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Author Topic: Bit-Depth understanding required  (Read 17512 times)
Dennis
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« Reply #20 on: July 22, 2006, 08:44:31 AM »
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...because digital cameras are very insensitive to red,
Digital cameras are very insensitive in the blue spectrum, that's why the blue channel ist usually the one containing the most amount of noise. The sensors of digital cameras are highly sensitive for the red spectrum - so sensitive, that they need a special IR cut filter to cut the light somewhere at 700nm. So, I'd rather say digital cameras are highly sensitive to red.
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John Sheehy
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« Reply #21 on: July 22, 2006, 04:41:56 PM »
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Digital cameras are very insensitive in the blue spectrum, that's why the blue channel ist usually the one containing the most amount of noise. The sensors of digital cameras are highly sensitive for the red spectrum - so sensitive, that they need a special IR cut filter to cut the light somewhere at 700nm. So, I'd rather say digital cameras are highly sensitive to red.
[a href=\"index.php?act=findpost&pid=71467\"][{POST_SNAPBACK}][/a]

The *sensor*, operating in monochrome mode, with no filters, is most sensitive to red.  The hot mirror drastically reduces red response, and the entire sensor/CFA/AAfilter/hotmirror sandwich is least sensitive to red, and that's all that matters when your're taking a picture.

It is a well know fact that RAW converters multiply the red channel data by about 2 when doing daylight WB, and about 1.4 for blue.

If blue is the noisiest channel, it is because the light source or subject is lacking in blue, such as incandescent lighting.  For incandescent lighting, the blue RAW data is usually multiplied by about 4 to achieve incandescent WB (red and green are very close in strength).

This is for Bayer RGB cameras in general.  There are individual small-sensor compacts that have blue as the least sensitive (2.25x for daylight WB).  They must be horrendous in incandescent light.
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jani
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« Reply #22 on: July 23, 2006, 06:14:45 AM »
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Well, our eyes and the camera both use RGB tristimulus values to simulate the color of the blue sky. For that color to be recorded by the camera or perceived by a human, it is necessary to have the red sensor activated. As you may recall, the Rho (red), Gamma (green) and Beta (blue) sensors of the eye have considerable overlap, especially for green and red. The camera sensors have less overlap.
http://www.photo.net/photo/edscott/vis00010.htm
Indeed.

But the human colour management apparatus is a bit more advanced than that.

This article from last year should provide some rather interesting additional insights.

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In the RGB model, an unsaturated blue must have red and green components. If the red sensor of the camera is activated, this is equivalent to saying that the unsaturated blue has a red component as well as a green component. It may all be a matter of terminolgy, so why don't we just drop this topic?
Well, it's rather obvious that an unsaturated blue must have (equal) red and green components in the RGB colour model, at least if you're following the RGB colour cube. That's how it is by definition, but it doesn't mean that the unsaturated blue has red it in it, colour-wise.
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Jan
jani
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« Reply #23 on: July 23, 2006, 06:20:43 AM »
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Not surprising! I work in ProPhoto and the image has been converted to sRGB and jpeg compressed. But 128 or 129 is nitpicking. The color is essentially as it was when created. It's no longer a clear blue because of the presence of green and red.
I'm sorry, that's simply not correct in the RGB colour cube model; it is equivalent to saying that e.g. 128,128,128 isn't a neutral grey.

As an experiment, create the inverse colour of a neutral 50% saturated blue, and invert it.

Which colour do you get?

Also, feel free to experiment with a different colour model where you're able to adjust the hue, saturation and brightness levels. Set the hue to pure blue, and then set brightness to 100% and saturation to 50%. Convert to RGB; what RGB values do you get?

