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kfir Simon
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Do we need Luminance for imaging? or only RGB?
      #5677061 - 02/13/13 06:16 AM

Hello everyone.

From time to time I get the feeling that maybe LRGB imaging is not necessary - and only RGB is sufficient enough - meaning no need for Luminance!

I know there are some imagers that image Nebulae only in RGB without Luminance and get great results.
My traditional way was to image Lum in Bin 1 and RGB in Bin 2 to color the Luminance.
With this in mind I took this image of M42 only in RGB (each channel for 9 minutes) - and took a separate Luminance image of 9 minutes to see if it really matters.

To my surprise - the Lum didn't add at all to the final result, on the contrary - it made the beautiful colors of the RGB wash out and didn't add to the details.
Therefore I decided to keep only the RGB image.

Please see here:

http://www.pbase.com/tango33/image/148764474

Some thoughts:
Does RGB imaging (Without Lum) work only on bright Nebulae?
Since we need the color data - isn't it better to image only in RGB and extract the LUM from it? (Since LUM is R+G+B)

I would like to hear your opinions and/or experience on the subject!

All the best,

Kfir


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Nicola
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Re: Do we need Luminance for imaging? or only RGB? new [Re: kfir Simon]
      #5677070 - 02/13/13 06:25 AM

Kfir, your thoughts are correct. RGB is enough provided:

- you shoot a total integration time for RGB that you would do with LRGB
- you have a fast f/ratio scope (and this seems to be your case)

People imaging with an f/10 will probably stick with LRGB binning 2x2 for colors.

But..it depends also on the object. While an RGB will fully describe an emission nebula, it will loose some info with reflection nebulas/dust clouds/galaxies as the total spectrum is a continuum.
BTW: With a fast scope such as yours I would not bin IMHO.

Edited by Nicola (02/13/13 06:26 AM)


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Nicola]
      #5677189 - 02/13/13 08:11 AM

I think the big benefit to LRGB is that you can do all the processing on just one layer "L". You really do not need to spend much time at all processing RGB if you do L. If your results were the same, it's probably because you did spend time on RGB, which, in my experience just isn't necessary, or even correct (for the reasons you mention) when adding an L layer, as the L will add all of the processed detail. The only thing you really need from the RGB is color, you do not need to spend time bringing out the detail in RGB, it eill, as you experienced look funny because when you do stretch and sharpen the RGB layers to bring out details, it changes the range, noise, saturation etc.. in ways not really compatible with the L layer.

Also, it is easier to see contrast and subtle detail in B/W, although this would apply to RGB if done separately in greyscale.

To me though, the magic is the L layer, that is where everything happens and really helps the processing workflow if used correctly.


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freestar8n
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Re: Do we need Luminance for imaging? or only RGB? new [Re: kfir Simon]
      #5677212 - 02/13/13 08:40 AM

I think a key misunderstanding about LRGB is that it does not, in fact, somehow "increase SNR". Instead, it just lets you make a more visually pleasing image by spending more time on luminance than color - because human color perception is not sensitive to high spatial frequency.

So - unlike noise models and so forth, you won't find LRGB used in professional work - because it only applies to aesthetic imaging - not quantitative scientific work where SNR is the primary concern.

So - the "theory" behind LRGB is that a high res L combined with quick, low res color should look great - but it never worked that well for me because it doesn't look as natural as a normal RGB - especially for something like a star cluster. And if people want to muck around with the stars and colors with heavy processing - they can do that with RGB anyway.

So my impression is that LRGB is less fashionable nowadays - especially with binned color since binning may not have much win with LRGB.

But what matters is how it looks after you process it the amount you want to process. Theoretically LRGB should look better than RGB with a given amount of imaging time - but I think it depends on personal taste and personal amounts of processing.

Frank


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Bob Gillette
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5677216 - 02/13/13 08:42 AM

Anyone Imaging with an OSC camera is shooting RGB, unless you take the added step of separating the channels and creating an artificial Lum, uwhich I suspect most of us don't.

I do both OSC and mono. On bright objects like M42, I find, as you did, no significant difference. But on fainter objects, I do see higher resolution and greater sensitivity with the mono Lum.

Try your experiment on a mag 9-11 galaxy and seen if you perceive a difference with the same total integration time.

