Profiles and gamuts

Golden writes …

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

You can do this but there are problems editing the files because the gamuts can be much wider than what your monitor can display.

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

The main problem is the saturated colors outside the gamut of your monitor are displayed clipped (the monitor profiles are relative colormetric, not perceptual) so you can be changing these colors radically yet not even know it until later when it’s time to print.

Here’s a link to a good article that discusses the good and bad sides of using wide gamut spaces …
http://www.creativepro.com/story/feature/8582.html?origin=st ory … here’s a quote from the article illustrating the dangers of shifting colors your monitor can’t display …
"(using) Kodak’s ProPhotoRGB, you can reduce saturation by about 20 to 25 points, and shift hue by about 7 to 8 degrees, without seeing any visible change in the display of strongly saturated colors." Except you WILL see the changes when you convert to a smaller space for output and usually you’ll be surprised by the results.

ProPhotoRGB is wider than the WideRGB you mention but it’s available in the Adobe RAW converter as a target space and digital camera users make use of it sometimes for saturated images, but one suggested workflow is to convert (or in your case, assign after scanning) to the wide space to preserve the subtle tonalities (using 16 bit/channel mode) and then convert again (using perceptual rendering) to something easier to work in, say AdobeRGB, so you are working in a space that better matches what the monitor can display.

This is a controversial topic so I’m expecting some to disagree with my post 🙂 What I do is use the wider spaces (often Ektaspace since it matches the gamut of slide films) with saturated films like Velvia or for digital RAW images with saturated colors, especially reds. But for most RAW conversions I just use AdobeRGB unless I’m worried about saturation issues. I personally stay away from ProPhoto since it’s much wider than Ektaspace and I’m used to Ektaspace from my film scan days (the gamut of ProPhoto is, I think, "all visible light") but others seem to like it.

At any rate, run a few scans and convert to different profiles for different types of images (ie, highly saturated vs not so saturated) and see what works for you … by the time you get to the printing stage you might be hard pressed to find the differences for most images, especially if they are printed small or don’t have highly saturated colors to begin with.

Bill

Golden writes …

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

You can do this but there are problems editing the files because the gamuts can be much wider than what your monitor can display.

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

The main problem is the saturated colors outside the gamut of your monitor are displayed clipped (the monitor profiles are relative colormetric, not perceptual) so you can be changing these colors radically yet not even know it until later when it’s time to print.

Here’s a link to a good article that discusses the good and bad sides of using wide gamut spaces …
http://www.creativepro.com/story/feature/8582.html?origin=st ory … here’s a quote from the article illustrating the dangers of shifting colors your monitor can’t display …
"(using) Kodak’s ProPhotoRGB, you can reduce saturation by about 20 to 25 points, and shift hue by about 7 to 8 degrees, without seeing any visible change in the display of strongly saturated colors." Except you WILL see the changes when you convert to a smaller space for output and usually you’ll be surprised by the results.

ProPhotoRGB is wider than the WideRGB you mention but it’s available in the Adobe RAW converter as a target space and digital camera users make use of it sometimes for saturated images, but one suggested workflow is to convert (or in your case, assign after scanning) to the wide space to preserve the subtle tonalities (using 16 bit/channel mode) and then convert again (using perceptual rendering) to something easier to work in, say AdobeRGB, so you are working in a space that better matches what the monitor can display.

This is a controversial topic so I’m expecting some to disagree with my post 🙂 What I do is use the wider spaces (often Ektaspace since it matches the gamut of slide films) with saturated films like Velvia or for digital RAW images with saturated colors, especially reds. But for most RAW conversions I just use AdobeRGB unless I’m worried about saturation issues. I personally stay away from ProPhoto since it’s much wider than Ektaspace and I’m used to Ektaspace from my film scan days (the gamut of ProPhoto is, I think, "all visible light") but others seem to like it.

