RE: [APML] Synthetic Luminance Channel for Film Images

["Sean Walker" <swalker@SkyandTelescope.com>]

I think the benefit here is that you can adjust, sharpen, depeckle, or
deconvolute the L shot without upsetting the color balance of an image when
doing a synthetic L shot, not that it adds signal. adding un-binned color
information also helps the signal. I don't recall implying it gives
something for nothing.

Sena Walker

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Jerry Lodriguss
Sent: Sunday, June 16, 2002 12:31 PM
To: astro-photo@seds.org
Subject: Re: [APML] Synthetic Luminance Channel for Film Images



>there was another question about this, answered by Stan Moore on the SBIG
>list, maybe it helps:


Hi Volker,

You left out the complete post by Paul Kane that Stan was replying to. I'm
going to try to put these posts in sequence with comments. Mike Cook was
kind enough to pass them on to me.

Right now, I am back to leaning towards thinking that a synthetic RGB
doesn't really gain anything. Common sense and Occam's razor would indicate
that you can't get something for nothing, and I think Paul explains it best.

I measured the s/n in a blank area of sky, but that seems to only be
measuring the apparent graininess of the image. There isn't any real signal
there that we would care about.

So if we can lessen the noise or graininess that's good, but if the process
lowers the contrast of the real image data in the process, and we have to
increase the contrast to get it back, then the s/n in the blank sky will go
back up.

I may go back and include a gray scale step wedge in the tests, but
non-linear curves are kind of a pain to duplicate by hand, so it may be
tough to match them exactly.





 From Paul K:

>I really doubt that using LRGB with a synthetic L channel can
>increase the real S/N. I believe that the results demonstrated on
>Jerry's page are simply due to the effect of linear and non-linear
>transformations used to construct the L channel.
>
>The Grayscale method mixes R,G, and B by applying a predefined linear
>scale. I'm guessing that so does the Desaturate method, although the
>linear scale is different.
>
>The Lab L channel is a non-linear combination of R,G, and B that is
>meant to be more perceptually accurate.
>
>As the result of applying these different L channels in an LRGB
>composite, different emphasis is given to lighter or darker areas and
>midtones. Such an change in contrast can create an artificially
>higher local S/N, but it certainly does not generate "more" signal.
>What's more, the same effect can be achieved by applying the right
>non-linear (curves) or linear (levels) transforms to the original RGB
>image, bypassing the LRGB step altogether, IMHO.


Me:

I think this is really what's going on. I ran into something very similar
when I explored different Photoshop methods of compositing multiple color
originals.  Photoshop has a lot of different compositing modes, like
screen, and softlight, that apply even more non-linear curves in the
compositing process, and they can be duplicated in curves.

The true gain comes from compositing two different originals. With a
synthetic luminance image we don't have separate original data.



 From Stan:

>But it does seem possible to transfer some of the chromatic S/N to
>the luminance, the penalty being lower chromatic spacial resolution
>(a familiar characteristic of LRGB <g>).


Me:

I thought the lower chromatic spatial resolution was because the color was
shot originally at a lower resolution because it was binned CCD data.

However, the idea of transferring some of the S/N from the chrominance to
the luminance seems possibly plausible, but it would seem to have to
balance back out when re-combined with the color data. I will have to think
about this some more.




 From Mike Cook:

>I think Stan is trying to rationalize how it might be possible to obtain
>higher S/N since you can't get something for nothing.  It is analogous to
>binning the color channels but instead making the trade by a processing
>technique.


 From Stan

>For a weakly colored object (like a galaxy), combining the RGB
>exposures into a greyscale produces S/N approx 1.7x higher than each
>filtered exp.


Me:

Yes, but a galaxy is a broadband source, so there is signal in all three
channels. I wonder if the s/n increase of a grayscale composite of the
individual RGB channels would yield as high a s/n increase for an emission
object.

Also, it seems obvious that three times the exposure in a composite of the
three individual channels would have better s/n than a single exposure of
one of the channels.


That's it so far.


Jerry

Astronomical photography: http://www.astropix.com



--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>