Camera profiling - matching values on Macbeth CC passport

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Originally Posted by Iliah Borg

Yes, but this has nothing to do with gamma. Gamma is the method of compensation for the non-linearity of capture and non-linearity of presentation. Generally speaking, a poor method, to boot.

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Or to put it another way, if we view a scene directly and then view the scene through a linear photographic system, we should get the same visual sensation in our brain, limited of course by the dynamic range and the processing of the camera system.

Welcome to the forum from me too Iliah, I've used your Rawdigger program for 2 or 3 years and found it very useful. I hope you stick around this forum to give us enightenment on raw capture and processing.

Dave

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Originally Posted by Iliah Borg

>your statements regarding our eye and nature need to be challanged.

Do you disagree with "Nature has linear gamma, so our eyes are expecting images in linear gamma"?
So, if the subject is lit with twice the luminous intensity, an exposure meter does not recommend to half the exposure?

Linear relation between the luminous intensity of the light source and the amount of reflected light from a subject is a fundamental law of Nature. Our eyes are expecting a linear scene to be presented, be it in Nature or on a media, with gamma = 1. That is why in photography gamma of the paper compensates for gamma of the film, for total gamma close to 1. The response of our eyes enter the equation at a later stage.

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I'm choosing my words quite carefully here and suggesting that while our camera sensors are linear devices, our eyes (connected to the way our brain processes the signal from our eyes) are not.

This is all about how we evolved and our eyes are primarily made up of B&W sensors (rods) which are quite useful for detecting predators that were lurking on the ground or in trees and ready to pursue our ancestors for their evening meal. Being able to detect subtleties in shadow detail obviously worked as there are lots of humans around and I believe 96% of our visual sensors are these monochrome receptors.

On the other hand roughly 4% are what provides our colour vision; the cones. Here the distribution is not linear either, as roughly half of the cones are sensitive to middle frequencies of light (green) and slightly more of the remaining are red sensitive and the smallest amount of sensors are blue sensitive. Again, looking at this from an evolutionary viewpoint, this too makes sense. Our ancestors would have been dealing in environments with high levels of green (trees and short plants). Ripe fruits and berries (predominantly red values) were nutritionally more important than unripe (green) ones. Human eyes are not particularly sensitive to blue light., and again this seems to make sense as the variations in the colour of the sky or water was less important in human evolution.

Again, the way our eyes / brain build an image is entirely different than how a camera sensor do. What we see is effectively a composite image that is built up through small adjustments in eye movement; a sharp centre and less sharp periphery. Our eye stops up and down fairly quickly as we scan between highlights and shadow areas. With a camera, it's all based on a single, stationary view.

My understanding of gamma is very much in line with both Jess Schewe's paper and the CiC article on gamma that I linked to in my previous response.

While I agree that from a recording standpoint, both the light meter and the camera sensor will record light in a linear fashion, in post processing we will often override these decisions and will introduce more subtlety to the image through processes like dodging and burning. The final image will look more like what our eyes see, versus what our cameras and screens record.

>suggesting that while our camera sensors are linear devices, our eyes (connected to the way our brain processes the signal from our eyes) are not.

That is not the justification for gamma.

> My understanding of gamma is very much in line with both Jess Schewe's paper and the CiC article on gamma that I linked to in my previous response.

Are those suggest that gamma is used to compensate for perception non-linearity? If so, your sources are wrong. (he is Jeff, not Jess, BTW).

It is indeed important to understand what tone mapping consists of, and why most of the raw converters work in linear gamma, and why a raw image tagged with an appropriate colour space and linear gamma is displayed as it should be in any ICC-savvy software.

> if we view a scene directly and then view the scene through a linear photographic system, we should get the same visual sensation in our brain

Yes. To quote Mees, "In order that tone reproduction shall be subjectively correct, it is necessary that the impression formed of the photographic print shall correspond at all points to the impression formed of the original subject."

Thank you for using RawDigger, and of course I would be glad to answer any questions related to raw capture, formats, and processing.

Iliah - I understand that you are a well respected authority through your work on RawDigger, RawView and dcraw.

it's nice of you to say my sources are wrong. In that case please direct me at sources that are correct. My understanding has always been (in part): https://www.cambridgeincolour.com/tu...correction.htm

"Our eyes do not perceive light the way cameras do. With a digital camera, when twice the number of photons hit the sensor, it receives twice the signal (a "linear" relationship). Pretty logical, right? That's not how our eyes work. Instead, we perceive twice the light as being only a fraction brighter — and increasingly so for higher light intensities (a "nonlinear" relationship).
linear vs nonlinear gamma - cameras vs human eyes.