Bob

B


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Bob Gillette]
      #5677394 - 02/13/13 10:44 AM

Just to add on one thing, I think I was not clear about. When you enhance contrast of the RGB layers and process them, you effectively add black and decrease the RGB signal in some areas. Right, that is what contrast is, a change between intense and less intense signal. So, the now darker areas, unless they perfectly match the dark areas of the L, will effectively cancel out some color because while the L may illuminate it, but there is no longer color there. And vice versa. So, you actually are usually better off with fairly low contrast on the RGB layers. I've found I get very satisfactory results doing little more than applying a mild Gaussian blur to the RGB, to remove some noise and adjusting the intensity to merge well. Going much further than this with the RGB layers usually results in disappointment in my experience. But it is preferable to focus on the L too, as then you get a sense of how everything fits together, in one layer. Trying to process the RGB separately, you don't really get that sense until you merge them. Once you merge them, and start processing, then your basically just using an expensive OSC with extra steps..

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BlueGrass
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5678475 - 02/13/13 10:13 PM

... but shooting RGB with a mono vs. OSC gives you much more control over how much of each channel to capture or add to the final image. OSC always captures more green if present than red or blue and that needs to be removed or adjusted in post processing.

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bill w
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Re: Do we need Luminance for imaging? or only RGB? new [Re: BlueGrass]
      #5678614 - 02/13/13 11:46 PM

yes
in my experience
mid to long focal length with poor transparency and light pollution.
makes a huge difference especially for faint galaxies and reflection nebulae. 3 times the signal per unit time.
less so for objects that are largely mono or bichoromatic emission nebulae.
need to be careful with LRGB combine that you have enough RGB. takes some skill. suggest RGB sub exposure length be 3x luminance. can combine this data with luminance with some improvement in background smoothness
your mileage may vary


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SL63 AMG
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Nicola]
      #5678645 - 02/14/13 12:21 AM

Quote:

BTW: With a fast scope such as yours I would not bin IMHO.




If the seeing is good, I don't bin my RGB subs, I shoot LRGB all binned 1x1. In fact, I am imaging M78 RGB right now at F/3.6 binned 1x1. The subs look great.

I have a fast scope, clear dark skies and good seeing.

I never really thought about not using Luminance data. Adam Block teaches to use deconvolution on almost all galaxy luminance masters. I wonder if deconvolution is necessary anymore if processing the luminance with PixInsight's HDR wavelets.

It's something to think about and discuss.

I look forward to hearing from the experts.


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alpal
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Re: Do we need Luminance for imaging? or only RGB? new [Re: bill w]
      #5678650 - 02/14/13 12:29 AM

The Luminance has a higher signal to noise ratio than any particular colour of RGB.

Therefore in the faint areas of the picture -
luminance will enable us to see the low s/n data which would otherwise be invisible in the noise.

The only test would be on a very faint object such as a galaxy & in particular it's faint arms.


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pfile
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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5678673 - 02/14/13 12:52 AM

i still do deconvolution on my L; it's the most "pure" way of sharpening the image. after DBE the next step is deconvolution thru a mask, to protect the low-SNR areas. then it's off to nonlinear-land...

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Leonardo70
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Re: Do we need Luminance for imaging? or only RGB? new [Re: pfile]
      #5678782 - 02/14/13 05:22 AM

Hello Kfir, my opinion (with in mind your fast scope) :

- R+G+B isn't equal to L due to the filters cut
- L is necessary for faint objects and better SNR (now i use always after one year i used a RGB only)
- Colors become washed if you work separately on L and RGB (mix togheter as soon as possibile) and work on it, better to use an L-LRGB approach
- Don't use a bin2 for color (you are fast and don't need)
- Use Sum instead of Mean for color stacking (i also use for L) , you'll gain more dinamic when stretching it according the luminance. This mean to use a good rejection tools.

All the best,
Leo


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freestar8n
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Leonardo70]
      #5678890 - 02/14/13 07:30 AM

Quote:

L is necessary for faint objects and better SNR




This is typically how LRGB is described, but the problem is that in color imaging, you just have R, G, B signals - and you want each signal accurately recorded and with low noise. If you multiply L by R to get a "better" R - you may get an R channel that looks better - but the signal has been corrupted by mixing in G and B data from the L, into R.

So L has more detail and so forth - but it doesn't do anything to improve the SNR of the color channels. One text that gets this right is Berry/Burnell's handbook where they say LRGB makes for a better looking image with a given total exposure time - by combining a good looking L channel with low res color. It's a perceptual thing - not an SNR thing.

If LRGB really did improve SNR, it would be used professionally to capture better data in a color channel. It's just used for making nicer *looking* images in a given time - and even then, some people like its results and others don't.

Frank


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Ken Crawford
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Re: Do we need Luminance for imaging? or only RGB? new [Re: freestar8n]
      #5679162 - 02/14/13 10:52 AM

The luminance channel is better for sharpening systems like Decon and others.