This is central in the colour theory document linked to by bjanes; if your claim had merit, changing the R and G values with the same amount would pull the blue colour off the surface of the colour cube.
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Jan
Dennis
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« Reply #24 on: July 23, 2006, 07:05:12 AM »
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If blue is the noisiest channel, it is because the light source or subject is lacking in blue, such as incandescent lighting.
In earlier times, Phil Askey's photographic noise tests at dpreview.com were split up to the three color channels. The shots were taken at daylight lightening, and the blue channel was always the worst one.
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John Sheehy
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« Reply #25 on: July 23, 2006, 07:29:03 AM »
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In earlier times, Phil Askey's photographic noise tests at dpreview.com were split up to the three color channels. The shots were taken at daylight lightening, and the blue channel was always the worst one.
[a href=\"index.php?act=findpost&pid=71529\"][{POST_SNAPBACK}][/a]

And  he also used camera-produced JPEGs in earlier times, didn't he?

Subject color can make a difference, too.
« Last Edit: July 24, 2006, 09:04:12 AM by John Sheehy » Logged
Ray
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« Reply #26 on: July 23, 2006, 10:01:37 AM »
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I'm sorry, that's simply not correct in the RGB colour cube model; it is equivalent to saying that e.g. 128,128,128 isn't a neutral grey.

[a href=\"index.php?act=findpost&pid=71527\"][{POST_SNAPBACK}][/a]

Interesting! When is blue not quite blue? When it comes to what's blue and what's not blue, I guess I rely upon my eyeballs.

In the following samples of blue, colors 1-4 are all shades of pure blue, 0, 0, B.

Of the 4 colors on the right, #6 looks bluest to me and that's got more green than red (128, 150, 255). #8 looks least blue to me and that's got more red than green (150, 128, 255). #5, which you describe as a clear blue (128, 128, 255) does not look as blue as #6 to my eyes.

Perhaps your argument is a bit like declaring a banana is not a fruit but a herb. I believe from a strictly botanical perspective, a banana is a herb, although it tastes pretty fruity to me   .

[attachment=836:attachment]
« Last Edit: July 23, 2006, 10:03:50 AM by Ray » Logged
jani
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« Reply #27 on: July 24, 2006, 04:07:22 AM »
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Interesting! When is blue not quite blue? When it comes to what's blue and what's not blue, I guess I rely upon my eyeballs.
And therein lies the trap.

Keep in mind the visual spectrum, which ranges from near-infrared to near-ultraviolet. Red and blue are at opposite ends, and violet is beyond blue. Yet many people seem to think of violet as a blue with a minor red tint to it. Why is that? I honestly don't know, I don't recall seeing an explanation for it.

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In the following samples of blue, colors 1-4 are all shades of pure blue, 0, 0, B.

Of the 4 colors on the right, #6 looks bluest to me and that's got more green than red (128, 150, 255). #8 looks least blue to me and that's got more red than green (150, 128, 255). #5, which you describe as a clear blue (128, 128, 255) does not look as blue as #6 to my eyes.
To my eyes -- on my monitor at work -- #6 looks markedly blue-green, #7 I'm unsure of, and #8 definitely is redder than the rest. Compared to #6, #5 looks like it's on the other side of blue, but since they're next to each other, it's impossible to tell.

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Perhaps your argument is a bit like declaring a banana is not a fruit but a herb. I believe from a strictly botanical perspective, a banana is a herb, although it tastes pretty fruity to me   .
Yes, it is a bit like that. And the RGB colour cube doesn't give a perfect simulation. In addition we have all the usual variables like monitor, human eye/processing system and language apparatus involved.

Don't take the following as anything but amusing anecdotes.

As a child, I had a few quarrels with other people regarding whether something was more yellow than green or vice versa. A British girl had quarrels with her mom about the colour of some surfaces, which the girl claimed were a deep red, while everyone else saw them as black. The Science Daily article sort of explains why that might have been the case.
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Jan
bjanes
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« Reply #28 on: July 24, 2006, 09:53:06 AM »
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Indeed.

But the human colour management apparatus is a bit more advanced than that.