Compared to a camera, we are much more sensitive to changes in dark tones than we are to similar changes in bright tones. There's a biological reason for this peculiarity: it enables our vision to operate over a broader range of luminance. Otherwise the typical range in brightness we encounter outdoors would be too overwhelming.

But how does all of this relate to gamma? In this case, gamma is what translates between our eye's light sensitivity and that of the camera. When a digital image is saved, it's therefore "gamma encoded" — so that twice the value in a file more closely corresponds to what we would perceive as being twice as bright."

My experience has been that many models use a linear approach because it it is simplest to model that way. Often this approach is certainly "good enough". If insolation is not linear, so how can the reflected light be linear?

I'm not trying to give you a hard time, I'm trying to understand why you are saying what you are.

> it's nice of you to say my sources are wrong

That is not what I said. There was an if clause.

> In that case please direct me at sources that are correct.

Mees is one of those. You may want also to read the classic http://www.poynton.com/notes/colour_...FAQ.html#gamma and possibly look at something like this, taken from http://http.developer.nvidia.com/GPU...ms3_ch24.html:

http://http.developer.nvidia.com/GPU...ks/24fig02.jpg

> "Our eyes do not perceive light the way cameras do. With a digital camera, when twice the number of photons hit the sensor, it receives twice the signal (a "linear" relationship). Pretty logical, right? That's not how our eyes work. Instead, we perceive twice the light as being only a fraction brighter — and increasingly so for higher light intensities (a "nonlinear" relationship).

The above is fine.

> linear vs nonlinear gamma - cameras vs human eyes.

First of all, study of vision and perception suggests our perception does not conform to a gamma curve, L* curve is closer. Second, it is not just eyes, it is the whole perception (eyes+brain+training).

Now, this is not what I'm discussing at all. Our perception is of the Nature, where gamma is absent (who is raising the gamma when we look through a window?). That is why accurate tone reproduction of a natural scene means that a similar, linear image must be presented to our perception.

> gamma is what translates between our eye's light sensitivity and that of the camera.

That is completely wrong, as explained above. In modern computing gamma is preserved for compatibility only. It is not needed for anything else; and it comes on the way when we need to colour- and tone-correct the image (white balance and exposure compensation operations need to be applied in linear space, that is why those are crude when the image is gamma-encoded). Even for JPEGs, here are 2 images, one is gamma 1, the other is gamma 2.2:
http://f.cl.ly/items/0z0l191Q0d0M0M0D3y0N/g1.jpg
http://f.cl.ly/items/2F3S261T2a1h3T0V2D3I/g22.jpg
If your web browser is colour-savvy, you should not see much of a difference.

I think the thing to remember here is that we never actually "see" the gamma encoded image, we always view it through a monitor that has the opposite gamma curve.

On the other hand, we do see the effects of a separate tone curve if we are using ACR for example. This is a curve applied by the camera profile as a default. It is similar to a gamma correction curve but not the same. It basically gives the image more pop. The two images below show the effects of the standard tone curve and a linear one. They need to be viewed in LiteBox to see the real difference.

Dave

http://i58.tinypic.com/x1brjs.jpg

http://i61.tinypic.com/1g62vo.jpg

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Originally Posted by GrumpyDiver

If insolation is not linear . . .

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By insisting on using the "L" word by itself, in spite of my earlier post, your meaning remains unclear.

And now the discussion has become restricted to insolation when there is no need to do so because the type of illumination is really not relevant to the discussion.

To keep it simple, consider a surface illuminated by 200lx (lux). Consider a surface of reflectance 0.5. The surface emittance is 0.5 x 200 = 100lx (I'm keeping the same units for simplicity). Move the source of illumination away from the surface such that illumination becomes 100lx and the surface emittance becomes 0.5 x 100 = 50lx and that is a linear relationship, namely, y = 0.5 times x. There is no exponent involved at all. And this is what Iliah is trying to say, no more than that.

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, so how can the reflected light be linear?

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I believe that you, Manfred, are referring to the energy/wavelength characteristic of the light source, known as the spectral emittance which is not a straight line for any source of light, natural or otherwise. However, the use of the term "linear" in that context is both unusual and confusing.

