Histogram shape and clipping

General discussion about StarTools.

Re: Histogram shape and clipping

Postby almcl » Sat Feb 01, 2020 1:43 pm

Thanks for responding to it, Ivo. You set my mind at rest, anyway (and as a bonus I now know how to examine histograms in minute detail courtesy of ImageJ).

As Alacant points out above the question was actually about colour. It was just unfortunate that the Photoshop brigade, who insist that the image background should be set to 25 (out of how many, I've never established, having forgotten to ask one of its leading proponents when he told me about it in a tutorial some years ago) in Photoshop rather muddied the waters.
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Re: Histogram shape and clipping

Postby admin » Sun Feb 02, 2020 5:57 am

almcl wrote:As Alacant points out above the question was actually about colour.

If there was any doubt, you can safely disregard the nonsense posted about colour.
There is a reason why star "colours" are not published in a convenient, autohritative CIE XYZ component table. That's because there is no one set way to derive XYZ tristumuls values from temperatures (let alone simple B-V values), without making a huge amount of assumptions.

I also see that Martin Meredith posted in that thread, proposing some star (intensity-less) colorings, based on a stackoverflow table/formula that converts GAIA survey photomteric values, a formula which itself is based on the vendian.org link I posted eariler in the thread.

The page at that link states;

All these numbers are approximate!

These are not magic values for pixel color. Each time I change the details of how things are handled, and then regenerate these web pages, different colors fall out. Quite similar, but different.

The choice of white point (D65 vs D50) makes a big difference. I.e., is the Sun bluish or pinkish. sRGB and Rec.709 both use D65.

The choice of gamma correction (sRGB vs Rec.709) makes some difference. And the chromaticities I derive from the various sources are not very tightly clustered. And I may well be making mistakes.

For each type/class, I am blindly averaging (in XYZ space) together the chromaticities of the spectra I have available ( Kurucz, Silva, Pickles ), or of the blackbody chromaticities ( Handbook, Tokunaga ) if I have none. This approach unfortunately creates color discontinuities as one jumps among data sets. I am also missing data on assorted class/types.

What are possible improvements? There are few enough class/types that one could hand craft a set of colors. I am unlikely to do this. One might interpolate missing class/type colors to get a more complete set. Look more carefully at the spectra to determine the sources of variance. Get more complete spectra sets (Kurucz comes to mind). Provide ranges of color


The colours in the table are only true for one gamma (close to 2.2), for one (arbitrary) white point (6500K) and for normalised intensities. Not for custom stretches, not for point lights of various intensities diffracted by atmosphere and optics, not for different white points.

But they, don't take it from me, because I apparently "don't understand" standard illuminants to begin with! :lol:
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Re: Histogram shape and clipping

Postby almcl » Sun Feb 02, 2020 9:37 am

I'll take your word for it Ivo!

I have Martin's Pretty Deep Maps and refer to it frequently when examining images but his astronomical, mathematical and programming skills go way beyond my present level of understanding.

May get there one day but for those of us who learnt to program by hand punching IBM cards, it may be a step too far:
Card punch.jpg
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Re: Histogram shape and clipping

Postby admin » Mon Feb 03, 2020 1:46 am

almcl wrote:I'll take your word for it Ivo!

I guess that's the thing - at the end of the day, I kind of wish people didn't take my or anyone else's word for it. :)
It is always preferable if people - on their own - would check any claims made, check sources, ask clarifying questions questions, and form a better understanding of the subject matter that way.

With regards to programming, you'd be surprised how relevant some of the fundamentals still are! :thumbsup:
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Re: Histogram shape and clipping

Postby admin » Wed Feb 19, 2020 1:05 pm

For those interested, I just happened on this brilliant YT video explaining how subjective colouring (and colouring perception) really is. It touches on all the points I tried to make in the SGL thread.

Very important point also at 14:44; wavelength (defining hue) is only part of how we perceive a colour, 2 other factors change how we perceive colours; brightness and saturation. That's why StarTools' Color Constancy mode tries to keep saturation changes due to non-linear stretching to a minimum, so that hue (representing wavelength) is preserved as much as possible in the face of non-linear brightness stretching. This way, objects emitting at similar wavelengths can be easily distinguished.

Well worth watching. :thumbsup:
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