICC for the white reference: D50).
    Perhaps I should directly ask Photoshop! But can you provide some URL's where it is stated that they use D65 (I tried to look in the help files but found nothing specific).

    Thanks for the feedback what can be done with Photoshop. I was relaying on someone else here. I now downloaded it myself so that I can check things myself.

    I understand from several sources that most RGB workspaces are pure power functions. One exception which is the sRGB, which has a linear part near the black region (so for low values it is not a power functions). I got this from URL's like:
    <http://www.poynton.com/papers/IST_SPIE_9801/index.html>

    I agree, I will to check this all with better charts, I have pictures with the Macbeth ColorChecker and IFRAO standard scale. But first I need to get my basics correct (likr the D? and standard observation angles, so measuring with colorimeter is at this moment being done).

    I have changed all your other remarks in my text.

    Thanks again and all the best,

    Victor
    vreijs@adobeforums.com Guest

  10. #10

    Default Re: reference white

    Hello Chris,

    Can you tell me where I can adjust the gamma in Photoshop? I can't find it? I still want to stay in the sRGB environment though. In ImageReady I can do it, but see nothing like that in Photoshop.

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  11. #11

    Default Re: reference white

    Hello Chris and Gernot,

    Here is some sRGB information: <http://www.srgb.com/>
    and: <http://www.srgb.com/c55.pdf>
    This last link is clearly stating that D50 should be used for sRGB and I understood Adobe is doing that also in the way it is stated in that doent. Do you have contacts with Adobe to verify this? I have no support contract or something like that.

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  12. #12

    Default Re: reference white

    Victor,

    the second link is well-known. Thanks for both.
    sRGB is primarily a working space, well defined by three primaries, D65 white point and a Tone
    Reproduction Curve (TRC) which is near to Gamma=2.2 (less 1% deviation).

    The authors are talking ADDITIONALLY about the question how to transform sRGB data into a so-
    called Profile Connection Space (PCS), which can be used as input for multi-dimensional look-up
    tables with CMYK outputs for printing.
    Graphics art is traditionally viewed "under" D50 light. The aim is: the printed paper under D50 should
    look equal to the monitor view. The monitor can reproduce sRGB more or less directly, if calibrated
    for D65.The (simplified) conversion is done by the Bradford matrix.
    There is an essential adaptation problem: if monitor and print are viewed one after the other, then
    eye+brain are adapted alternating to D65 and D50 and the perception of the SAME physical color
    would be different. The balance for equal appearance is (hopefully) done by Bradford.
    sRGB itself is defined for D65 white point.
    The appreciated authors shouldnīt assign D50 to "sRGB for printing". These transformations be-
    long to the PCS, IMO, and to the definition of the two viewing conditions.

    There is hardly anywhere an algorithmical and understandable explanation. The ICC specification
    is still rather confusing (after reading the important parts twenty times).

    Best regards --Gernot Hoffmann
    Gernot_Hoffmann@adobeforums.com Guest

  13. #13

    Default Re: reference white

    Hello Gernot,

    Thanks for your explanation, I hope I understand it. So now in my words;-).
    I understand sRGB is defined as being in the D65 space as according to the standard. I also understand the e.g. a programa like Paint Shop Pro is also using this. From one person (Bruce Lindbloom, who is quite knowlegdable on this subject) I heard that Photoshop does not use the default white point for sRGB but uses D50 (or perhaps I misunderstand his pages: <http://www.brucelindbloom.com/index.html?WorkingSpaceInfo.html> section 'Adapted Primaries').
    I understand from you and Chris that this is not true, but that Photoshop is using for sRGB, as defined in the standard, D65.

    So this is where I am stuck. I hope that Photoshop can provide the final answer on this (I have no support contract, so I am not able to ask this officially). Are you willing to do this?

    In the meantime, I have changed my web-page in such a way , that I don't link Photoshop anymore to the D50 (except in a note with a question mark). I have now two columns one for sRGB/D65 (standard) and one for sRGB/D50 (non standard). As soon as I know what precisely Photoshop uses, I will update the table.