Furthermore reflective surfaces, natural or otherwise, have spectral reflectivities that are all different, often unpublished, and are of course not "linear" in your terms. But still, in most cases, their reflectivity to illumination is constant and therefore the relationship of 'light in' to 'light out' is linear, QED.

Not trying to give you a hard time either, but from my corner, yourself and Iliah are using the word "linear" for two different things, hence the misunderstanding.

> I believe that you, Manfred, are referring to the energy/wavelength characteristic of the light source, known as the spectral emittance which is not a straight line for any source of light, natural or otherwise.

But that has nothing to do with gamma?

Isn't this getting way off-topic wrt the original question?

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Originally Posted by xpatUSA

Not trying to give you a hard time either, but from my corner, yourself and Iliah are using the word "linear" for two different things, hence the misunderstanding.

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Thanks Ted. I was starting to come to that conclusion as well. That has always been the problem with words that can be used in more than one context.

My aim is to understand gamma and I suspect that there may be a similar issue here as well. Iliya has made some statements I am trying to understand, because they seem to be somewhat inconsistent with what other experts say; specifically Jeff Schewe and (the late) Bruce Fraser, etc.

Iliya - If I am to understand what you are saying is that a gamma correction was encoded into an image primarily to correct the non-linear characteristics of CRT displays. Now that CRTs have largely been replaced with technology that does not have this issue, we continue to do gamma encoding for backwards compatibility reasons. Modern screens use lookup tables to emulate the gamma curve for this reason.

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Originally Posted by Iliah Borg

> gamma is what translates between our eye's light sensitivity and that of the camera.


That is completely wrong, as explained above. In modern computing gamma is preserved for compatibility only. It is not needed for anything else; and it comes on the way when we need to colour- and tone-correct the image (white balance and exposure compensation operations need to be applied in linear space, that is why those are crude when the image is gamma-encoded).

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This really sounds like a bad engineering decision, but then that wouldn't be the first time that legacy has won over reason...

It also seems to run counter to what other well known experts have said. In a 2004 paper, seems to be saying what I have understood.

https://www.adobe.com/digitalimag/pdfs/linear_gamma.pdf


Is this because gamma is being used in a different context here or am I missing something obvious again. This article seems to suggest something different from what you are saying. I had always viewed Bruce Fraser as a knowledgeable source.

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Originally Posted by Iliah Borg

> I believe that you, Manfred, are referring to the energy/wavelength characteristic of the light source, known as the spectral emittance which is not a straight line for any source of light, natural or otherwise.

But that has nothing to do with gamma?

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I agree - nothing.

Gamma is just the greek symbol for an exponent used in a relationship such as y = x^ɣ.

Therefore, if people in this thread continue to use the "g" word by itself, thereby forcing us to rely on context to determine it's meaning, confusion will continue to reign.

Time flies like an arrow; fruit flies like a banana . . .

This article on Adobe's site needs some serious corrections. I'm highly doubtful this article is anything more than oversimplification, and it is hard for me to believe Bruce wrote all of it - he payed much more attention to details in every publication he authored. The article contains minor mistakes - like you may see that the "equal distribution" gradient counts less divisors than the "linear" one. It also contains a fundamental mistake in colour management, the one I addressed in my previous post: assigning correct gamma to a linear capture data does not change RGB numbers, it only tells the CMMs how to interpret the existing RGB numbers. As you can see, it takes care of the "linear images being dark" and the two images are practically indistinguishable. Unfortunately, Bruce is not with us anymore, and his last period of life was so painful that I considered it to be inappropriate to draw his attention to the article.

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Originally Posted by Iliah Borg

This article on Adobe's site needs some serious corrections. I'm highly doubtful this article is anything more than oversimplification, and it is hard for me to believe Bruce wrote all of it - he payed much more attention to details in every publication he authored. The article contains minor mistakes - like you may see that the "equal distribution" gradient counts less divisors than the "linear" one. It also contains a fundamental mistake in colour management, the one I addressed in my previous post: assigning correct gamma to a linear capture data does not change RGB numbers, it only tells the CMMs how to interpret the existing RGB numbers. As you can see, it takes care of the "linear images being dark" and the two images are practically indistinguishable. Unfortunately, Bruce is not with us anymore, and his last period of life was so painful that I considered it to be inappropriate to draw his attention to the article.

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I've noted that at the end of the article there is a statement that the article was "adapted" from a book by Bruce Fraser entitled Real World Camera Raw. It would be interesting to know whether the adaptation deviates from the book. Does anyone have a copy?