This article from last year should provide some rather interesting additional insights.
Well, it's rather obvious that an unsaturated blue must have (equal) red and green components in the RGB colour model, at least if you're following the RGB colour cube. That's how it is by definition, but it doesn't mean that the unsaturated blue has red it in it, colour-wise.
[a href=\"index.php?act=findpost&pid=71526\"][{POST_SNAPBACK}][/a]

Jani,

You can obfuscate the discussion by bringing in the opponency color models such as CIE LAB or even the theories about color perecption--colors exist only in the brain. However, with our cameras we are measuring light, not color, and the components of light that we are measuring are red, blue, and green as measured through relatively wide pass color filters.

Yellow exists as a pure spectral color with a certain wave length. If we used very narrow band RGB filters in the camera, yellow light would be recorded by neither the blue nor green sensors of the camera and the result would be black (R = G = B = 0). However, with wider pass filters, the yellow light is recorded by both the red and green sensors. Combining red and green wave lenghts of light does not produce yellow light with its characteristic wave length, but the color is perceived as yellow, since it activates the red and green sensors in the eye, producing a metameric match.

Now, does yellow light consist of a mixture of red and green light? No, but it it can be matched by a combination of red and green. In the RGB tristimulus model (Young- Helmholtz) it has red and green components.

Now in the case of an unsaturated blue, the blue is mixed with white light, which does contain RGB components along with the other colors of the rainbow. That unsaturated blue does have a true red component.
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jani
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« Reply #29 on: July 24, 2006, 02:57:33 PM »
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Now in the case of an unsaturated blue, the blue is mixed with white light, which does contain RGB components along with the other colors of the rainbow. That unsaturated blue does have a true red component.
Yes, but it's a meaningless statement. That -- plus the fact that you didn't have the RGB caveat in your original post -- is my point.
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Jan
bjanes
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« Reply #30 on: July 24, 2006, 05:58:52 PM »
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And therein lies the trap.

 The Science Daily article sort of explains why that might have been the case.
[a href=\"index.php?act=findpost&pid=71595\"][{POST_SNAPBACK}][/a]

The science daily article explains nothing with regard to this discussion. We are dealing with the recording of RGB values of a blue sky by a sensor, not perception of color.
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bjanes
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« Reply #31 on: July 24, 2006, 06:00:07 PM »
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Yes, but it's a meaningless statement. That -- plus the fact that you didn't have the RGB caveat in your original post -- is my point.
[a href=\"index.php?act=findpost&pid=71647\"][{POST_SNAPBACK}][/a]

Jani,

Your whole arguments are meaningless. I will waste no more time replying to your posts. Go argue with the Brittish girl.  
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Ray
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« Reply #32 on: July 25, 2006, 10:54:28 PM »
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I make no claim to special insights into color matters, but the friction between Jani and Bill Janes makes me think there's something of interest to be resolved here.

Why should a red and green light, mixed together, produce yellow? Off the top of my head, I would say, because the shorter wave lengths of red are yellow and the longer wavelengths of green are also yellow. Combine them both and you get a reinforcement of the frequencies in common which produce the sensation of yellow in our minds.

Yet, doing some Googling research on the matter, I came across the following reference from the most amazing of all encyclopedias, Wikipedia, which is in a constant state of revision and correction by anyone who thinks he has the knowledge. (Great concept!)

[attachment=840:attachment]

As you will see from the above image, there is no overlap between the red and green frequencies, so the question arises, if red and green light with the exact frequencies as shown in the Wikipedia image were superimposed, or mixed, would yellow result? If so, how?
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Ray
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« Reply #33 on: July 25, 2006, 11:34:14 PM »
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To make it clearer for those who might be confused by tables, red plus orange is  a longer wavlength than yellow. Green is a shorter wavelength than yellow.

How the heck does yellow appear as a result of the combination of the two.
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Jack Flesher
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« Reply #34 on: July 25, 2006, 11:44:38 PM »
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Why should a red and green light, mixed together, produce yellow?

