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Author Topic: New MaximDL plugin release - LRGB via H.S.L. color space  (Read 4705 times)

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Offline John Winfield

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New MaximDL plugin release - LRGB via H.S.L. color space
« on: 00:11:48, 29 October, 2006 »
This pair of plugins for MaximDL allows the composition of an LRGB image in the H.S.L. (Hue, Saturation, Luminance) color space.

MaximDL contains a built-in method for creating LRGB luminance layered compositions, using the L.a.b. color space. This converts the RGB image into the L.a.b. color space and then replaces the computed Luminance channel with the one supplied by the user, before converting back into the RGB color space.
Whilst this works reasonably well, it results in a color shift from the original due to the way the L.a.b. color space works.

In the L.a.b. color space, the three channels are not completely separate. The "a" channel is tied loosely with the L channel, but the "b" channel is not significantly connected. The result is that if you modify the L channel only, you will still change the "a” and “b" channels proportionately. The "a" channel controls the reds and oranges, and the "b" channel controls the greens and blues. Since the "b" channel is not significantly altered, it does not change hue as the brightness of the L channel goes up. However, as you increase the brightness of the L channel, the "a" channel shifts its hue. The result is that reds become more orange resulting in the infamous "salmon pink" coloration, while blues stay pretty much the same.

In addition to the hue shift which affects only the "a" channel, as the Luminosity in the "L" channel increases, both the "a" and "b" channels will loose their saturation. Since this is a non linear function, it is quite complex to predict a correction factor.

An alternative method is to translate the RGB image into the H.S.L. color space and then replace the computed Luminance channel with the one supplied by the user, before converting back into the RGB color space. This has the advantage of not changing the image hue at all, thus preserving the original image colors accurately.

You can download these free plugins from my web site as normal:

http://winfij.homeip.net/maximdl/index.html

If you use these plugins, please post feedback on here or send to me direct via email.

Regards,

John
10" LX200R, Pentax SDHF-75 on AP900
ART11002 + TT Filter wheel, DSI Pro.
WO ZS66SD + PST.
Old kit:
C11 on EQ6 mount. Meade SN8 on LXD75 mount, ART285

Offline the fordster

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #1 on: 12:45:32, 29 October, 2006 »
Great work, John! :urock: Made this a sticky - Thanks for sharing 8)

Offline mcrossley

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #2 on: 17:00:08, 29 October, 2006 »
John

What scheme are you using for extracting the Lum?  I've seen two methods refered to:

a) Lightness = (max(r,g,b) + min(r,g,b)) / 2
b) Intensity = (r + g + b) / 3
Mark
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Offline John Winfield

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #3 on: 17:05:24, 29 October, 2006 »
Mark - I used the first of the two algorithms - it was by far the most common in the references I found.
Where did you find the second one?

John

PS. I believe Photoshop actually uses the second one.
10" LX200R, Pentax SDHF-75 on AP900
ART11002 + TT Filter wheel, DSI Pro.
WO ZS66SD + PST.
Old kit:
C11 on EQ6 mount. Meade SN8 on LXD75 mount, ART285

Offline synner

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #4 on: 20:05:08, 29 October, 2006 »
For a bit of background to these plugins...

This all came out of a thread that started off about filter weighting and ended up about LRGB combination.

Here's the thread :D
Nick
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Offline mcrossley

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #5 on: 20:44:50, 29 October, 2006 »
John, it appears I am talking about two different things.

Formula a) is HSL, where L = Lightness
Formula b) is HSI, where I = Intensity

So are the luminance frames we normally capture L or I, I would suspect full spectrum images would be I?

One other thing to bear in mind is that HSL in Photoshop can only show a fraction of the all the possible 8 bit RGB values because it uses integer values for the hue (0-359).  If it doesn't already and you can I would suggest that your plugin uses floating point values for H - or scale them up a factor of 100 to integer encode the hue to two decimal places?

Cheers
Mark
Mark
C14 SCT + Hyperstar » SV4 Apo » MI-250 + Gemini » QHY8 » QHY5 » ToUcam
http://www.wilmslowastro.com/

Offline John Winfield

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #6 on: 20:49:31, 29 October, 2006 »
One other thing to bear in mind is that HSL in Photoshop can only show a fraction of the all the possible 8 bit RGB values because it uses integer values for the hue (0-359).  If it doesn't already and you can I would suggest that your plugin uses floating point values for H - or scale them up a factor of 100 to integer encode the hue to two decimal places?

The plugins both use all double floating point values, so they should be accuate enough.

I've seen "HSB" (Brightness) referenced in some places, although not "HSI". Perhaps they're the same.

I'm not too sure which would be more appropriate.

John
10" LX200R, Pentax SDHF-75 on AP900
ART11002 + TT Filter wheel, DSI Pro.
WO ZS66SD + PST.
Old kit:
C11 on EQ6 mount. Meade SN8 on LXD75 mount, ART285

Offline Bertha

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Re: New MaximDL plugin release - LRGB via H.S.L. color space
« Reply #7 on: 22:45:27, 17 November, 2011 »
Cool plugin, I still use it to this day.

 

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