RE: [APML] Synthetic Luminance Channel for Film Images

[Thor Olson <Thor.Olson@EFI.COM>]

> -----Original Message-----
> From: Roland Roberts [mailto:roland@astrofoto.org]
..
>     Thor> In this case equal weights gives std=5.05, [...]
> 
> I'm sorry to nit pick (but only a little or I wouldn't post at all...)
> 
> Equal weighting gives mean = 31.09, std 4.49.

Hi Roland,
Sorry, you are absolutely correct.  I transcribed Jerry's summary
result without checking.  The Photoshop mechanism of blending layers
at 50%, then 33% seems to be an imperfect averaging method.  The relative
results still stand however: equal weighting yielded a higher sigma than
conventional luminance weighting, which in turn was higher than the 
optimized weighting.


> The somewhat non-intuitive result that the stdev has dropped is due to
> taking the average instead of the sum.  the stdev of the sum is, of
> course, higher than any particular channel; stdev = 13.46.  The S/N of
> the sum (or average) has increased to 6.9 vs the original best-case
> (green) of 5.4.

The reason I am skeptical of taking the S/N ratio is that the background
"black" level is subject to biases.  Are these signals, or offsets?  
A nonzero offset in the blue channel for example, can introduce enormous
errors
in "signal to noise" when really we are just trying to minimize the grain.
At levels close to zero, the absolute variances are more believable to me 
than their ratio to some arbitrary offset.


>     Thor> But since truly calibrated colorimetric images are so
>     Thor> difficult, we've already crossed the "correct color"
>     Thor> boundary and should be looking for ways to make attractive
>     Thor> (but uncalibrated) pictures.  And this seems like another
>     Thor> tool to apply to the cause.
> 
> I agree completely; if you are doing photometry color calibration is
> important but for most of the pictures we do I think "reasonable
> color" and "attractive" are the things to keep in mind.

Thanks for your nitpick!  Until I see that color images are
obtained by proper matrix mixing of the photometric channels, we are not
entitled to claims of "true color".  I have processed a few Hubble
frames, keeping colorimetric calibration.  So far, my visual preference
is for the pseudocolor original rather than my colorimetrically-correct 
equivalent!

Thor.

Thor Olson                            
web-res astrophotos at:             http:\\www.nightscapes.net

   "Man has ten, computers have two, Nature has e fingers."


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