At any rate, run a few scans and convert to different profiles for different types of images (ie, highly saturated vs not so saturated) and see what works for you … by the time you get to the printing stage you might be hard pressed to find the differences for most images, especially if they are printed small or don’t have highly saturated colors to begin with.

Bill

On 23 Aug 2005 15:10:13 -0700, "Golden"
wrote:

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

Specifically, I’m thinking of Wide RGB and a Nikon V.

It depends on what you’re starting with and what you intend to do with it. More info will result in a more useful reply.

—

Hecate – The Real One

Fashion: Buying things you don’t need, with money
you don’t have, to impress people you don’t like…

Golden wrote:

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

Specifically, I’m thinking of Wide RGB and a Nikon V.

Thanks!

Golden

If you use the widest gamut that is available, converting back into sRGB/AdobeRGB may look very ugly.

Waldo

"Golden" wrote in message

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

You should use the narrowest gamut that still accomodates your capture device’s output.

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

Don’t play safe by using the widest you can find, because that will compromise accuracy for most colors, instead of for just a few outliers.

Specifically, I’m thinking of Wide RGB and a Nikon V.

Depending on the material you scan, the Ektaspace profile may be a good choice. A better choice is a profile you specifically make for the scanner + film-dye family in question.

Bart

Bill Hilton wrote:

Golden writes …

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

You can do this but there are problems editing the files because the gamuts can be much wider than what your monitor can display.

That’s basically why I’m asking. I like to preserve as much color data as possible but I can’t see most of it because the gamut is shrunk before it is displayed.

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

The main problem is the saturated colors outside the gamut of your monitor are displayed clipped (the monitor profiles are relative colormetric, not perceptual) so you can be changing these colors radically yet not even know it until later when it’s time to print.

That’s it again. Making editing decisions based on a small subset I can see doesn’t seem like a very good idea if those changes are then applied to data I do not see. But as I say I don’t know much about this so that’s why I ask.

Here’s a link to a good article that discusses the good and bad sides of using wide gamut spaces …
http://www.creativepro.com/story/feature/8582.html?origin=st ory … here’s a quote from the article illustrating the dangers of shifting colors your monitor can’t display …

Thank you very much for that. It looks like I’ll be going back to sRGB.

Golden

Waldo wrote:

Golden wrote:

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

If you use the widest gamut that is available, converting back into sRGB/AdobeRGB may look very ugly.

I noticed! 😉 The histogram ain’t too hot either with all types of artefacts after the conversion. That’s what prompted me to ask.

Golden

Bart van der Wolf wrote:

You should use the narrowest gamut that still accomodates your capture device’s output.

I’m still knew at all this but am I correct in saying that "scanner profile" is basically my capture device’s (Nikon V) output? Is that the pure data from the scanner? When I compare that to sRGB (with sRGB as my working space) I see virtually no difference.

If all that is correct it would suggest that Nikon V output can be fully expressed in sRGB color space. Is that correct?

Depending on the material you scan, the Ektaspace profile may be a good choice. A better choice is a profile you specifically make for the scanner + film-dye family in question.

I’m not a pro so that’s probably going too far for me. I have a wide variety of materials from some very old B&W film to all sorts of negatives (local film I bought in the country I was at) to mostly Kodachrome slides but there are some no name slides there as well.

So basically it looks like I should just stick to sRGB.

Golden

Golden wrote:

I’m still knew at all this but am I correct in saying that "scanner profile" is basically my capture device’s (Nikon V) output? Is that the pure data from the scanner? When I compare that to sRGB (with sRGB as my working space) I see virtually no difference.

That is because you are looking at the image on a monitor, so you are not really looking at the original colors, but a representation of those colors in the monitors color space (which is close to sRGB).

If all that is correct it would suggest that Nikon V output can be fully expressed in sRGB color space. Is that correct?

No, see above. The scanner probably has a wider color space, at least AdobeRGB. The problem is that you cannot see the difference on the average monitor.

