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alexit3748321
alexit3748321
Inspiring
October 27, 2023
Question

What tone curve does Photoshop use for sRGB?

  • October 27, 2023
  • 1 reply
  • 1444 views

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 reply

    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
      alexit3748321Author
      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 28, 2023

        BTW - since ACES is mentioned here, some clarification. ACES-based color management (OCIO) is used in the film/video industry. It operates on exactly the same principles as icc-based color management - but with one important distinction: there is no measurement of the display, no monitor profile. The screen output is just defined as a range of preset standards.

         

        https://helpx.adobe.com/after-effects/using/opencolorio-aces-color-management.html 

         

        In terms of accuracy, icc based color management takes it one step further by using a monitor profile. It makes sense to not do that in video, since a full monitor profile conversion for each frame may not be very practical.

         

        ACES-based color management does not do a full remapping. So there you need to match file and display as closely as possible beforehand, and any variation from the preset tone curve will be visible. With icc-based color management that is not necessary, full remapping is done in real time.

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