Display gamma vs RGB working space gamma

[knweiss]

I've got another gamma-related question which keeps me thinking for a while now: What is the relation of the inherent gamma of a RGB working space and the display gamma?

In their tutorials Jeff and Michael often recommend ProPhotoRGB as a RGB working space in Photoshop (at least for 16 bit/channel images). ProPhotoRGB has a gamma of 1.8. On the other hand my display is calibrated to 6500K and 2.2 gamma. Is this a problem? Is there a relation between both gammas? Would I be better of with AdobeRGB as a RGB working space which has an inherent gamma of 2.2 which matches the display's gamma?

(To be honest I do not have a practical image processing problem. I'm very happy with my prints and in my eye they match the monitor color even when I use ProPhotoRGB. I would just like to understand the theory behind it, too.)

[teddillard]

I've got another gamma-related question which keeps me thinking for a while now: What is the relation of the inherent gamma of a RGB working space and the display gamma?

In their tutorials Jeff and Michael often recommend ProPhotoRGB as a RGB working space in Photoshop (at least for 16 bit/channel images). ProPhotoRGB has a gamma of 1.8. On the other hand my display is calibrated to 6500K and 2.2 gamma. Is this a problem? Is there a relation between both gammas? Would I be better of with AdobeRGB as a RGB working space which has an inherent gamma of 2.2 which matches the display's gamma?

(To be honest I do not have a practical image processing problem. I'm very happy with my prints and in my eye they match the monitor color even when I use ProPhotoRGB. I would just like to understand the theory behind it, too.)
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Gamma, strictly, is simply input signal vs output signal. It's a basic ratio of what you feed something and what you get out.  (dammit, sorry..  I can't find the actual equation, right off...) The gamma of a color space deals with what you start with for color values, and where they end up after being mapped...  the display gamma is about what you feed the card for a signal, and what it outputs.  Printer gamma, the same ratio- feed it a signal, it outputs a response.

This is pretty oversimplified...  if you're doing a lot of heavy lifting in color science, you're probably going to jump all over me.  But in terms of a photographer understanding display, color space and other "gamma", I think it's fair to say that they are all different, yet related, and although "gamma is gamma", they are not the same...

[Guillermo Luijk]

The colour profile gamma (1.8 in ProPhoto RGB, 2.2 in Adobe RGB, a modified version of 2.2 in sRGB) will only affect you in the way the RGB values of your image will spread along the histogram. No matter which profile you use, Photoshop will take its gamma into account so the image will display the same in any of those colour profiles (leaving aside the larger or smaller colour gammut of course).

So in brief: you don't have to care about matching your colour profile gamma to the display gamma.

I trend to avoid sRGB and use Adobe RGB instead, not only because of sRGB's insufficient gammut in many cases, but also because I feel more comfortable with a pure 2.2 gamma (Adobe RGB) than the special sRGB tone curve, which produces a higher compression in the low end of the histogram so deep shadows are more difficult to handle than in Adobe RGB.

These are the histograms of the same image in:

sRGB


Adobe RGB


Both display the same, but the low end of the Adobe RGB histogram is easier to edit.

BR

[madmanchan]

So in brief: you don't have to care about matching your colour profile gamma to the display gamma.

Bingo.

[bjanes]

The colour profile gamma (1.8 in ProPhoto RGB, 2.2 in Adobe RGB, a modified version of 2.2 in sRGB) will only affect you in the way the RGB values of your image will spread along the histogram. No matter which profile you use, Photoshop will take its gamma into account so the image will display the same in any of those colour profiles (leaving aside the larger or smaller colour gammut of course).

So in brief: you don't have to care about matching your colour profile gamma to the display gamma.

I trend to avoid sRGB and use Adobe RGB instead, not only because of sRGB's insufficient gammut in many cases, but also because I feel more comfortable with a pure 2.2 gamma (Adobe RGB) than the special sRGB tone curve, which produces a higher compression in the low end of the histogram so deep shadows are more difficult to handle than in Adobe RGB.

These are the histograms of the same image in:

sRGB


Adobe RGB


Both display the same, but the low end of the Adobe RGB histogram is easier to edit.

BR
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Here is a look at aRGB and sRGB as rendered by ACR 4.5 using a linear tone curve.


[attachment=8168:attachment]

[knweiss]

So in brief: you don't have to care about matching your colour profile gamma to the display gamma.

Thanks for your helpful reply!

[mbalensiefer]

Ok, good info. Thank you.