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/

"Golden" wrote:

In order to preserve as much color data as possible for editing in Photoshop

You should, in the first place, acquire and edit in the 16-bit/c mode.

should I use the widest gamut profile available on my
scanner?

In order to absolutely preserve as much color data as possible you would of course need to optimize the acquire space according to the material you scan but doing that that is not an easy thing to do and is gratuitous especially when you work in 16-bit/c. In this case using the largest gamut that the acquire device provide is the very best and the most safest approach.

Are there any pitfalls to using a gamut which is too wide

RGB gamuts are 3-dimensional volumes, they are either large or small, they are all equal in "height" (luminance) but their "width" (or the form of their cross-section plane) depends on the luminance level that the measurement is done and their "width" at a particual luminance level is way different depending on between what two edge colors the "width" is measured.

Larger gamuts are good in that they have more room for saturation of the hues at the higher luminance levels (near the whitepoint) than what the smaller gamuts provide.

(unwanted dynamic range compression, for example)?

No, when you acquire in 16-bit/c.

There are other aspects to the selection of the RGB space gamut, especially to the selection of the working-space gamut. By experimenting you can easily notice that the exactly same editing operation can give quite different results in different RGB working-spaces. So …then… this immediately raises the question: What is the correct result, they can not all be correct, and hence what is the correct gamut to choose for the RGB working-space.

Timo Autiokari
http://www.aim-dtp.net/

On Wed, 24 Aug 2005 11:44:09 +0200, Waldo wrote:

Golden wrote:

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

Specifically, I’m thinking of Wide RGB and a Nikon V.

Thanks!

Golden

If you use the widest gamut that is available, converting back into sRGB/AdobeRGB may look very ugly.

It can do, but not if you use the mechanism described in Don Margulis’ Professional Photoshop where you can end up converting between WideRGB, CMYK, LAB and another RGB.

—

Hecate – The Real One

Fashion: Buying things you don’t need, with money
you don’t have, to impress people you don’t like…

Hecate wrote:

On Wed, 24 Aug 2005 11:44:09 +0200, Waldo wrote:

Golden wrote:

In order to preserve as much color data as possible for editing in Photoshop should I use the widest gamut profile available on my scanner?

Are there any pitfalls to using a gamut which is too wide (unwanted dynamic range compression, for example)?

Specifically, I’m thinking of Wide RGB and a Nikon V.

Thanks!

Golden

If you use the widest gamut that is available, converting back into sRGB/AdobeRGB may look very ugly.

It can do, but not if you use the mechanism described in Don Margulis’ Professional Photoshop where you can end up converting between WideRGB, CMYK, LAB and another RGB.

Therefore I said the word "may". It really depends on the content of the image.

Also, converting between different color spaces is always done via a "profile connection space" (PCS), a device independant color space. Mostly XYZ and LAB are used for this purpose.

Waldo

Waldo wrote:

If you use the widest gamut that is available, converting back into sRGB/AdobeRGB may look very ugly.

I doubt that very much. No matter how wide the color space, you are looking at the image *in your monitors color sapce* already. That is usually very close to sRGB and even the best monitors are not more than AdobeRGB. So, you are already looking at a ‘converted image’ if you are looking at it on your monitor. If it doesn’t look ugly on your monitor right now, there is no reason why it would look ugly after conversion to sRGB or AdobeRGB. In fact, you probably wouldn’t even notice any difference, for the above reason.

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/

Johan W. Elzenga wrote:

Golden wrote:

I’m still new at all this but am I correct in saying that "scanner profile" is basically my capture device’s (Nikon V) output? Is that the pure data from the scanner? When I compare that to sRGB (with sRGB as my working space) I see virtually no difference.

That is because you are looking at the image on a monitor, so you are not really looking at the original colors, but a representation of those colors in the monitors color space (which is close to sRGB).

That takes me back to my original question, how do I edit an image in a working space which is narrower than the color space of the image? What do you pros do? Is it a 6th sense or are there some practical things I can draw on?