Huh???  Maybe because of Physics 101?  All colors of light are composed of the three primary colors for light, Red, Green and Blue.  From those three primaries you can make ALL colors of light.  Combine any two of the primaries and you get the secondary colors: Green and Blue combined in equal parts make Cyan; Blue and Red make Magenta and voilla, Red and Green combined in equal parts make yellow. Amazing how that works!   (But I suspect you really did already know this and are just teasing us  )


Now for the real kicker -- and IMO why discussions on bit-depth always break down to arguments...  

The simple answer is RGB color models suck    

The longer answer is Numerical RGB (and CMYk for that matter) models by themselves are meaningless.  IOW saying 100, 110, 244 is color Bxxx will NEVER be accurate in and of itself.  The reason is the numbers have no reference point unless you first define the size of the space they are representing.  So any RGB numerical model also must have a color space designation to have meaning -- Ah hah!    All bigger bit depth gives us is more numbers to define the colors with and thus we can define them more accurately, especially as the color space itself gets bigger.  BUT! ALL spaces will still contain an infinite number of colors between neighboring numerical coordinates and hence can only be APPROXIMATELY defined by the numerical representation, regardless of how big it is.  (Though 16-bits per channel can describe a BUNCH of different colors -- and most importantly, far more shades of colors than can be distinguished with the Human eye.)

This is the main reason LAB or HSB makes a better model for discussing color, as they are both absolute color designations: Having a color space designation for them is irrelevant, because any color we define with LAB or HSB is either inside that space or not, thus the limits of the space can simply be designated directly by absolute values in either system and the space definition become superfluous.

Cheers,
« Last Edit: July 25, 2006, 11:58:53 PM by Jack Flesher » Logged

Jack Flesher
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« Reply #35 on: July 26, 2006, 12:12:28 AM »
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To make it clearer for those who might be confused by tables, red plus orange is a longer wavlength than yellow. Green is a shorter wavelength than yellow.

How the heck does yellow appear as a result of the combination of the two.
[a href=\"index.php?act=findpost&pid=71732\"][{POST_SNAPBACK}][/a]

Uhhhh...  Let's see if I can rephrase for you: red is a longer wave than yellow, green is a shorter wave than yellow and you are asking why if we average the red and green waves together we get yellow?  The same yellow that has a wavelength part-way between red and green?  

Well Ray, if you are serious this really does explain a *lot* about the nature of your posts  
« Last Edit: July 26, 2006, 12:16:47 AM by Jack Flesher » Logged

opgr
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« Reply #36 on: July 26, 2006, 03:34:20 AM »
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Uhhhh...  Let's see if I can rephrase for you: red is a longer wave than yellow, green is a shorter wave than yellow and you are asking why if we average the red and green waves together we get yellow?  The same yellow that has a wavelength part-way between red and green? 

Well Ray, if you are serious this really does explain a *lot* about the nature of your posts  
[a href=\"index.php?act=findpost&pid=71734\"][{POST_SNAPBACK}][/a]


Well, actually that is a valid question, and I am afraid it has nothing to do with averaging. The reason we perceive a combination of those two frequencies as yellow is because our perception seems to work as a trichromatic scanner. That is: our perception can be thought of as filtering light with three colored filters just as an RGB scanner might do.

And these filters are broadband filters, not monochromatic frequency detectors. In other words: if you excite 2 of the filters, both with a single frequency, it will appear to us as if we see a combination of the filter colors, not the frequency colors, because frequencies do not have the property "color".

And this also  means that two other frequencies may result in the same perceived color, because the 2 filters are excited similarly. hence: metamerism.


Also, the OP's problem was a relative problem particularly the difference of any single primary over a graduated color range. The sky for example might go from an almost white to a desaturated blue. And even though the red component will likely show the largest difference, it may still be less than necessary for a perceived smooth gradation. The usual perceptual calculations (delta E = 3 etc) do not apply, because if we are dealing with gradations of a single color, our perception becomes more sensitive to differences.

