Linear Raw Editing, Display Gamma

[Dave Ellis]

I have read in a number of places that when a raw file is edited in ACR, the edit adjustments are done on the linear raw data (rather than gamma encoded data). Presumably though, gamma must still be applied to the image data before it is sent to the display hardware to ensure it doesn't look dark and un-natural. Am I correct in assuming that this is how it works ie edit adjustments on raw data but then display data is gamma encoded ?

The next question - what happens when this developed raw file is directly opened in PS ? Does the image data go across to PS as linear data or gamma encoded. If the former, is the editing in PS done on linear data as in ACR ?

Finally, if a jpeg is opened in ACR or PS, is the editing is done on gamma encoded data (which is built in to the jpeg image) ?

Dave

[D Fosse]

I won't pretend to know the internal workings of ACR, but display color management works the same as Photoshop and any other color managed application. The source color space - in this case linear ProPhoto - is converted/transformed into the display color space (display profile). It's a standard profile conversion (although the technical term is "transform", probably to emphasize that it's not user initiated, but automatically performed by the application, on the fly).

This is no different than 1.8 ProPhoto converted into a 2.2 display profile. One TRC (gamma) remapped into the other. Remember that the display has an inverse native TRC, so the net result is 1.

Opening from ACR into PS is technically an export, same procedure, straight color space conversion.

[Dave Ellis]

I won't pretend to know the internal workings of ACR, but display color management works the same as Photoshop and any other color managed application. The source color space - in this case linear ProPhoto - is converted/transformed into the display color space (display profile). It's a standard profile conversion (although the technical term is "transform", probably to emphasize that it's not user initiated, but automatically performed by the application, on the fly).

This is no different than 1.8 ProPhoto converted into a 2.2 display profile. One TRC (gamma) remapped into the other. Remember that the display has an inverse native TRC, so the net result is 1.

Opening from ACR into PS is technically an export, same procedure, straight color space conversion.

Thanks for your reply. I guess it is difficult to know what's going on inside a software application but I think your comments add weight to the argument that gamma encoding or transformation for display purposes is, at least on some occasions, handled separately from what happens in the editing space.

Dave

[digitaldog]

Presumably though, gamma must still be applied to the image data before it is sent to the display hardware to ensure it doesn't look dark and un-natural.

Not at all. As long as the ICC profile that describes the linear data is intact, it will preview just fine. If no profile exists, it then looks dark and unnatural. I've got linear capture from a very old Kodak DCS camera that I've used for years in seminars to illustrate that when viewed untagged, it looks rather awful. Assign Profile command, select the correct profile that defines the actual capture, the numbers don't change at all, the color appearance looks correct. Be happy to dig up the file and profile for anyone who wants to see this in action. But the point is this, you can build and use a 1.0 TRC profile and data that is 1.0 and it doesn't look dark and unnatural. Lie to the app and make it 'think' this is gamma corrected data, it then looks ugly.

The next question - what happens when this developed raw file is directly opened in PS ? Does the image data go across to PS as linear data or gamma encoded. If the former, is the editing in PS done on linear data as in ACR ?

Conversion from one color space to the other. Just as you could convert from say ProPhoto RGB who's TRC is 1.8 to sRGB who's TRC is 2.2 without issue. You could if you wanted, render from ACR or LR into a 1.0 TRC variation of ProPhoto RGB (build it in Photoshop). It will look just fine. Not sure why you'd want a 1.0 TRC of ProPhoto, but it's quite easy to do.

[Dave Ellis]

Not at all. As long as the ICC profile that describes the linear data is intact, it will preview just fine. If no profile exists, it then looks dark and unnatural.

Thanks for your comments Andrew. We may be talking at cross purposes here - I was simply suggesting that if the data is linear, gamma encoding must be applied to it somewhere before it reaches the display, otherwise it will look dark. I was not meaning this to be a manual step though, but rather it be done automatically by the software. I assume the ICC profile is used for this purpose. Or is the gamma encoding done in the display hardware (video card) ?

Dave

[digitaldog]

-I was simply suggesting that if the data is linear, gamma encoding must be applied to it somewhere before it reaches the display, otherwise it will look dark.

It has to be applied in any case or the image will not appear correctly. It can look too light too if the two are out of sync in the opposite fashion.
The compensation between the two happens between the profile that describes the image and the profile that describes the display (Adobe calls this:Display Using Monitor Compensation).

[Tim Lookingbill]

...I was simply suggesting that if the data is linear, gamma encoding must be applied to it somewhere before it reaches the display, otherwise it will look dark...

Dave

I believe the curve you're really referring to here pertaining to why a Raw image doesn't look initially dark due to its linear encoded data is not technically a gamma curve at all but this red curve shown here...

http://www.jpereira.net/images/stories/dng/tone_curve.png

...viewed in Adobe's DNG Profile Editor of the Base Tone curve embedded in all Default camera profile driven previews in ACR/LR. You'll note every Raw file viewed in ACR/LR has to have some camera profile which can't be turned off meaning the red base tone curve.

[Dave Ellis]

It has to be applied in any case or the image will not appear correctly. It can look too light too if the two are out of sync in the opposite fashion.
The compensation between the two happens between the profile that describes the image and the profile that describes the display (Adobe calls this:Display Using Monitor Compensation).

Thanks Andrew, I will read up on Display Using Monitor Compensation.

Dave

[Dave Ellis]

I believe the curve you're really referring to here pertaining to why a Raw image doesn't look initially dark due to its linear encoded data is not technically a gamma curve at all but this red curve shown here...

http://www.jpereira.net/images/stories/dng/tone_curve.png

...viewed in Adobe's DNG Profile Editor of the Base Tone curve embedded in all Default camera profile driven previews in ACR/LR. You'll note every Raw file viewed in ACR/LR has to have some camera profile which can't be turned off meaning the red base tone curve.

Thanks Tim, I have not used DNG Profile Editor before. I have downloaded it and will experiment with it to see if I can get a handle on how it's tone curve works.

Dave

[D Fosse]

gamma encoding (...) handled separately from what happens in the editing space.

gamma encoding must be applied to it somewhere before it reaches the display

Just rephrasing Andrew's reply, but for clarity - the display profile is gamma encoded (usually but not necessarily 2.2).

Again, display color management is a lot less complicated than most people think, and perfectly equivalent to any other standard color management operation: source/document > destination/display. In this operation, source TRC/gamma is remapped into destination TRC/gamma. Each profile just needs to be an accurate description of the color space it refers to.

[D Fosse]

An interesting point: Gamma encoding's main purpose is to maximize bit information content, since human vision is more sensitive to differences in low luminosities. So more bits are needed there than up in the highlights.

And as it happens, the native behavior of a CRT monitor is pretty much the inverse of that.

From Wikipedia:

"The similarity of CRT physics to the inverse of gamma encoding needed for video transmission was a combination of luck and engineering, which simplified the electronics in early television sets".

http://en.wikipedia.org/wiki/Gamma_correction

[Tim Lookingbill]

Thanks Tim, I have not used DNG Profile Editor before. I have downloaded it and will experiment with it to see if I can get a handle on how it's tone curve works.

Dave

The DNG Profile Editor is a great "understanding" tool to show how ACR/LR render tone and color of Raw files. Other Raw converters implement something similar to the red base tone curve, but I'ld have to warn you that turning this curve off or editing it will not get you very far in improving the image.

I've messed with this base tone curve in other Raw converters several years ago and it wasn't worth the PITA especially when trying to bring out shadow detail close to absolute black without kicking up a lot of noise and posterization.