One of the ways to improve the total signal is if you take unbinned RGB data, you can combine the RGB into a synthetic Luminance then combine your deep luminance and the RGB luminance together for a combined signal Luminance. This method is used by Jay GaBany and myself for the professional Star Stream Team survey to enhance the contrast of the very faint streams. This method we us has been explained in several AJ papers that we have produced on the subject.

That way you are not just using the RGB result to colorized the Luminance, you are using the RGB + L combined to produce a better result.

You can then use the RGB result to colorize the "Super L" and use the standard L to Decon and apply as a sharpening layer.


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679201 - 02/14/13 11:09 AM

Quote:


So L has more detail and so forth - but it doesn't do anything to improve the SNR of the color channels.





It seems it is improving "luminance SNR" though, especially if used in a way such as Ken described. However, perhaps skewing the color rendering or "color SNR". It sort of depends what "signal" we're interested in I guess. From an EE perspective (I/m not an EE, so, could be wrong), I think usually signal is consider as both intensity and frequency/wavelength, so, then in that view, perhaps it would be fair to say it improves sampling of the intensity at the expense of wavelength sampling?

Edit: I changed "frequency" to "intensity" above

Edited by Inverted (02/14/13 12:03 PM)


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freestar8n
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679226 - 02/14/13 11:20 AM

If you are taking L and also taking R, G, B, then sure you can improve the SNR of your L by summing the RGB into the L - assuming L is really equal to the sum of R, G, B. But the result is a higher SNR L - not a higher SNR R, G, B. If you just wanted a high SNR L, you would have been better off not exposing R,G,B at all and instead do all as L.

In the AJ2010 paper - which I assume is representative of what you are referring to, as far as I can tell the RGB info is only used to make a reference color image to enhance the aesthetics of the figures - but the analysis is based strictly on L. If you have a paper that describes the SNR improvement of the R,G,B channel information by combining with L I'd be happy to take a look at it.

Frank


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Ken Crawford
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Re: Do we need Luminance for imaging? or only RGB? new [Re: freestar8n]
      #5679269 - 02/14/13 11:40 AM

The RGB Sum + L was used to produce higher contrast in the streams but the Lum only was used for the professional measurements and analysis. You are correct that you could keep going longer in L only for the "best" results but the idea was to use the RGB signal since we have it to improve contast in the streams - which we do for display only. This way the RGB signal was not just used to tint the Lum channel. In other words, we have the data we might as well use it!

The Max-Planck guys measured my L only channel on NGC4216 down to 29.3 mags / sq/ arcsec and the combined RGB + L gave a higher s/n ratio of the streams.


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freestar8n
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679280 - 02/14/13 11:47 AM

Quote:

This way the RGB signal was not just used to tint the Lum channel. In other words, we have the data we might as well use it!




Sure - that makes perfect sense. But people think that LRGB is a way actually to improve the color accuracy of an image - by somehow increasing the SNR of each R,G,B channel. I claim this is incorrect, and cite Berry/Burnell as a source that gets it right. LRGB relies on a perceptual trick to make a color image look better - by spending more imaging time on luminance than color. The result just looks better - but the inherent color accuracy or "SNR" has not been improved.

So people shouldn't feel there is an inherent SNR win by doing LRGB vs. RGB. But there is a chance the end result would be perceived as higher quality and look better - or not.

Frank


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Ken Crawford
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679282 - 02/14/13 11:50 AM

Frank,

See page 5 of this paper that describes the RGB combined method.

http://www.cosmotography.com/images/ngc5907_def.pdf

I joined the team after this paper and is where I learned the method. Once it was described, we did not have to keep talking about it. But as you see, the combined data was used and we still will use now for contrast enhancement.

We just take the data and carefully process it to specs, we don't do the analysis - the PI and other pros do.

Regards,


Regards,


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Ken Crawford
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Re: Do we need Luminance for imaging? or only RGB? new [Re: freestar8n]
      #5679288 - 02/14/13 11:53 AM

Quote:

Quote:

This way the RGB signal was not just used to tint the Lum channel. In other words, we have the data we might as well use it!




Sure - that makes perfect sense. But people think that LRGB is a way actually to improve the color accuracy of an image - by somehow increasing the SNR of each R,G,B channel. I claim this is incorrect, and cite Berry/Burnell as a source that gets it right. LRGB relies on a perceptual trick to make a color image look better - by spending more imaging time on luminance than color. The result just looks better - but the inherent color accuracy or "SNR" has not been improved.

So people shouldn't feel there is an inherent SNR win by doing LRGB vs. RGB. But there is a chance the end result would be perceived as higher quality and look better - or not.

