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Author Topic: converting from prophoto to sRGB: tip  (Read 32281 times)
digitaldog
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« Reply #40 on: November 25, 2007, 06:47:31 PM »
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You keep saying yes and no to some of my remarks without providing a a proper answer.

Because your statements are black and white and there's more to it than that.


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"that ProPhoto RGB (like all the other RGB working spaces) are synthetically manufactured using simple math'

All the RGB working space are synthetic color spaces.

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You also state
'In the case of ProPhoto RGB, there ARE colors defined that fall outside human vision.'

mmm  I think most people would challenge you on that one!

All you have to do is look at a CIE chromaticity diagram to see this. There are at least two primaries that fall outside the spectrum locus.

http://www.luminous-landscape.com/tutorial...photo-rgb.shtml
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« Reply #41 on: November 25, 2007, 07:14:40 PM »
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You also state 'In the case of ProPhoto RGB, there ARE colors defined that fall outside human vision.' mmm  I think most people would challenge you on that one!
Colours outside the HVS????

You betcha! Challenge accepted, since it is so easy:
http://www.luminous-landscape.com/tutorial...photo-rgb.shtml

Look at Figure. 3b  (about half way down) ProPhoto RGB "actually exceeds it (visible spectrum) in the deep greens and deep blues. What this means is that colours can be pushed into areas which can neither be seen nor reproduced, producing very nasty looking results within the visible spectrum. User beware."

It is worth reading the rest of the article too. - Angst
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digitaldog
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« Reply #42 on: November 25, 2007, 07:25:24 PM »
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Look at Figure. 3b  (about half way down) ProPhoto RGB "actually exceeds it (visible spectrum) in the deep greens and deep blues. What this means is that colours can be pushed into areas which can neither be seen nor reproduced, producing very nasty looking results within the visible spectrum. User beware."
[a href=\"index.php?act=findpost&pid=155953\"][{POST_SNAPBACK}][/a]

Well I don't know if user beware is compulsory, we don't want to scare people because its necessary to have such large working spaces.

Forgive my standard copy and paste, but it puts these kinds of color spaces into perspective.

I call it, Gamut mismatch (fitting round pegs in square holes)

It IS true that the wider the granularity in a color space, the harder it is to handle subtle colors. This is why wide gamut displays that can't revert to sRGB (current LCD technology doesn't allow this.) are not ideal for all work (ideally you need two units).

There are way, way more colors that can be defined in something like ProPhoto RGB than you could possibly output, true. But we have to live with a disconnect between the simple shapes of RGB working space and the vastly more complex shapes of output color spaces to the point we're trying to fit round pegs in square holes. To do this, you need a much larger square hole. Simple matrix profiles of RGB working spaces when plotted 3 dimensionally illustrate that they reach their maximum saturation at high luminance levels. The opposite is seen with print (output) color spaces. Printers produce color by adding ink or some colorant, working space profiles are based on building more saturation by adding more light due to the differences in subtractive and additive color models. To counter this, you need a really big RGB working space like ProPhoto RGB again due to the simple size and to fit the round peg in the bigger square hole. Their shapes are simple and predictable. Then there is the issue of very dark colors of intense saturation which do occur in nature and we can capture with many devices. Many of these colors fall outside Adobe RGB (1998) and when you encode into such a space, you clip the colors to the degree that smooth gradations become solid blobs in print, again due to the dissimilar shapes and differences in how the two spaces relate to luminance.

So, since we're dealing with simple shapes here (synthetic RGB working spaces), you have to move the primaries father apart, in the case of ProPhoto RGB, that means two are outside human gamut. So yes, you can define colors you can't see, let alone work with colors that fall outside your display gamut. Its just the compromise we have to deal with working with large gamut encoding color spaces. But I don't know that we need to be too scared of this (user beware now is probably appropriate)
« Last Edit: November 25, 2007, 07:26:45 PM by digitaldog » Logged

Andrew Rodney
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« Reply #43 on: November 26, 2007, 10:36:02 AM »
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You also state 'In the case of ProPhoto RGB, there ARE colors defined that fall outside human vision.' mmm  I think most people would challenge you on that one!
Colours outside the HVS?Huh

You betcha! Challenge accepted, since it is so easy:
http://www.luminous-landscape.com/tutorial...photo-rgb.shtml

Look at Figure. 3b  (about half way down) ProPhoto RGB "actually exceeds it (visible spectrum) in the deep greens and deep blues. What this means is that colours can be pushed into areas which can neither be seen nor reproduced, producing very nasty looking results within the visible spectrum. User beware."

It is worth reading the rest of the article too. - Angst
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yes ive looked at that article.
first the author  states referring to figure 3b
 'Below is a more traditional colour gamut chart. The large horseshoe shaped area shows the LAB colour space.'

The horseshoe does not show the CIELAB colourspace. It shows the CIE x,y chromaticity diagram. This is derived from CIEXYZ colour space.