    Thanks again for your valuable input!

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  14. #14

    Default Re: reference white

    Victor - I don't have URLs, I'm looking at the source code and the sRGB profile. (there is a reason my name appears in the splash screen)

    No, most profiles do NOT use pure power functions. In fact, I don't know of anyone other than yourself (and one net troll) who makes that mistake.

    The Gamma adjustment is the middle slider in the levels dialog, same as in ImageReady.

    I don't know how you're getting D50 for sRGB - it is very clearly using D65, always.
    I'll have to check with Bruce and see how he made a mistake thinking that Photoshop uses something other than the standard sRGB profile.
    Chris_Cox@adobeforums.com Guest

  15. #15

    Default Re: reference white

    OK, Bruce has it correct. But you are misreading what he wrote.

    In ICC profiles, all the primaries are adapted to the PCS whitepoint - which is defined as D50. The sRGB whitepoint (in a separate tag) is still D65.
    Chris_Cox@adobeforums.com Guest

  16. #16

    Default Re: reference white

    Hello Chris,

    I am glad you can confirm that the sRGB in Photoshop is indeed using D65. It was what I always thought, but could not get the correct arguments in place towards Bruce (even though it is in his first table!). I think we had a basic misunderstandig, due to my inexperience perhaps.
    Thanks for confirming Photoshop's implementaion!

    Now that I am certain about the workspace and how to convert them using colorimeter measurements, I will improve my optimization method. Including another gamma function is not problem, and hopefully it will make the optimization process even more stable.
    The gamma option in Photoshop is very essential, so I am glad it is there (will check this night). Now the use of Photoshop becomes much more fluent.
    I hope that my methodology of course works for this simple standard scale (i know about other scales, but it seems that the IFRAO scale is also widely used). I had my personal doubts on its effeciveness, two doubts I had: a) no proper reference color definitions (so only RGB values of Photshop4 were existing) and b) NO procedure how to use it for optimization!!!
    I am now trying to solve these points. And your feedback is helping me greatly.

    I am just a layman wanting to be sure that my time in using the IFRAO card at rock/megalithicart is not wasted!

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  17. #17

    Default Re: reference white

    Hello Chris,

    But thanks to you, I found the Gamma function.
    I re-checked the help files, the word 'Gamma' is not used in the Levels dialog help files. It is differently describe as: "You can use the middle Input slider to change the intensity values of the middle range of gray tones without dramatically altering the highlights and shadows." That made me miss the relation with Gamma, sorry (Photoshop was up to now not my default program).

    Another thing, I can do different functions for gamma's in my optimization program, no problem! But does Photoshop support different gamma functions (so with linear parts), looking at the supported gamma in the Levels it seems to be the pure power functions.
    To be realisitic my optimization program uses the same functions as available under Photoshop/PSP.

    All the best,

    Victor
    vreijs@adobeforums.com Guest

  18. #18

    Default Re: reference white

    I would like to add a question:

    According to my test program IccInspect by LittleCMS the working space Adobe RGB (98)
    uses a pure power function with exponent 2.2 without a linear slope.
    Is this a misinterpretation by IccInspect ?

    The tone reproduction function can be defined by
    a) 3 tables
    b) 3 parametric functions of different complexity.

    Best regards --Gernot Hoffmann
    Gernot_Hoffmann@adobeforums.com Guest

  19. #19

    Default Re: reference white

    The gamma adjustment in Levels is a predefined power function with a linear slope, plus a small spline segment to smooth the meeting of the two functions.

    Sorry that the doentation left out the word gamma. We'll have to work on that.

    Gernot - the ICC profile spec's it as a power function, but all CMMs that I know of add the linear slope.
    Chris_Cox@adobeforums.com Guest

  20. #20

    Default Re: reference white

    Chris,

    "linear slope for Adobe RGB(98)"
    This not according to the standards. The CMS has to use the features as defined
    in the profile.

    Best regards --Gernot Hoffmann
    Gernot_Hoffmann@adobeforums.com Guest

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