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how is ProPhoto converted to sGRB, and is Wide Gamut monitor necessary if only posting on web

Forum|Forum|8 years ago
September 11, 2017
5 replies
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Hi All,

Could you please help with my two questions?

1) How is a ProPhoto image converted to sGRB? Which case below is true?

a) Simply clipping away the colors outside of sGRB space, retaining the colors inside the sGRB space, and keeping each color's relative intensity intact? (sorry I don't fully understand the CIE color space definition. I assume CIE color space only defines the mix of wavelength for each given color but not the intensity of that color).

b) Clipping away the colors outside sGRB space, but adjusting the remaining colors' (relative) intensities so as to mimic the original ProPhoto image as much as possible?

If the answer is a), then as a result some images will look quite differently after the conversion, depending on how much of the original image is outside the sGRB space, correct?

Again, if a) is the answer, then my second question is:

2) Does a wide gamut monitor help in photo editing, if I only export in sGRB for the web? If so, in which way?

One way I can think of is - during the editing, because colors outside sGRB space are visible to user, they help the user to make a better decision in editing. But again, there will be a color shift after conversion so this 'benefit' will more or less cancel itself out in the end.

Only if the answer to question 1) is b), then I can see the clear benefit of using a wide gamut monitor in editing for web publishing (sGRB exports).

Sorry for the long questions. Searched for days and couldn't get the relevant answers to the exact questions.

Thank you all very much!

Richard

This topic has been closed for replies.

Correct answer D Fosse

For standard color space profiles, only relative colorimetric rendering intent is available. You can choose other intents, but relcol is what you get.

That means all colors outside the target gamut are hard clipped. All colors inside the target gamut are preserved unchanged.

What you see on screen at any time is already clipped to the monitor color space. So if you have a standard gamut monitor (sRGB-ish), you will not see any on-screen change. ProPhoto to sRGB will look exactly the same.

For web, where you want to stay in sRGB consistently, a wide gamut monitor makes no sense - and will probably get you into a lot of trouble. The thing about wide gamut monitors is that they require full display color management at all times, no exceptions. It'll work for web as long as you use a fully color managed web browser like Firefox, but the instant color management stops, it blows up in your face.

Wide gamut monitors only make sense for print work, where you can work with print profiles that exceed sRGB.

SnP-jtEvJOAuthor

Participant

Sorry a follow-up question about "hard clipping":

Let's say at a specific location of a picture (a spot) of which the color belongs to a position in the color space that is outside of sRGB. After hard clipping, that color is simply deleted. Now what happens to that location in the picture? White? Black? Interpolation from neighbors if they are already in sRGB space? But interpolation is a type of 'intent', isn't it? So is it, if replacing the deleted with a color from the nearest edge of sRGB space, right?

thanks and sorry for the additional troubles,

Richard

D Fosse

Community Expert

Hard clipping means clipped to the gamut boundary. Out of gamut colors are reproduced as the nearest in-gamut, reproducible color.

If you have a standard gamut monitor, and work in Adobe RGB or ProPhoto, you have hard clipping right in front of you all the time. Most of what you see on screen will contain clipped areas, small or large.

So it's not a show-stopper in itself. It just produces a rather unpleasant visual effect where texture and detail is lost, and the overall impression is very dense, opaque and lacking "air".

SnP-jtEvJOAuthor

Participant

Thank you!

SnP-jtEvJOAuthor

Participant

Hi D Fosse, davescm, Stephen_A_Marsh, Test Screen Name,

Thank you all very much for your kind teaching and very quick and clear responses. I am truly grateful for all your help!

Richard

T

Test Screen Name

Legend

You might be interested to know how ICC conversion works. There are just two "standard" colours in ICC, called Lab and XYZ. These colours are "absolute", not dependent on colours in a screen or printing inks. These two colours are called Profile Connection Space or PCS.

A profile has, in an ideal case, six conversion rules in it. The rules might be a table of values, a mathematical formula, or some mixture. There are three conversions from the profile colour to PCS and three conversions from PCS to profile colour.

Your original question shows a keen insight as they describe a real choice. The choice is called "rendering intent" and there is (in the ideal case) a conversion rule for each of the three rendering intents. But according to the other discussions, Standard RGB profiles tend not to include some of them, except for "absolute", the one which keeps accurate colour by just clipping.

davescm

Community Expert

Hi

How the conversion of colours are treated , particularly those outside or near to the gamut limit of the color space, depends on the chosen rendering Intent.

The two most commonly used with photographic images are Relative and Perceptual. See link below for an explanation of each:

Introduction to the ICC profile format

If all your images will only end up as sRGB and never be used for anything else - then the value of a Wide Gamut monitor is limited. Indeed, it could be a hindrance as you must use color managed applications throughout (that includes viewers , browsers etc) with such monitors.

I hope that helps

Dave

D Fosse

Correct answer

Community Expert

For standard color space profiles, only relative colorimetric rendering intent is available. You can choose other intents, but relcol is what you get.

That means all colors outside the target gamut are hard clipped. All colors inside the target gamut are preserved unchanged.

What you see on screen at any time is already clipped to the monitor color space. So if you have a standard gamut monitor (sRGB-ish), you will not see any on-screen change. ProPhoto to sRGB will look exactly the same.

For web, where you want to stay in sRGB consistently, a wide gamut monitor makes no sense - and will probably get you into a lot of trouble. The thing about wide gamut monitors is that they require full display color management at all times, no exceptions. It'll work for web as long as you use a fully color managed web browser like Firefox, but the instant color management stops, it blows up in your face.

Wide gamut monitors only make sense for print work, where you can work with print profiles that exceed sRGB.

davescm

Community Expert

https://forums.adobe.com/people/D+Fosse wrote
For standard color space profiles, only relative colorimetric rendering intent is available. You can choose other intents, but relcol is what you get.

I just learnt something new there Dag

Dave

D Fosse

Community Expert

Yep, it's a little known fact. The new version 4 spec supposedly supports perceptual, but so far only an sRGB v4 spec has surfaced, and is not well supported at that. It remains to be seen whether it will be more widely adopted. Most of these profiles are simple matrix-based. For perceptual to work you need table-based (LUT) profiles, which are much more complex.

The same goes for monitor profiles. AFAIK V4 monitor profiles are still only relcol across the board.

Print profiles, in contrast, usually offer a choice, and here it can make a substantial difference to the final result. The ultimate solution, however, is still a full gamut remapping, but that can be a lot of work and may not be worth the effort.

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