If all that is correct it would suggest that Nikon V output can be fully expressed in sRGB color space. Is that correct?

No, see above. The scanner probably has a wider color space, at least AdobeRGB. The problem is that you cannot see the difference on the average monitor.

What I’m still confused about is how does this conversion happen?

Please bear with me because this is hard to explain. Let’s just for the sake of the argument imagine two gamuts: "Big" with 100 colors and "Lil" (a subset of Big) with 50 colors. Let’s also assume that scanner gamut is identical to "Lil" gamut. Finally, let’s say that the available dynamic range can handle 50 colors.

Given all that it appears to me that using Lil gamut is actually better than using Big gamut. Why? Because expressing the scanner’s gamut with Lil gamut will utilize the available dynamic range fully. Big gamut will only use a fraction (50%) of its available dynamic range to express Lil and the rest will end up unused. In other words, Lil gamut will have to be compressed to fit the space allocated to it within Big gamut.

Does that make sense?

Golden

Timo Autiokari wrote:

"Golden" wrote:

In order to preserve as much color data as possible for editing in Photoshop

You should, in the first place, acquire and edit in the 16-bit/c mode.

Yes I definitely do that.

In this case using
the largest gamut that the acquire device provide is the very best and the most safest approach.

That’s was my initial instinct but now I’ve been rattled.

(unwanted dynamic range compression, for example)?

No, when you acquire in 16-bit/c.

What about the case I raise in my answer to Johan?

Each gamut utilizes the available dynamic range fully. Right?

If that’s true then using a bigger gamut to express a smaller one will only end up compressing the smaller gamut and so make matters worse. Yes, there will be more room at both ends but I start with fewer colors.

Golden

"Golden" wrote in message

Johan W. Elzenga wrote:

Golden wrote:

What I’m still confused about is how does this conversion happen?

It is nornally done by an ‘on the fly’ profile to profile conversion between your working colour space (e.g. Adobe RGB etc.) and your monitor’s profile (as defined by your monitor calibration) using relative colorimetric rendering. Relative colorimetric rendering causes colours which are common to both working space and monitor to display correctly, but colours in the working space which are beyond the monitor’s capability are clipped to the nearest hue. Hence 2 distinct colours in your working space will look identical on your monitor. However, the data in your file is not affected – as I said, the conversion to monitor space is done ‘on the fly’.

Please bear with me because this is hard to explain. Let’s just for the sake of the argument imagine two gamuts: "Big" with 100 colors and "Lil" (a subset of Big) with 50 colors. Let’s also assume that scanner gamut is identical to "Lil" gamut. Finally, let’s say that the available dynamic range can handle 50 colors.

Given all that it appears to me that using Lil gamut is actually better than using Big gamut. Why? Because expressing the scanner’s gamut with Lil gamut will utilize the available dynamic range fully. Big gamut will only use a fraction (50%) of its available dynamic range to express Lil and the rest will end up unused. In other words, Lil gamut will have to be compressed to fit the space allocated to it within Big gamut.

Does that make sense?

Gamut is *not* about ‘number’ of colours or dynamic range – it is about the *range* of colours available, i.e. how saturated a colour can be represented. Technically, a scanner does not have a gamut – it will return a set of RGB values for any colour it sees; only a calibration (i.e. scanner profile) gives meaning to those RGB values by assigning them to *device independent* colours as defined by Lab or XYZ. This calibration will then define the *effective* gamut of the scanner.

As an example, let’s say the scanner was calibrated with a Velvia calibration slide. The slide will contain the full range of colours available on Velvia film. I am sure you are aware that Velvia film can produce colours far more saturated than the average monitor which has an sRGB like characteristic. If the scanner has been calibrated with such a target, it will have an effective gamut like Velvia film – far in excess of sRGB. If you decide to use sRGB as your working space, the the colours from your scanner need to be converted to fit that smaller colour space. This can be done *perceptually* – i.e. by compressing the entire Velvia gamut into the limits of sRGB (which will reduce the saturation of *ALL* the colours, both in and out of gamut), or *colorimetrically* – i.e. by clipping *only* out of gamut colours to the nearest sRGB hue and hence losing the separation between some of the colours.