Frank




Agreed !! It does not help the color channels at all. I have found however that with today's processing tools, you will never have a problem pushing color as long as you have enough of it

Great stuff Frank!


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679346 - 02/14/13 12:20 PM

First, by the way, I modified my post above a bit, I realize really I meant to say "intensity" when I said frequency.

Anyways, just getting back to SNR, as I find this metric fascinating, I guess, I still haven't been convinced that SNR is really the best metric to describe image quality as we talk about it. For one, as mentioned in the exposure length thread, it ends up being a biased estimate, if skyglow goes up for example, it can actually improve on paper.


But also, regardless, pretending "observed SNR", did actually measure "true SNR" perfectly, the whole idea of talking about SNR as a metric, even under perfect conditions seems to fail somewhat. This is because when we do talk about 'image quality" we are talking about our perception of quality, not just means and standard deviations in the data. When we're talking about luminance, the means and standard deviations do seem to make more sense because of the way our eyes perceive grey scale and metrics used to describe luminance take into account the weighting of our eyes.

When we start talking about color though, this changes significantly because we are not considering the weight anymore. From a scientific perspective, it seems that we do want to know the means and standard deviations of the colors. However, our eyes can can be 20x or more, more sensitive to say green than red or blue. So, when we start talking about color, I think that there is a bigger discrepancy between the numbers and our perception. The luminance takes into account our perception, so, we can use that to boost our perception of the data and make a "better" image, although, as mentioned from a pure numbers perspective, it probably isn't lower variance, i.e. higher SNR.


So, when we start talking about color data, it seems we should almost use a luminance weighted estimate of the SNR. Does that make sense to others? And does anyone already use such an estimate?

Edit: also by the way, how do most programs measure SNR when using color data. For example, do they only look at variance of pixel to pixel intensity and ignore color? I think so, as when in Maxim DL, when I put the little curser over part of the image, it doesn't give me seperate L, R, G and B SNRs. So, when your talking about improvements to SNR etc... to lay ears such as mine, it may help to know the specifics of how it is being measured. If we are just measuring luminance SNR, then it seems it would improve with extra, separate L data. And if we're discussing SNR, with regards to what are we referring, just the measured value, or the theoretical, underlying value?

Edited by Inverted (02/14/13 12:52 PM)


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freestar8n
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Re: Do we need Luminance for imaging? or only RGB? new [Re: Ken Crawford]
      #5679396 - 02/14/13 12:53 PM

Thanks Ken - That all looks good to me. I think it's great you were able to incorporate the color channels to improve the depth for the scientific goals of the project - and at the same time get nice color images of the galaxies to provide context for the star streams.

As for LRGB - I'm a believer that it *should* work to make a nicer looking result with more detail and better colors - but I think a key problem for me is slight spherochromatism in my c11 with reducer that makes it hard for the star sizes in different colors to match exactly so the end result looks natural. I assume I could mess with it in processing - but my main message is for people to give it a try, but it may not work as automagically as hoped - and there is no *inherent* improvement in the actual color SNR.

Thanks,
Frank


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Ken Crawford
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Re: Do we need Luminance for imaging? or only RGB? new [Re: freestar8n]
      #5679469 - 02/14/13 01:24 PM

Quote:

Thanks Ken - That all looks good to me. I think it's great you were able to incorporate the color channels to improve the depth for the scientific goals of the project - and at the same time get nice color images of the galaxies to provide context for the star streams.

As for LRGB - I'm a believer that it *should* work to make a nicer looking result with more detail and better colors - but I think a key problem for me is slight spherochromatism in my c11 with reducer that makes it hard for the star sizes in different colors to match exactly so the end result looks natural. I assume I could mess with it in processing - but my main message is for people to give it a try, but it may not work as automagically as hoped - and there is no *inherent* improvement in the actual color SNR.

Thanks,
Frank




I even get some star size variances from seeing differences from night to night with my RC. I normally images several nights on a target. What I do is a very mild positive constraint Decon on the two largest FWHM color channels to push them down to the smallest of the color channel. I am talking very mild 10-20 iterations with a proper PSF. I then find my color fringing is less in the RGB master.

I also stretch around a star mask to keep the color from blowing out the Lum margins.

This is really good stuff Frank, your comments gives me inspriation for this hobby as I have not been as active as I would like to be on the imaging side. I learn from these types of interactions and gain more fire in the belly

Kindest Regards,


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alpal
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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5680271 - 02/14/13 09:49 PM Attachment (9 downloads)

Quote:

The Luminance has a higher signal to noise ratio than any particular colour of RGB.

