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Author Topic: Accurate Astro Color - Adaptation  (Read 2681 times)

Jim Kasson

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Re: Accurate Astro Color - Adaptation
« Reply #20 on: March 21, 2018, 01:05:16 PM »

Interesting questions, Jack. In my experience, what I see -- or saw, since it's been a long time -- when looking through a 14 inch or smaller telescope (I've never had the pleasure of riding high on a 200 inch one) was dramatically different from what the professional astronomers turned out. Even with a 14-incher, there are a lot of objects that you look at out of the corner of your eye to make them appear bright enough, and now you're using strictly scotopic vision (which is why you perform that little trick in the first place). Even bright nebulae look pale through a small scope compared to the shots the big boys (or the Hubble) take. I've also noticed that when looking at the Milky Way with bare eyes that it looks a lot paler than all the shots that I see now that digital photography has made taking images of our galaxy falling-off-a-log easy.

So I question the whole idea of the viewer's adaptation when looking through a telescope as the basis for adjusting the colors in astro images. I think they are what they are in terms of color, and any way that you get them to look "right" (and right is now defined for many as "like all the other astro images, and maybe in particular by the ones from the Hubble) is fair game.

Jim

Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #21 on: March 22, 2018, 06:47:05 AM »

Good comments Jim, even though they tend to defeat the purpose of this thread :)

I was surprised to learn that most astrophotographers process their images for maximum impact, they don't even try to show approximately accurate tones (including many images out of the Hubble).  For instance, they stretch contrast logarithmically and often use faux color for emphasis so their objective has more to do with emphasizing different features than anything else.  Someone read one of the posts that documents what I am learning about color and asked me to comment on someone else's page that claimed to provide 'true' astro colors that looked wrong to this person.   It was clear right away that that page was based on an incorrect understanding of CMFs thus resulting in grossly incorrect tones no matter the underlying assumptions.  However it got me thinking about what it would take to do it right, hence the question on adaptation in this thread.

The concept of 'resting' adaptation, to use Oscar's terminology, is an interesting one.  Here is the question again, slightly reformulated:

Quote
Say I have been sitting in a dark room, totally dark except for the power LED on the TV (say red, narrowband, around 600nm, just inside Adobe RGB's gamut). I am dark adapted but I see the normally bright LED well and recognize its color as the 'correct' shade of red (correct in the sense that it looks to me to be roughly the same red as when there is light in the room).

Now say I want to calculate the Adobe RGB coordinates of the LED directly from its spectrum. I take the 600nm narrowband SPD and dot-multiply it by the 1931 2 deg CIE XYZ CMFs of choice*. Do I need to adapt to D65 before projecting to Adobe RGB? If so, from what white point?

In other words what's our 'resting' adaptation when there is no ambient light?



What do you think?

Jack

*My impression by looking at said LED is that it looks somewhat more saturated in the dark than when there is light in the room but the hue is about right - so it would seem that CIE's photopic CMFs is possibly not too far off in this situation.
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GWGill

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Re: Accurate Astro Color - Adaptation
« Reply #22 on: March 22, 2018, 08:51:16 AM »

The concept of 'resting' adaptation, to use Oscar's terminology, is an interesting one.
I've been assure by someone who should know (Mark Fairchild) that given long enough, you will adapt to any illuminant.

Given that the underlying chromatic adaptation mechanism is assumed to be the independent light level adaptation of the L, M & S cones, this makes sense. It would also explain why most individuals have a similar sense of what "white" is, even though there is significant individual variation in the density of L, M & S cones in the retina.

So the expected answer if you wait long enough, is that there is no such thing as a 'resting' adapation.
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opgr

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Re: Accurate Astro Color - Adaptation
« Reply #23 on: March 22, 2018, 09:43:33 AM »

I've been assure by someone who should know (Mark Fairchild) that given long enough, you will adapt to any illuminant.

Yeah, but the problem at hand is "no illuminant".

Given that the underlying chromatic adaptation mechanism is assumed to be the independent light level adaptation of the L, M & S cones, this makes sense. It would also explain why most individuals have a similar sense of what "white" is, even though there is significant individual variation in the density of L, M & S cones in the retina.

Well, we've recently seen some internet images with a dress and a shoe that suggest this doesn't entirely hold.

In addition, what I find most intriguing in relation to this is the workings of auto-white-balance and metering on digital camera's. It's very easy to test that the average color of the entire image is not a good indicator of perceived white-balance. Neither linear space average, nor perceptual average. Similarly for median. There appears to be something happening in our perception that prioritises bright areas or perhaps bright peaks for whitebalance determination and thus possibly also for adaptation.
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Jim Kasson

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Re: Accurate Astro Color - Adaptation
« Reply #24 on: March 22, 2018, 10:57:10 AM »

I've been assure by someone who should know (Mark Fairchild) that given long enough, you will adapt to any illuminant.

I haven't seen Mark in a long time -- say "Hi" to him for me, please -- but I can tell you that after two hours straight in the darkroom under a Thomas sodium vapor safelight, it still looks yellow. Maybe to get adapted to that, I'd have to bring in food and a cot.

Jim
« Last Edit: March 22, 2018, 11:36:14 AM by Jim Kasson »
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Jim Kasson

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Re: Accurate Astro Color - Adaptation
« Reply #25 on: March 22, 2018, 10:59:11 AM »

.

*My impression by looking at said LED is that it looks somewhat more saturated in the dark than when there is light in the room but the hue is about right - so it would seem that CIE's photopic CMFs is possibly not too far off in this situation.

That experiment combines changing the luminance of the surround as well as its chromaticity, doesn't it? 

Jim

Jack Hogan

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Re: Accurate Astro Color - Adaptation
« Reply #26 on: March 22, 2018, 11:19:38 AM »

Quote
*My impression by looking at said LED is that it looks somewhat more saturated in the dark than when there is light in the room but the hue is about right - so it would seem that CIE's photopic CMFs is possibly not too far off in this situation.

That experiment combines changing the luminance of the surround as well as its chromaticity, doesn't it? 

Yes.  I was thinking that in addition to adaptation two other effects might be at play: contrast and desaturation.  We know even from post processing that increased contrast typically makes things look more colorful (think fireworks at night).  And we also know that adding more 'white' illuminant light to a patch of color desaturates it (think chromaticity diagram).

Jack
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Doug Gray

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Re: Accurate Astro Color - Adaptation
« Reply #27 on: March 22, 2018, 11:35:19 AM »

I haven't seen Mark in a long time -- say "Hi" to him for me, please -- but I can tell you that after two hours straight in the darkroom under a Thomas Sodium vapor safelight, it still looks yellow. Maybe to get adapted to that, I'd have to bring in food and a cot.

Jim

Could be a cognitive effect.  See pg. 149 of Mark Fairchild's book "Color Appearance Models"
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GWGill

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Re: Accurate Astro Color - Adaptation
« Reply #28 on: March 22, 2018, 09:26:24 PM »

-- but I can tell you that after two hours straight in the darkroom under a Thomas sodium vapor safelight, it still looks yellow. Maybe to get adapted to that, I'd have to bring in food and a cot.
Mark implied that it may well take a very long time i.e. possibly days :-)
I guess it may involve a level of neurological adaptation for extreme illuminant hues, analogous to the "mirror glasses" type experiments where the subjects report seeing the world upside down for some time, until it flips.
(I chatted to him about this specific subject at a recent CIC, as it has a bearing on something I've been working on.)
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