The range of colours available, as defined by the gamut of the colour space, is divided up by the *number* of available colours as defined by the bit depth (e.g. 8-bit, 16 bit). A wide gamut colour space divides a *wider* range of colours into the same number of increments as a narrow gamut colour space. Hence the *jumps* between individual colours in a wide gamut space are greater than in a narrow space. Hence the recommendation to use 16 bit colour when using wide gamut colour spaces.

—
John
Replace ‘nospam’ with ‘todnet’ when replying.

Golden wrote:

Please bear with me because this is hard to explain. Let’s just for the sake of the argument imagine two gamuts: "Big" with 100 colors and "Lil" (a subset of Big) with 50 colors. Let’s also assume that scanner gamut is identical to "Lil" gamut. Finally, let’s say that the available dynamic range can handle 50 colors.

No, as John explained, it’s not about MORE colors, it’s about a more saturated color range.

Given all that it appears to me that using Lil gamut is actually better than using Big gamut. Why? Because expressing the scanner’s gamut with Lil gamut will utilize the available dynamic range fully. Big gamut will only use a fraction (50%) of its available dynamic range to express Lil and the rest will end up unused. In other words, Lil gamut will have to be compressed to fit the space allocated to it within Big gamut.

Does that make sense?

In theory, yes but in practise, no. If your scanner has a color gamut that is close to AdobeRGB, and you scan in ProPhotoRGB, a lot of the ProPhotoRGB space will remain unused. That is indeed true and it makes sense: Using ProPhotoRGB does not somehow increase the maximum color saturation your scanner can capture, so the most saturated colors of ProPhotoRGB will not be really present. However, if you scan in 16 bits, the ‘excess’ compared to 8 bits is so big, that this doesn’t really matter.

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/

"Golden" wrote:

Each gamut utilizes the available dynamic range fully. Right?

Dynamic range usually refers to the gray axis, that axis is exactly the same in what ever RGB gamut.

With colors (with all the other codes than the grays that are the same as R=G=B) there is also dynamic range but it is better related to saturation and to the maximum available luminance for the particual hue. Larger gamut can hold both higher saturation and higer luminance values for hues.

If that’s true then using a bigger gamut to express a smaller one will only end up compressing the smaller gamut

Yes it is true.

and so make matters worse.

Not so if you work in the 16-bit/c space.

There is more quantization for colors of course but the 16-bit/c space (that in Photoshop is only 15-bit/c + 1 level) can easily withstand that. For example, in the worst case, there is quantization of only 1 bit between the gamuts of adobeRGB and CIE1931 (that is a very very large gamut). So converting image data in Photoshop 16-bit/c space from AdobeRGB to CIE_1931_D65_Gamma_1 profile will result effectively 14-bit/c (since it really was in 15-bit/c space). And that quantization does not happen all over the gamut but only locally here and there.

Now, none of us have such image data that truly is 15-bit/c, typically we have something like 10 to 14 bit/c A/D conversion _from_ sensor data that is effectively comparable to less than 10bit/c. So there is extra room in 16-bit/c to hold the image data in large gamut RGB spaces. E.g in case you have the typical 12-bit/c A/D converter in your scanner or camera then the image data that is destined to Photoshop 16-bit/c mode is multiplied by 8, so :

original 12-bit level 0 becomes level 0*8=0 in the file Photoshop sees original 12-bit level 1 becomes level 1*8=8 in the file Photoshop sees original 12-bit level 2 becomes level 2*8=16 in the file Photoshop sees

and so on. So nothing is lost if the quantization is doubled, it could be doubled several times.