Therefore in the faint areas of the picture -
luminance will enable us to see the low s/n data which would otherwise be invisible in the noise.

The only test would be on a very faint object such as a galaxy & in particular it's faint arms.




I tested the above theory out on some actual single frame RAW data from galaxy NGC 253.

I used a single Luminance frame & a Green frame.
Both frames were 3 minute exposures under light polluted skies with no ALP filter.
I cropped each of them out of the full size pic.
I then pasted them onto a new pic.
I stretched both of them equally as one pic.
I then stretched the Green version ( on the right hand side )
so that the background was equalised with the luminance frame. ( I chose 40 with the eyedropper in PS curves )
The area outlined as a box is then expanded later for the next pic to view right down at pixel level.


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alpal
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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5680276 - 02/14/13 09:51 PM Attachment (12 downloads)

Next is the expanded pixel version of just the box.
Notice the faint arms are hardly visible in the Green version on the right hand side.
The faint arms are lost inside the noise.


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blueman
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Re: Do we need Luminance for imaging? or only RGB? new [Re: kfir Simon]
      #5680358 - 02/14/13 10:44 PM

I have not used true luminance for quite a while. I make sythetic luminance from the RGB. But I take a lot of RGB and then I have 30-60 frames to make a sythetic luminance. I have not found that luminace in needed for any of my images.
Blueman


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Re: Do we need Luminance for imaging? or only RGB? new [Re: blueman]
      #5680565 - 02/15/13 01:53 AM

Well if you create the luminance from the RGB this will not help in comparison with pure RGB. If you shoot separate luminance you get the whole wavelength spectrum. If you only take R, G and B you will miss some wavelengths because the filters' spectrums do not overlap completely. At least this is my understanding.

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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5680600 - 02/15/13 02:38 AM

Hi-

There is no question that L will probably have higher SNR than G for the same exposure time. But in LRGB, the claim is that multiplying G by L will increase the SNR of G - and that is why LRGB is good. That part is not correct because multiplying G by L doesn't improve the accuracy of G at all - in fact it corrupts it by mixing in R and B with the new G signal. But it ends up looking better in the end because you can really smooth the color channels before you do the LRGB combine - and your eye/brain won't realize it.

A big reason your G signal is fainter is that some of the structure in there looks like Ha stuff - in the red. It's supposed to be missing in G. If you mix L with G to make a 'better' G, you would be incorporating Ha into the G - but that makes no sense. Your G signal may look better and have less noise - but it's not accurately capturing G. So your signal is corrupted and your SNR for the G signal would go down - even if the end result looks better.

This is even more extreme when people do HaRGB. Some people like it, others don't - but you can take a nice, contrasty Ha image with a lot of detail - and multiply it by RGB and get an incredibly colored image. But did you improve the accuracy of the G signal? No - you just mixed G with Ha and got a colorful image. If you like the result that's fine - but it didn't improve the accuracy or SNR of the G signal - even though the colors look "better".

Frank


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alpal
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Re: Do we need Luminance for imaging? or only RGB? new [Re: freestar8n]
      #5680605 - 02/15/13 02:42 AM

Hi Frank,
Yes I would probably have to repeat the test with Red to be sure.
Thanks for the insight.


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5680772 - 02/15/13 07:25 AM

Hi Apal, also, I think you should also display as green, not grey scale. When you display it as green, the perceived difference will be different than in the precieved difference in grey scale. The luminance layer of an image is really adjusted to the sensitivity of our eyes. However, this will have the least impact for green, as our eyes are most sensitive to green. Well, actually to green/yellow, our visual perception peaks at about 550nm.

However, then, our eyes are not as sensitive to red and blue, so, if you take that green image and convert it from green to blue or red, I would expect the perceived "SNR" (or more properly stated in this case apparent detail) would be less.

This is an interesting result. It seems in these threads the focus is on SNR, as that is perceived as a quantifiable metric and "perception" of quality is apparently not. However, I'm not sure that is true. The characteristics of the human eye and perception of color are well defined by the photopic luminosity function. We can actually quantitatively determine color perception of an average human eye. And actually, when you convert color to a luminance layer, the software will generally weight accordingly, so, what you get in luminance is (assuming the software is weighting properly) is a representation of the perceived color, not a 1:1:1 scaled representation of actual color.

So, I realize when I asked about a color weighted SNR, a luminance layer actual is.

Anyways, that's just my .02. Years ago I worked in a molecular biology lab studying retinal rod and cone progenitor cells. So, this is one of the few topics in the hobby I actually should have good intuition about and perhaps even know something LOL.


