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alexit3748321
alexit3748321
Inspiring
October 27, 2023
Frage

What tone curve does Photoshop use for sRGB?

  • October 27, 2023
  • 1 Antwort
  • 1424 Ansichten

Hi,

Simple question really... what tone curve does PS apply when displaying sRGB images? Is it the piecewise sRGB encoding function or a pure 2.2 power curve?

    1 Antwort

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    October 27, 2023

    First of all, the premise for the question is faulty. Any tone curve will be remapped through standard color management, so that the net visual result on screen is linear. It's a standard profile conversion from the document profile into the monitor profile. Whether it's sRGB, gamma 2.2 Adobe RGB, or gamma 1.8 ProPhoto, it will all be displayed correctly on screen. You don't "see" the document profile tone curve.

     

    Without color management, however, the document's native encoded tone curve is sent directly to screen uncorrected.

     

    The sRGB specification is a custom and irregular tone curve, not a regular gamma function. Among other things, it has a flat "toe" section near black. The reason for this is that sRGB was originally specified as a description of a CRT monitor.  It was, in a sense, the seed for modern color management.

     

    The sRGB tone curve is also used in Apple's Display P3, and Photoshop's Image P3 (those two are identical).

     

    Modern LCD panels don't behave quite like the CRT monitors did, and so it's a bit less useful. But it has been close enough to survive to this day.

      alexit3748321
      alexit3748321Autor
      Inspiring
      October 27, 2023

      No, it's not a faulty premise. The question is whether Photoshop applies a piecewise inverse EOTF or a 2.2 power forward EOTF. Which transfer function does Photoshop use for display output?

       

      SRGB uses an encoding function with an offset 2.4 power with a linear section, however the EOTF is a pure 2.2 power function. This, by the way, is exactly how Apple handles it in both sRGB and Display P3. You can find a good discussion here with references to the actual standard.

      sRGB piece-wise EOTF vs pure gamma - Discussions - ACESNext / VWG – Output Transforms - Community - ACESCentral

        D Fosse
        Community Expert
        D FosseCommunity Expert
        Community Expert
        October 30, 2023

        Sorry, but that makes no sense. You can't just ignore the encoding and aåply the inverse of whatever your monitor transfer function is.


        @alexit3748321 

         

        Yes, that's just what you can. Of course it's not "ignored" - it's corrected for.

         

        I see you're coming from aces/ocio, and as I already said, the big difference is that icc-based color management uses a monitor profile based on actual measurement of the display.  Aces/ocio doesn't do that, so the concept may be unfamiliar.

         

        I suggest you read up on display color management. My co-poster here, Conrad Chavez, has written some excellent stuff, maybe he can point you to something.

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