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alexit3748321
Inspiring
October 27, 2023
Question

What tone curve does Photoshop use for sRGB?

  • October 27, 2023
  • 1 reply
  • 1399 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
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
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
Community Expert
October 28, 2023

Neither, you still misunderstand. Photoshop applies your monitor profile.

 

Standard color management remaps the tone curve from the document color space to the monitor color space.

 

Are you using a calibrator to make your monitor profile? A calibrator measures your display and builds an icc monitor profile based on that measurement. The transfer function in the profile is whatever your monitor does.