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5680861 - 02/15/13 08:42 AM Attachment (7 downloads)

Hi "Inverted",
Here is the same pic but green is green - what do you think?


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Re: Do we need Luminance for imaging? or only RGB? new [Re: alpal]
      #5680906 - 02/15/13 09:04 AM

At least my opinion is that there is still fainter visible detail in the luminance frame which is not present in the green channel.

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Re: Do we need Luminance for imaging? or only RGB? new [Re: neptun2]
      #5681063 - 02/15/13 10:14 AM

Yes, the green I think is actually even a little less visible now than before when shown in grey (as would be expected). Now, if we took the green and converted it to red or blue, it should look different still. However, the kicker is, that in all instances, it's the same image and the SNR is the same. When we're looking at greyscale, we're just looking at contrast of the image, which our eyes see differently. It isn't effected by our perception of different colors, in the same ways and the SNR does not have a linear effect with respect to our perception of all colors.

When we take a color image though, the SNR is reduced because the filter is clipping some light. The SNR of the mono image is not effected in this way. Then we work on luminance and apply it to the color image. So, even though the SNR will not effect perception of colors linearly, increasing it (by utilizing more wavelengths as we are not clipping any with the filters) will still boost our perception of the colors.

I guess as Frank mentioned, you now may be seeing an increase in color because there are extra wavelengths being captured by the mono channel. I'm not entirely sure I'd agree this is "less accurate though" The mono "luminance" channel is the most accurate representation of the signal, as no signal is being clipped by the filter, however, they colors may not end up perfectly "correct". Although, I'd note, that when using filters, the filters we use aren't perfect, there is overlap and gaps etc.. so, the color also isn't "perfectly correct". With monochrome imaging though, with no filters, you are capturing more signal, so, the SNR should be greater for a given exposure and therefore there should be more contrast in the final image (even if the color isn't perfectly represented).


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681249 - 02/15/13 11:44 AM Attachment (4 downloads)

At least on photoshop R,G,B can be layered and blend as "screen", then merge. isn't the same as Luminance concept?

by the other hand showing in this diagram that luminance fill some gaps between the colors, I might be a bad interpretation of a wrong color at all.


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Inverted
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Re: Do we need Luminance for imaging? or only RGB? new [Re: JoseBorrero]
      #5681316 - 02/15/13 12:16 PM

I looked into how Photoshop processes luminance and it looks like if you go from an RGB image to a "luminance" image, it scales it as Luminance = 0.3 R + 0.59 G + 0.11 B. So, as expected, it's not a 1:1:1 ratio. It appears the values are an attempt to scale according to the luminosity function, based on the base values of the 3 colors as used.

Going the other way, I am not as sure about the scaling.

Also, I'm not quite following what you were saying about the graph?


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681337 - 02/15/13 12:24 PM

PixInsight allows you to extract Luminance from RGB using 1:1:1 RGB ratio.

Peter


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Peter in Reno]
      #5681385 - 02/15/13 12:45 PM

Quote:

PixInsight allows you to extract Luminance from RGB using 1:1:1 RGB ratio.

Peter




but then it isn't "luminance" By definition, "luminance" is the weighted power intensity, weighted to the sensitivity of the human eye. The SI definition of luminosity is candella per square meter, where a candella is based on the photopic luminosity function as shown here :



Also, I think technically, we probably shouldn't call a monochrome image a "luminance image" until it has been processed and stretched to be better perceived by human vision. Hobby usage may vary though...

The processing steps we use then are sort of backwards, which I think is why it is perceived as "not as accurate". Although, I'm not sure color rendering in the hobby is done super accurately regardless. Perhaps somewhat if we do RGB and white balance, but it seems to really be accurate, the filters should match the response of the ccd, or be adjusted to it and not have much overlap, or gaps. I could be wrong, but I think scientific filters try to do this more thoroughly.


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681453 - 02/15/13 01:08 PM

It was suggested to me from PixInsight forum to use 1:1:1 RGB ratio if you want to create and process pseudo Luminance and combine with RGB. I find this method to work quite well and better than the ratio you listed.

It's mostly all about making the images look prettier. It's a personal preference. I live in pretty high LP area and I have to make some adjustments to get what I want to match as closely as other great images. Everyone's method is different based in their equipment and location. But if I live under a very dark sky like New Mexico, then processing RGB alone would be very easy without added Luminance. JWalk's examples are very good and he uses OSC camera. I have processed other people's images taken from dark skies and they were very easy to process because data is much better from dark skies and very little work to take care of unwanted signal (LP) or noise.

