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 Post subject: Is there a plugin - hue/saturation adjustment
PostPosted: Mon Aug 22, 2011 3:25 pm  (#1) 
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Is there a plugin for GIMP (well duh..) that can adjust the hue/saturation of an image while preserving the luminosity?

The built in Hue/Saturation/Lightness adjustment adjusts a layer based on HSV values. The HSV system is flawed in that two colours with the same value and saturation can have very different perceived brightness. For example, if you have a layer of bright blue (0000ff) and you adjust the hue a half circle, you get bright yellow (ffff00) which is way more bright than the blue. Both colours have the same saturation and value in the HSV system, but the other is many times brighter actually...

So... is there an existing plugin that would do the same as the built-in hue/saturation/lightness adjust, but would work in H/C/Y (hue-chroma-luminosity) colour... or, allow adjusting the hue/saturation while preserving luminosity?

For reference: luminosity refers to the perceived brightness of a colour, it's different from both lightness and value. The desaturate -function has an option of using luminosity when converting to grayscale.

I just want to know if there is an existing plugin/script for this before I start writing one, because writing one would not be a small task...


Last edited by dd on Mon Aug 22, 2011 7:45 pm, edited 2 times in total.

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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Mon Aug 22, 2011 7:12 pm  (#2) 
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I don't know if this helps. Have you looked at the saturation equalizer? It's in the registry and included in the "useful plugins" that I provide.

Partha


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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Mon Aug 22, 2011 7:45 pm  (#3) 
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partha wrote:
I don't know if this helps. Have you looked at the saturation equalizer? It's in the registry and included in the "useful plugins" that I provide.

Partha


While that seems interesting and useful in itself, it isn't quite what I'm looking for... thanks though.


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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 11:04 am  (#4) 
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Well, I figured out a pretty easy way to do this.

First, take the layer you want to adjust. Take a copy of it, desaturate the copy with luminosity setting and name the copy "lum".

Then make the hue/saturation adjustments you want on the original.

Now, make another copy of the original, desaturate it again with luminosity, set it on grain extract and merge with the original. Now the original has all luminosity removed from it, and the hue has been adjusted... but we still have the "lum" layer that has the original luminosity information. Simply set it on grain merge, merge it with the original, and voilá.

Now I'll just have to write a script for this.


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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 12:57 pm  (#5) 
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Quote:
For example, if you have a layer of bright blue (0000ff) and you adjust the hue a half circle, you get bright yellow (ffff00) which is way more bright than the blue. Both colours have the same saturation and value in the HSV system, but the other is many times brighter actually...


I believe just impossible have a color seen by human eyes as a "dark yellow", meaning dark as blue...because yellow will be never felt as dark , or if percived as dark would be no more felt as yellow.(but will be see as brown )

you may give more or less brightness to blue,or red, but for yellow you may only add some brightness (resulting in a whiter yellow) but as soon you start to lower its brightness the human eyes refuse to see the resulting color as something close to yellow

Anyway maybe you may found something useful in the "color" submenu of GMIC, as the varius color mixer

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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 1:05 pm  (#6) 
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Quote:
I believe just impossible have a color seen by human eyes as a "dark yellow", meaning dark as blue...because yellow will be never felt as dark , or if percived as dark would be no more felt as yellow.(but will be see as brown )


Well, it depends on the surrounding colours as well. Colour perception is not absolute, it's about the frame of reference.

For example, if you have a photograph of a room with yellow curtains on the window, and a photograph of the same room in the dark / low lighting... then the actual colour of the curtains in the photo can be "brown" but with the frame of reference you can still perceive it as yellow.

Anyway, I'll be writing a script for this when I have time.

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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 1:28 pm  (#7) 
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Men usually do not distinguish and so the color subtleties ....
Very waiting for scrript - and agree with an opinion of women...color belongs to the Women's...


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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 2:38 pm  (#8) 
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MareroQ wrote:
Men usually do not distinguish and so the color subtleties ....
Very waiting for scrript - and agree with an opinion of women...color belongs to the Women's...


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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 2:53 pm  (#9) 
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:clap That is so right, RobA. :lol

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 Post subject: Re: Is there a plugin - hue/saturation adjustment
PostPosted: Tue Aug 23, 2011 3:10 pm  (#10) 
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Excellent Rob. lol

I know it's wrong, but I always classify cyan as blue too; sometimes though, I call it sky blue. ;)

:)

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