Even so, I don't believe the difference between 12bit and 16bit is noticeable or relevant in the original problem, so I don't think it is related to bit differences perse, but perhaps OP is shooting JPEG for example... or the colorspace differences between scanner and camera are such that the screen representation produces more banding issues etc...
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Oscar Rysdyk
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PeterLange
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« Reply #37 on: July 26, 2006, 06:05:38 AM »
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This is the main reason LAB or HSB makes a better model for discussing color, as they are both absolute color designations: Having a color space designation for them is irrelevant...

HSB is a pure RGB derivative,
and therefore dependent on the underlying color space.

At least thats how Photoshop computes
(see also http://en.wikipedia.org/wiki/HSV_color_space)

Peter

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bjanes
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« Reply #38 on: July 26, 2006, 07:01:08 AM »
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I make no claim to special insights into color matters, but the friction between Jani and Bill Janes makes me think there's something of interest to be resolved here.

Why should a red and green light, mixed together, produce yellow? Off the top of my head, I would say, because the shorter wave lengths of red are yellow and the longer wavelengths of green are also yellow. Combine them both and you get a reinforcement of the frequencies in common which produce the sensation of yellow in our minds.

Yet, doing some Googling research on the matter, I came across the following reference from the most amazing of all encyclopedias, Wikipedia, which is in a constant state of revision and correction by anyone who thinks he has the knowledge. (Great concept!)

[attachment=840:attachment]

As you will see from the above image, there is no overlap between the red and green frequencies, so the question arises, if red and green light with the exact frequencies as shown in the Wikipedia image were superimposed, or mixed, would yellow result? If so, how?
[{POST_SNAPBACK}][/a]

Ray,

The answer to your question is that mixing red and green light does not produce yellow light for the reasons you cite, but the mixture is perceived as yellow because the red and green sensors of the eye have a broad spectral response centered on red and green respectively, but actually responding to a wide range of colors. If you look at the reference I posted previously both the red and green sensors have a strong response to yellow light.

[a href=\"http://www.photo.net/photo/edscott/vis00010.htm]http://www.photo.net/photo/edscott/vis00010.htm[/url]

It is interesting that the red and green cone response curves are releatively close together and overlap, but the blue response is considerably to the left on the graph and there appears to be a gap in sensor response between blue and green. An interesting article in this month's Scientific American magazine (an American magazine for the intelligent layman) explains the reason for this in evolutionary terms.

It turns out that birds have a fourth sensor between the blue and green and have better and more extended color vision than humans. As mammals evolved from the common ancestor, they lost two color sensors since they were operating on the forest floor where light was dim and color vision was not useful. These mammals relied mainly on the responses of the rods.

As primates evolved, they ascended into the trees and needed color vision in order to pick out colored fruits and they regained a sensor. However, there is still a gap where the fourth sensor was originally, and this explains the gap between the blue and green sensors.

For color perecetion, the brain intetegrates the responses from all the sensors and determines the color by a process similar to triangulation.

Bill



http://www.photo.net/photo/edscott/vis00010.htm
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Ray
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« Reply #39 on: July 26, 2006, 07:04:10 AM »
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And these filters are broadband filters, not monochromatic frequency detectors. In other words: if you excite 2 of the filters, both with a single frequency, it will appear to us as if we see a combination of the filter colors, not the frequency colors, because frequencies do not have the property "color".



As I imagined. Thanks for the explanation, opgr. If a narrow band of red light (700 nm) is combined with a narrow band of green (550 nm), the wavelengths of those 2 frequencies do not change, or average, and no frequency which we might ascribe to pure yellow (580 nm) necessarily exists as a result of that combination. The visual cortex is so constructed that a single frequency of pure yellow (580 nm) will have the same (or similar) effect to a combination of two quite different frequencies, 700 nm and 550 nm. Interesting!
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