Either way, processing with pseudo or mono Luminance is not accurate in terms of color. Like Frank said, it allows you to make pretty pictures but not necessarily accurate colors.

Peter

Edited by Peter in Reno (02/15/13 01:16 PM)


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Peter in Reno]
      #5681477 - 02/15/13 01:20 PM

For sure. I'm not talking about what works best, I'm just trying to take a step back and look at what we are actually talking about and how they relate to "pretty pictures".

Really, there isn't much green in space, so, weighting green 59% probably doesn't make much sense as it is probably mostly noise. So, using this function, we may be distorting our overall SNR unnecessarily for example. Speaking of PixInsight, I think there is a noise reduction function (I forget what it's called) that targets green specifically, as this is considered to likely be noise. The point though is metrics like SNR only work so well, and start breaking down when we move from quantitative data land to more qualitative human vision land...


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681509 - 02/15/13 01:30 PM

Also, by the way, the monochrome CCD image I think is 1:1:1 until we stretch it. Once we stretch it, then it doesn't necessarily match the 1:1:1 RGB ratio anymore, but does provide the background for our perception of the RGB ratio (i.e. luminance). So, it is perhaps "less accurate". That's been mentioned by myself and others, previously, but I'm not sure how clearly...

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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681594 - 02/15/13 02:03 PM

You might be thinking of PixInsight's SCNR which removes unwanted green cast caused by LP from the images. You can also select to remove red or blue as well with this tool. Be careful when using this tool because it can also remove "good" green as well.

Peter


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Peter in Reno]
      #5681655 - 02/15/13 02:26 PM

Yeah, that's it. And I'm not providing any opinion or recommendation, just describing how it is described in the PI documentation.
http://pixinsight.com/doc/legacy/LE/21_noise_reduction/scnr/scnr.html

From it:
"The rationale for SCNR is quite simple. We know that, with the exception of some planetary nebulae, there is no green object in the deep-sky. There are no green stars. Emission nebulae are deeply red. Reflection nebulae are blue. Oxygen III emission corresponds to a mix of blue and green. We may conclude that if we find green pixels on a color balanced, deep-sky astrophoto, they are noise. Fortunately, removing green noisy pixels from most deep-sky images is not difficult and can be accomplished very efficiently."


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5681669 - 02/15/13 02:31 PM

I use SCNR to remove excessive green when I was processing Hubble Palette of Bubble Nebula since Ha is mapped to Green channel. You can adjust the amount of green to reduce. This simple tool did a neat trick. You can see my result at Bubble Nebula in HST .

Peter


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Peter in Reno]
      #5681697 - 02/15/13 02:45 PM

That's stunning. And very interesting usage of SCNR

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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5682116 - 02/15/13 06:30 PM

Quote:

.... We know that, with the exception of some planetary nebulae, there is no green object in the deep-sky. ....




Take a look at Hanny's Voorwerp.

dan k.


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Re: Do we need Luminance for imaging? or only RGB? new [Re: HunterofPhotons]
      #5682156 - 02/15/13 06:46 PM

>>>>>Take a look at Hanny's Voorwerp.

Or any of the classic Martians or other little green men.

Alex


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Re: Do we need Luminance for imaging? or only RGB? new [Re: HunterofPhotons]
      #5682184 - 02/15/13 07:09 PM

Quote:

Quote:

.... We know that, with the exception of some planetary nebulae, there is no green object in the deep-sky. ....




Take a look at Hanny's Voorwerp.

dan k.




They mentioned O-III in the paragraphs I quoted "Oxygen III emission corresponds to a mix of blue and green. "

If I remember correctly, it tends to fall end up registering twice in OSC cameras for that reason, because it ends op on the blue and green pixels. So, that is sort of a false color even without luminance as your doubly counting it (or triply maybe as green is counted twice on an OSC). So, there is false color without a luminance layer


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Alex McConahay]
      #5682186 - 02/15/13 07:10 PM

Quote:

>>>>>Take a look at Hanny's Voorwerp.

Or any of the classic Martians or other little green men.

Alex




Everyone knows that's a myth, the Martians are grey duh


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5682281 - 02/15/13 08:24 PM

oh on the PI thing, it actually extracts L* which is the perceptual lightness. if you leave your rgb image in it's original weighting and gamma then L* is as in the CIE model. but by setting the rgb weights to 1:1:1 you get something closer to luminance, rather than being green weighted to match the response of the human eye.