Timo Autiokari
http://www.aim-dtp.net/

On 25 Aug 2005 06:56:27 -0700, "Golden"
wrote:

That takes me back to my original question, how do I edit an image in a working space which is narrower than the color space of the image? What do you pros do? Is it a 6th sense or are there some practical things I can draw on?

Which brings me back to the question I asked you which you never replied to: for what purpose? For example, I use AdobeRGB all the way through my workflow (though I will convert in and out of other spaces for specific reasons). Why? Because it maps most closely to CMYK and I’m producing work for print.

—

Hecate – The Real One

Fashion: Buying things you don’t need, with money
you don’t have, to impress people you don’t like…

John wrote:

Gamut is *not* about ‘number’ of colours or dynamic range – it is about the *range* of colours available, i.e. how saturated a colour can be represented.

That’s what I mean even though my terminology is way off. This is what I was thinking:

The number of colors in the real world is virtually infinite. But in practical terms even the widest gamut is limited by the available dynamic range used to express it because there is only a finite number of colors available even in 16-bits. Ignoring specialist high definition range image file formats this puts a ceiling on how many visible colors can be expressed so in that sense this infinite gamut is expressed using only 64K distinct colors. What I was confused about is how different color spaces use this available limited range of only 64K distinct colors and how these color spaces are mapped relative to each other within this limited and finite range.

Technically, a scanner does not have a gamut – it will return a set of RGB values for any colour it sees;

From what I understand these values are returned when scanning in what

Nikon calls "scanner RGB" when there is no profile attached to the resulting file.

What I did is open such as file in two working color spaces sRGB and Wide using "Assign Working RGB" when the dialog box pops up. The sRGB version looked fine but the Wide version looked weird. Actually it looked exactly the same as when I open a file tagged with sRGB in Wide working color space. (I can desaturate monitor colors but that gets me even further away from what’s really in the file.)

From all this I drew the conclusion probably incorrectly that this

"scanner RGB", which as I understand contains all available scanner colors, could be expressed fully in sRGB alone.

The range of colours available, as defined by the gamut of the colour space, is divided up by the *number* of available colours as defined by the bit depth (e.g. 8-bit, 16 bit). A wide gamut colour space divides a *wider* range of colours into the same number of increments as a narrow gamut colour space. Hence the *jumps* between individual colours in a wide gamut space are greater than in a narrow space. Hence the recommendation to use 16 bit colour when using wide gamut colour spaces.

That’s exactly what I was getting at only I wasn’t able to express it due to my lack of terminology and knowledge in general.

Thank you for taking the time to explain all this, John! Things are slowly starting to make sense but I still have a very steep learning curve ahead of me and that includes rereading this message a few more times.

Golden

Johan W. Elzenga wrote:

Golden wrote:

Given all that it appears to me that using Lil gamut is actually better than using Big gamut. Why? Because expressing the scanner’s gamut with Lil gamut will utilize the available dynamic range fully. Big gamut will only use a fraction (50%) of its available dynamic range to express Lil and the rest will end up unused. In other words, Lil gamut will have to be compressed to fit the space allocated to it within Big gamut.

Does that make sense?

In theory, yes but in practise, no. If your scanner has a color gamut that is close to AdobeRGB, and you scan in ProPhotoRGB, a lot of the ProPhotoRGB space will remain unused. That is indeed true and it makes sense: Using ProPhotoRGB does not somehow increase the maximum color saturation your scanner can capture, so the most saturated colors of ProPhotoRGB will not be really present. However, if you scan in 16 bits, the ‘excess’ compared to 8 bits is so big, that this doesn’t really matter.

Which means that my instinct to just automatically go for the largest available gamut without thinking is definitely wrong. While it may represent a larger range of colors most of them will not be used. But by using 16-bits, and I do, this becomes a bit of a moot point because of the large number of colors available in 16-bits.