The author also fails to mention that ProPhoto (ROMM RGB)  primaries were chosen to encompass the gamut of real world surface colours  without using unnecessary encoding space ie coordinates outside the visual locus.

Again there are NO COLOURS outside the spectral Locus!!!

For a reference on Prophoto (ROMM RGB)
look at the chapter

Implementation of Device-Independent Colour At Kodak

by Kevin SPAULDING and Edward GIORGIANNI

Colour Engineering- Achieving Device Independent Colour
Eds Phil Green and Lindsay MacDonald.
2002 John Wiley and Sons
Chichester
England

or go to kodak and search ROMM  two pdf by by Kevin SPAULDING

[a href=\"http://search.kodak.com/?pq-locale=en_US&global=en&q=ROMM]http://search.kodak.com/?pq-locale=en_US&global=en&q=ROMM[/url]


Far more informative and correct.
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digitaldog
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« Reply #44 on: November 26, 2007, 11:20:05 AM »
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The horseshoe does not show the CIELAB colourspace. It shows the CIE x,y chromaticity diagram. This is derived from CIEXYZ colour space.

Two primaries do fall outside the spectrum locus. Here's an illustration made in ColorThink Pro in 3D. You can easily see the spectrum locus and the red shape of ProPhoto RGB and its clear it falls outside this plot. ColorThink allows you to plot using Yxy or Luv, it plots this over the spectrum locus.

Are you on a Mac? Double click on a profile to open the ColorSync utility.
Select New Utility Window, then click on the Profile button.
Select ProPhoto RGB and you can view the gamut in LAB (have Show Spectrum on), look at the plot. Now toggle to Yxy, to see the plot, which clearly shows the primaries falling outside the locus.

This isn't earth shattering news either.
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« Reply #45 on: November 26, 2007, 12:01:56 PM »
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If you read my posts carefully you will find that I have not disputed the fact that two of the primaries lie outside the spectral locus.

What I have been saying is that there are NO COLOURS outside the Spectral Locus. It is only encoding space and should be called as such.

Using phrases such as "COLOURS we cant see' is wrong and very misleading and adds confusion to the reader.

Its a matter of semantics. Very important in colour science.
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digitaldog
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« Reply #46 on: November 26, 2007, 12:21:20 PM »
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What I have been saying is that there are NO COLOURS outside the Spectral Locus. It is only encoding space and should be called as such.
Its a matter of semantics. Very important in colour science.
[a href=\"index.php?act=findpost&pid=156133\"][{POST_SNAPBACK}][/a]

Yeah OK, I'll buy that. We can't see 'colors' outside the spectrum locus so, I'm OK saying they are not colors (heck, if I can't see them, they are invisible).

Note we can define numerically a 'color' in ProPhoto RGB we can't see, there's no reason to argue that. But is that a color?

If a tree falls in the forest and no one is around... never mind. <G>
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« Reply #47 on: November 26, 2007, 12:44:21 PM »
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No. We can't numerically define a colour we cant 'see'.

We can define a coordinate in a colour encoding space that is outside the spectral locus but it is not

quote' a colour we cant see'

This implies that 'colour' exists outwith human perception.

Again the use of 'invisible colours' is wrong.

Using terms like the above will only help to cloud peoples understanding of colour science and colour reproduction etc.
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digitaldog
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« Reply #48 on: November 26, 2007, 12:48:58 PM »
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No. We can't numerically define a colour we cant 'see'.
[a href=\"index.php?act=findpost&pid=156143\"][{POST_SNAPBACK}][/a]

Well I think I'm going to have to disagree. In Photoshop, we can define the two primary colors (example R0/G0/B255) in ProPhoto RGB. But such a color would fall outside the locus. And here lies somewhat of an issue.
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« Reply #49 on: November 26, 2007, 01:59:51 PM »
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No. We can't numerically define a colour we cant 'see'.

Rubbish. We numerically difine and measure all sorts of things we can't see. Human perception is not a good yardstick for defining whether or not something exists. Infrared and ultraviolet light exists, 100KHz sounds exist, and so do neutrons. All of those thing have been precisely defined, and can be measured with the right equipment. But none of us are capable of perceiving such things directly with our senses. Given that, I think it's a bit narrow-minded to define a "color" as something perceivable by human vision, as the exact range of wavelengths that any given human eye can perceive varies from person to person, and then there's the whole issue of color-blind people...
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« Reply #50 on: November 26, 2007, 02:01:11 PM »
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Quote  'In Photoshop, we can define the two primary colors (example R0/G0/B255) in ProPhoto RGB. But such a color would fall outside the locus. And here lies somewhat of an issue.'


No you are defining a coordinate in a colour encoding space NOT a COLOUR!

Its a number and nothing but a number. You keep referring to it as a colour though, when its outside the HVS.