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Re: Do we need Luminance for imaging? or only RGB? new [Re: pfile]
      #5682759 - 02/16/13 02:26 AM

In photoshop you can easily control the ratios of R, G, B, yellow, cyan and magenta. Every channel can be set from -200 to +300%. The default configuration is as follows:

R - 40%
Yellow - 60%
G - 40%
Cyan - 60%
Blue - 20%
Magenta - 80%

This is maybe the so called "visual" convertion. There is also option for 0% on all channels called "Maximum black" which looks like the 1:1:1 ratio. There are also other presets and you can create and save your own. All this is accessible from Image->Adjustments->Black & White menu. This is with latest photoshop cs6. I don't know if the older versions have all these options.


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Re: Do we need Luminance for imaging? or only RGB? new [Re: kfir Simon]
      #5682886 - 02/16/13 07:04 AM

Quote:

Do we need Luminance for imaging? or only RGB?




According to an S&T article (Dec. 2012, page 71)...

"You create the color image by aligning and combining these (RGB) color-filtered stacks into a master RGB image called chrominance.

Although the chrominance image provides the beautiful color, it's the luminance image (shot without color filters) that can contribute some of the sharpest detail."


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Re: Do we need Luminance for imaging? or only RGB? new [Re: neptun2]
      #5682967 - 02/16/13 08:40 AM

Quote:

In photoshop you can easily control the ratios of R, G, B, yellow, cyan and magenta.




For sure, but then you have "false color". As mentioned we can think of an image in 2 parts, luminance and chrominance. An nebula or galaxy etc.. has a certain ratio of color "chrominance". And we then perceive that color based on the relative intensities and sensitivity of our eyes to that color this is "luminance.

So, by definition, an image should have a certain amount of luminance which by definition should match "chrominance" in a specific ratio. In our hobby we modify luminance (and "chromonance") to enhance visualization of features and detail in the image. So, I think we are really generally always working with "false color" images. That is why I don't think there is a strong argument to using RGB to begin with and extracting the luminance (unless you just prefer processing that way, or like the results better).. Unless we only do a linear stretch of the data, equally on all channels (once white balanced), and maintain the luminance in a 3:59:11 ratio, then we really do have false color.

So, there are two benefits to using L i think. If we collect a separate luminance image, this allows us to increase the SNR of the image, as no light is being blocked by the filters for the L. And two, we can work on a luminance image, which is essentially adjusted to remove the limitations of our vision. with respect to sensitivity to and perception of certain colors.

So, yeah, we do sort of end up with a false color image, but I seems that the way we proccess images in the hobby, we do regardless. Also, as mentioned, we are again only really increasing the SNR of the L, not the color, but do we care if it looks better? The L is where the detail is, and the RGB is distorted either ways.... So, to me, I'm perfectly happy distorting it a bit more to sample the detail a bit better.


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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5683162 - 02/16/13 10:30 AM

Yes i completely agree that luminance should be used. I wonder if we will get any improvement if we extract another luminance from the combined R + G + B channels with 1:1:1 ratio and stack it with the normal luminance creating one let's say "super luminance". After this we can normally process the "super luminance" and RGB frames as ususal. I think that the added "color" luminance should create higher SNR combined "super" luminance. Of course this will be valid only if you take L , R , G and B frames with equal rsolution and duration and not binned the color.

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Re: Do we need Luminance for imaging? or only RGB? new [Re: Inverted]
      #5683361 - 02/16/13 12:04 PM

Quote:

Quote:

Quote:

.... We know that, with the exception of some planetary nebulae, there is no green object in the deep-sky. ....




Take a look at Hanny's Voorwerp.

dan k.




They mentioned O-III in the paragraphs I quoted "Oxygen III emission corresponds to a mix of blue and green. "





if memory serves, the voor werp is "red shifted" towards pure green

clearly an exception to the rule


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Re: Do we need Luminance for imaging? or only RGB? new [Re: blueman]
      #5683564 - 02/16/13 01:49 PM Attachment (5 downloads)

Here is an RGB image, using the RGB to make luminace in CCDStack using my own process.

I can not compare it to LRGB, but I have done that with other images and I can not tell the difference.

The synthetic luminance is from 63 10 minute RGB images.

http://www.astrophotogallery.org/showfull.php?photo=10984

Blueman


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Re: Do we need Luminance for imaging? or only RGB? new [Re: blueman]
      #5683569 - 02/16/13 01:52 PM Attachment (6 downloads)

Here is the Synthetic Luminance, small size due to image size constaints.
Blueman


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Re: Do we need Luminance for imaging? or only RGB? new [Re: blueman]
      #5684646 - 02/17/13 05:24 AM

Thank you all for such great inputs.
As I understand, it isn't as "Black and White" in favor of LRGB over RGB as I suspected in the beginning.

Keep on the good ideas!

Kfir


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