Color loss: working with sRGB or Prophoto

ProPhoto maintains more color information during operations than sRGB.  It is lije making calculations with sone decimals instead of integers.

If you work without decimals, you round numbers at evey step  (like divisions) and inverse ooerations will have lost the decimals.  Working with decimals bring them back and you round only the final result.  In integers, 25/100 = 0, with decimals, .25.  If you later remultiply by 10, the result will be rounded at 3, much better than 0.

same goes with bit depth and colour calculations.

And if we take it further, as happens in image editing, we not only saturate once but thousands of times saturate and desaturate, we use curves for everything. That means that these 5 colors that the sRGB had generated me when making changes again would happen to me from 5 to 3 because sRGB would not have where to take them, and when making more changes I would spend 3 to 1. doing the same thing in Prophoto would start with 10 (not with 5) and maybe it would end with 6. Then it is much more faithful to compress 6 colors to 5 (prophoto to sRGB) than to stay in sRGB with 1 color repeated 5 times. it's not like that?

Hey people. My problem is not visualization. I could do the same theoretical exercise without a screen. I am asking about Photoshop's internal calculations, not its viusalization. The visualization during the work will be with proof colors in sRGB despite using Prophoto and the final viusalization will be in sRGB so it does not matter if my screen does not even reproduce adobeRGB. Nor does it make sense to think of painters, they did not use bits or make calculations between bits to obtain color. They had infinite bits to work and never had banding.

We'll see.

We imagine that for a pixel value the distance between two greens is 10 shades of green. If I saturate the image I increase the distance from 10 to 20 green tones. Among these 20 shades of green there are 20 different green colors making the progression. If I did the same in sRGB when it increased from 10 to 20, sRGB could only provide me with 5 new more greens (its an imaginari example, not the reality). So when I increase the saturation from 10 to 20, sRGB could only fill 5 green interleaved instead of 10. The other 5 spaces would be interspersed without color because sRGB has no more colors to interpolate. So when I move from Prophoto to sRGB I am interpolating his 10 new colors to 5 colors and i finish with only 5 colors like if I had done it from the beginning with sRGB. The question is: is it tighter to make perceptual compression from 10 colors in Prophoto to obtain 5 colors in sRGB, or is it better to generate 5 colors directly in sRGB?

I think it is more tight to compress 10 colors because you can choose which colors compress more, than not to generate 5 colors in a more limited color space that does not allow you to choose between 10 to decide which of the 5 suits you best.

What do you think?

Hello everyone and thanks for the replies. I do not need to know what Prophoto is used for, I just want to understand the internal operation of photoshop according to the color space, to be able to make informed decisions. I just asked if working with an image, for example applying saturation + curves, how is the internal calculation of photshop?

Option 1: Working with Prophoto I decide to apply infinity of curves, contrasts and saturations to edit an image. As prophoto has more colors, it will have more possibilities to distribute the colors resulting from the calculation of the curves. Then I will convert to sRGB in perceptual mode and send it to the client.

Option 2: Working in sRGB I will apply the infinity of cruvas and saturations doing exactly the same edition as before and send it to the client.

My question is whether with option 1 do I get exactly the same as with option 2? Is it true that with option two the margin of error is added after each calculation of curves and the final result of the relationship between the colors is poorer than having used prophoto?

Thank you

In fact ProPhoto has very few advantages, except in a very intermediate editing stage where you need to contain certain extremely saturated colors to avoid premature clipping - until you decide what to do with them.

Lightroom/ACR is a case in point. Very often you get processing artifacts in the form of extremely saturated patches. That doesn't necessarily mean those colors are "real" or even worth keeping. But you don't want to just hard clip them, that doesn't look good. You need to remap.

Further down the line, as you get closer to a specific output, ProPhoto just causes a lot of problems. You still have all these saturated colors that you need to remap for output. The less remapping you have to do, the easier your life gets.

It is often argued that the bigger the gamut, the better. Hence ProPhoto is supposedly always "better". I strongly disagree. Good color has nothing to do with absolute saturation. It's about building relationships. Painters have made great works with pigments of limited purity throughout history. 

ProPhoto is a useful tool in some very specific scenarios. That doesn't mean it's always the best tool for any job.

One very specific practical problem with ProPhoto is the extremely compressed shadow values (huge gamut means something else has to go. Gamma 1.8 exaggerates it). That means subtle shadow adjustments are very difficult.

ProPhoto is only useful in a print workflow where the output device gamut includes color that is outside of the sRGB space, which can be a considerable percentage of a typical CMYK space. Even then you probably wouldn’t want to use ProPhoto in a lowend print workflow that goes directly to press with no contract proofing or understanding of the destination CMYK output numbers. ProPhoto would have no advantage when the only destination is a web page.

The Prophoto working colourspace has various disadvantages, you already know it's 16 bit only (because its so massive) - it also potentially contains significant data that cannot be displayed properly on a monitor display screen.

Next - the conversion from Prophoto to sRGB isn by necessity, a colorimetric rendering (no matter what Photoshop seems to offer), so it can have none of the promised (but not always functioning as expected) gamut mapping features of Perceptual rendering.

In a nutshell, this means that during the conversion from Prophoto to sRGB colours that fit within the destination colour space volume will not be desaturated, any colour values within Prophoto, but outside sRGB, will be mapped to "the skin" of sRGB, basically, this means that detail in high saturation areas can be lost - since previously varying values sitting along the same hue angle are mapped to a single value.

Its called 'clipping' and can be seen in the histogram post conversion. 

I hope that aids your decision making?

I hope this helps

if so, please "like" my reply and if you're OK now, please mark it as "correct", so that others who have similar issues can see the solution

thanks

neil barstow, colourmanagement.net

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