As someone said earlier, and I think now I’m starting to understand it, the optimum gamut is the one which expresses all colors present in the scanner. If the gamut is larger then the scanner gamut will get compressed although that may not be much of a problem because 16-bits have lots of overhead. If the gamut is smaller then it may not be able to express all colors present in the scanner gamut but this will be offset in part by getting a truer representation on the screen of what’s in the file which means fewer problems caused by editing and no conversions for printing or display.

So the bottom line is there are no easy answers because it’s all pretty complicated with conflicting things pulling in different directions depending on usage. Exactly what I did *not* want to hear! 🙂

Golden

Hecate wrote:

On 25 Aug 2005 06:56:27 -0700, "Golden"
wrote:

That takes me back to my original question, how do I edit an image in a working space which is narrower than the color space of the image? What do you pros do? Is it a 6th sense or are there some practical things I can draw on?

Which brings me back to the question I asked you which you never replied to: for what purpose? For example, I use AdobeRGB all the way through my workflow (though I will convert in and out of other spaces for specific reasons). Why? Because it maps most closely to CMYK and I’m producing work for print.

Sorry I got sidetracked by all these new things I’m learning.

My use is fairly pedestrian. I plan to keep all my scans in their pure form before I do any touching up. I will then make various copies for different uses for example reduce them to JPGs for viewing or print them out for framing. But I want to keep the original scans as pure as I can so that’s why I wanted to capture as much as possible and why I wanted to use the largest gamut.

Golden

Timo Autiokari wrote:

"Golden" wrote:

If that’s true then using a bigger gamut to express a smaller one will only end up compressing the smaller gamut

Yes it is true.

and so make matters worse.

Not so if you work in the 16-bit/c space.

Yes I do. That’s a decision I made right on the outset.

There is more quantization for colors of course but the 16-bit/c space (that in Photoshop is only 15-bit/c + 1 level) can easily withstand that. For example, in the worst case, there is quantization of only 1 bit between the gamuts of adobeRGB and CIE1931 (that is a very very large gamut). So converting image data in Photoshop 16-bit/c space from AdobeRGB to CIE_1931_D65_Gamma_1 profile will result effectively 14-bit/c (since it really was in 15-bit/c space). And that quantization does not happen all over the gamut but only locally here and there.

How much difference is there between sRGB and adobeRGB, or even Wide? I know you wrote that "sadRGB" is very limited but how many bits would I lose by using it over adobeRGB or Wide?

My scanner is nominally 14-bits and I’m thorn between conflicting things pulling in different directions. I don’t want to sacrifice accuracy my scanner can deliver but I also don’t want the risk of editing such highly accurate data by looking at the narrow information my monitor shows me.

And thank you also Timo for a detailed explanation and for taking the time.

Golden

"Golden" wrote:

How much difference is there between sRGB and adobeRGB, or even Wide?

It took some effort to answer that question. First I explain what I did:

Calculations were performed in double precision floating point in Excel. For the smaller of the two gamut that are compared I created 29791 sample RGB patches, so each channel has 31 numerically equally spaced points from 0.001 to 1. This means that I used the range of 1000:1 (this was to avoid zero channel values) .

Then converted this RGB dataset to the larger gamut.

Then took the divisions Rs/Rl, Gs/Gl and Bs/Sl for each of the 29791 RGB pairs where index "s" means the smaller gamut value and index "l" means the larger gamut value. Because both the spaces have the same amount of digital levels per channel these results are in essence the multipliers with what the per channel quantisation steps in the smaller gamut are multiplied for that particular color when looking at the situation in the larger gamut.

I then took max, average, variance and stdev over these multipliers (over the 3*29791 result values).

An example (using 8-bit notation for simpicity): In case we have RGBsmall=100,50,25 that converts to RGBlarge=50,80,30 then for this color (the color that we see is the same in both the spaces, just the RGB values are different) the red channel in the larger gamut has 100/50 = 2 times the quantization what the R channel of the same color has in the RGBsmall. And for this color the green channel in the larger gamut has 50/80 = 0.625 times the quantization what the G channel of the same color has in the RGBsmall. And for this color the blue channel in the larger gamut has 25/30=0.833 times the quantization what the B channel of the same color has in the RGBsmall.