 If I've calibrated my PC's monitor to a gamma of 1.8 and my Mac's to a gamma of, say, 2.0--and then host my photoshop-edited images online...when others view them in any of their own Web browsers--would this be sufficient for color management purposes?

[teddillard]

Ok, good info. Thank you.

 If I've calibrated my PC's monitor to a gamma of 1.8 and my Mac's to a gamma of, say, 2.0--and then host my photoshop-edited images online...when others view them in any of their own Web browsers--would this be sufficient for color management purposes?
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OK, first...  the PC/MAC gamma difference is a total myth.  The short story is you should calibrate any display to 2.2.  (I actually have that myth part from the horse's mouth, Bill Atkinson, about when they were trying to design the first Applewriter, actually a Canon printer.  They changed the card to match the output.  In Bills words, "It was the stupidest thing..."  Every test I've seen on MAC/PC cards comes out with virtually identical numbers.)

Second, most web browsers are not color-managed applications, so all bets are off, but you probably know that.  A few new ones claim to be, but whether they are or not, actually, seems to be a point of debate.  In working "for the web", the safest assumption is to go for the lowest common denominator, rather than hope for the highest state of the art.

Finally, to answer the question...  probably yes.    If you calibrate properly, (at 2.2, with a white point of 6500), set up your color management policies in Photoshop to one of the two "defaults" (NA Prepress or NA General Purpose) , that is basically the best you can hope for.  You are starting from a good, color managed, industry standard.  What happens out in the world, well, happens.

At the very least, if a client complains about the images appearance on the web, you are backing yourself up from a position of industry standard...

[Guillermo Luijk]

In working "for the web", the safest assumption is to go for the lowest common denominator, rather than hope for the highest state of the art.

I agree a lot with this. In fact many times I have wondered: does a very good monitor give me any benefits for image edition when the output is just intended for Web applications? and second question even goes further: could a standard medium quality monitor be even better than a really good monitor for Web image edition?

I have heard about users with expensive high quality monitors that were very dissapointed when seeing their work on regular monitors, surely worse looking than if they had edited the images on the destination monitor.

Unfortunately this sets a disagreement between quality of image in RGB numbers and quality of image in the final application.

[teddillard]

I agree a lot with this. In fact many times I have wondered: does a very good monitor give me any benefits for image edition when the output is just intended for Web applications? and second question even goes further: could a standard medium quality monitor be even better than a really good monitor for Web image edition?

I have heard about users with expensive high quality monitors that were very dissapointed when seeing their work on regular monitors, surely worse looking than if they had edited the images on the destination monitor.

Unfortunately this sets a disagreement between quality of image in RGB numbers and quality of image in the final application.
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I like to think in terms of AM radio (OK, I'm old, and did, in fact, once listen to music this way), or even newspaper reproduction.  A great old Motown recording story goes that they actually used to take stuff out to the car in the lot to listen to the final mix, to make sure it would sound ok on the radio...  

If you really need to check it, you need a similar test...  look at it on an array of crappy to superlative displays.  Quite a few web designers actually do that, they set up testing stations of a few middle-of-the-road systems.  

But the fallback is the newspaper strategy.  Only give them a few tones to screw up...  the so-called "web-safe" approach.  When printing for newsprint I used to give them about 5 shades of gray, on the theory that they could screw up less.

[madmanchan]

Ok, good info. Thank you.

 If I've calibrated my PC's monitor to a gamma of 1.8 and my Mac's to a gamma of, say, 2.0--and then host my photoshop-edited images online...when others view them in any of their own Web browsers--would this be sufficient for color management purposes?
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In general, no, because most people around the world viewing images in their web browser will run into 2 problems: (1) their monitors aren't calibrated & profiled and (2) even if they are, their browsers don't support color management.

[mbalensiefer]

"...set up your color management policies in Photoshop to one of the two "defaults" (NA Prepress or NA General Purpose)".

 Thank you.

 How can I do this? I am currently saving my images in AdobeRGB...

V/R
Michael

[teddillard]

"...set up your color management policies in Photoshop to one of the two "defaults" (NA Prepress or NA General Purpose)".

 Thank you.

 How can I do this? I am currently saving my images in AdobeRGB...

V/R
Michael
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I posted a quick video on the YouTube a while back, showing this...  take a look.
[a href=\"http://www.youtube.com/user/rawpipeline]http://www.youtube.com/user/rawpipeline[/url]