Why?
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digitaldog
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« Reply #51 on: November 26, 2007, 02:10:57 PM »
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No you are defining a coordinate in a colour encoding space NOT a COLOUR!

Its a number and nothing but a number. You keep referring to it as a colour though, when its outside the HVS.

Why?
[a href=\"index.php?act=findpost&pid=156167\"][{POST_SNAPBACK}][/a]

Everything we do on a computer is a number. That this number defines what you don't want me to call a color (because its outside human gamut) doesn't make it much different from the perspectives of the user, the application where you define the color or the numbers does it?
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Andrew Rodney
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« Reply #52 on: November 26, 2007, 02:17:18 PM »
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You keep referring to it as a colour though, when its outside the HVS. Why?

Because insisting that a "color" be defined as something perceivable by human vision is arbitrary, and ignores the reality that not all humans perceive "color" in the same way (color blindness for example) and not all humans visually perceive the exact same frequency range of light waves anyway. In a dark room, I can faintly see the emissions of some IR remote control LEDs (same with the IR data port on some Palms & laptops), and can tell when a button is being pressed or data is being transmitted. Many people cannot. Is IR a color then, or not? I can see it, poorly, yes, but some IR falls within my range of visual perception. At what point does a "color" become a "color" instead of a mathematical construct? When 10% of the population can visually perceive it? 20%? 50%? 99?% Who put you in the position of authority to decide?
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digitaldog
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« Reply #53 on: November 26, 2007, 02:21:53 PM »
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Because insisting that a "color" be defined as something perceivable by human vision is arbitrary, and ignores the reality that not all humans perceive "color" in the same way (color blindness for example) and not all humans visually perceive the exact same frequency range of light waves anyway. In a dark room, I can faintly see the emissions of some IR remote control LEDs (same with the IR data port on some Palms & laptops), and can tell when a button is being pressed or data is being transmitted. Many people cannot. Is IR a color then, or not? I can see it, poorly, yes, but some IR falls within my range of visual perception. At what point does a "color" become a "color" instead of a mathematical construct? When 10% of the population can visually perceive it? 20%? 50%? 99?% Who put you in the position of authority to decide?
[a href=\"index.php?act=findpost&pid=156175\"][{POST_SNAPBACK}][/a]

Good point!

Well this at least is a civil and I have to say interesting conversation. Is a color someone can't see a color or not?
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Andrew Rodney
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« Reply #54 on: November 26, 2007, 02:25:14 PM »
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Maybe we should all be using visible color when defining colors within the locus?

I'm pinging a color scientist on this. It will be interesting to hear what they have to say about this (science or semantics)?
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« Reply #55 on: November 26, 2007, 02:34:33 PM »
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Maybe we should all be using visible color when defining colors within the locus?

Even that is measuring with a rubber ruler given the person-to-person variations in what wavelengths of light are visually perceivable and how they are perceived.

Is an 30KHz tone a sound? Some humans can hear frequencies that high to some extent, and bats and dogs can do so fairly easily. But most humans can't. Does that mean it isn't a "sound"?
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« Reply #56 on: November 26, 2007, 02:44:05 PM »
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Even that is measuring with a rubber ruler given the person-to-person variations in what wavelengths of light are visually perceivable and how they are perceived.

Visible color as defined by the CIE, not necessarily everyone on the planet. The Standard Observer if you will.

We've all seen the plots of frequency of electromagnet energy that define visible light, or the spectrum locus. I'm suggesting visible colors are defined by such general statements, stuff outside such plots are colors, but simply not known to be visible to humans. Of course, YMMV.
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« Reply #57 on: November 26, 2007, 02:45:30 PM »
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Even that is measuring with a rubber ruler given the person-to-person variations in what wavelengths of light are visually perceivable and how they are perceived.

[a href=\"index.php?act=findpost&pid=156190\"][{POST_SNAPBACK}][/a]

I think the way in which the rubber ruler issue gets resolved is by scientific method - e.g. to replicate visual experiments amongst a large number of volunteers, such that with a large enough sample of observers of the same phenomina the sample of responses becomes representative, reliable averages and their variances are calculated and that defines the standard, is it not?
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Mark D Segal (formerly MarkDS)
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« Reply #58 on: November 26, 2007, 02:59:38 PM »
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You can apply such methods to determine what percentage of the population can perceive a particular wavelength of light, but you still have the problem of defining what the cutoff percentage is before defining whether a color is "visible" or not. That's where the arbitrariness comes in. Do you define something as "visible" when 10% of the test subjects can see it, 25%, 50%, or 90%? No matter which of those values you choose to draw the boundary line, there will still be enough exceptions that using that boundary as a criteria for defining yes/no "colorness" attribute to a given point in a color space is pointless.
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« Reply #59 on: November 26, 2007, 03:15:18 PM »
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Yes, at some point judgments about signifigance need to be made, but how did the CIE approach this one back in 1931?
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Mark D Segal (formerly MarkDS)
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