In addition I used D65 whitepoint, absolute colorimetry and since we are interested about the effect of the gamut volume alone the gamma was set to 1.0 for all the color spaces.

Now to the results, they are quite interesting:

sadRGB to adobergb(1998):
max=1.3978, average=0.9585, variance=0.049, stdev=0.2213

sadRGB to Widegamut:
max=3.1208, average=0.932, variance=0.0934, stdev=0.3056

sadRGB to CIE XYZ:
max=2.3017, average=0.9323, variance=0.1311, stdev=0.3621

adobergb(1998) to Widegamut:
max=3.4334, average=0.9245, variance=0.0578, stdev=0.2404

adobergb(1998) to CIE XYZ:
max=1.6471, average=0.9229, variance=0.1058, stdev=0.3252

(multiplier 0.5 means one bit gain, multiplier 1 means no change, multiplier 2 means one bit loss, multiplier 4 means two bits loss to the gradation).

So, when going from a smaller gamut to a larger gamut there are some colors (some subvolumes inside the whole gamut volume) that suffer a little (since the max value is > 1) however in al the cases the average is a little less than 1 so, on average, there is a little benefit (the gradation steps are, on average, a little smaller). The small variance means that the multiplier are rather close to the average so the subvolumes of the gamut where the gradation goes close to the max are very small.

Now, these results seem to to be impossible, until it is realized that the dataset that was used was limited to the to smaller gamut. This is the only way how the gradation in the RGB channels can be examined since the rest of the colors that the larger gamut holds are naturally out-of-gamut colors for the smaller gamut (so can not be defined there at all). This means that the gradation of those colors that can only be defined in the larger gamut must be more coarse so that the larger gamut size comes true.

So, this evaluation tells that there is absolutely nothing to worry about when converting from a smaller gamut RGB working-space to a larger one in regards of quantization as long as the data is kept in the 16-bit/c mode. Even the very large "CIE 1931 D65 Gamma 1.0" profile that I have been using, for over a year now, as my RGB working-space does not introduce any problems.

I did this evaluation using my AIM.XLA, it is freeware Excel Add-In for colorimetric and spectral color calculations, available at http://www.aim-dtp.net/aim/technology/aim_xla/index.htm

My scanner is nominally 14-bits

So it has 14-bit analog to digital converter. However the sensor in your scanner has something about 10-bit effective dynamic range at the best so the extra 4 bits do not contain useful image data.

Timo Autiokari
http://www.aim-dtp.net/

Timo Autiokari wrote:

"Golden" wrote:

How much difference is there between sRGB and adobeRGB, or even Wide?

It took some effort to answer that question.

Thank you very much for that, Timo! I feel guilty you went through all that trouble. It will take me some time to digest it all and I really appreciate the effort.

So, this evaluation tells that there is absolutely nothing to worry about when converting from a smaller gamut RGB working-space to a larger one in regards of quantization as long as the data is kept in the 16-bit/c mode.

Thanks again for clarifying that. As I said I do use 16-bit mode.

I did this evaluation using my AIM.XLA, it is freeware Excel Add-In for colorimetric and spectral color calculations, available at http://www.aim-dtp.net/aim/technology/aim_xla/index.htm

I have often been to your site. I particularly like the gamma curves because they produce much more visually pleasing results than Photoshop’s gamma. I find that your gamma files produce smoother gradients.

My scanner is nominally 14-bits

So it has 14-bit analog to digital converter. However the sensor in your scanner has something about 10-bit effective dynamic range at the best so the extra 4 bits do not contain useful image data.

I did notice that because excessive noise is often a problem when scanning difficult film and Coolscan V doesn’t have multiscanning.

Golden