How to: Selective colour correction ?

[LigH]

Can anyone suggest how to efficiently do selective colour correction (e.g., only enhance the saturation of blue tones)?

I could guess a workaround like: mask by hue, tweak saturation, overlay by mask. But there might be a ready plugin, maybe?

[Inc]

http://forum.doom9.org/showthread.php?t=73522&highlight=red

I also asked that some time ago ;)

[LigH]

Well - finally, at the end of that thread, TweakColor (http://forum.doom9.org/showthread.php?s=&threadid=74334) sounds very promising! Thank you!

[brabbudu]

Look at http:\\expsat.sourceforge.net: it seems i've maked it for you! ;)

[LigH]

http://expsat.sourceforge.net/ -- interresting...

But it requires RGB input? That's just what I wanted to avoid.

[BBugsBunny]

I think you can't enhance the saturation of the blue tones/channel because the blue channel has already got a saturation of 100%. It's only possible to reduce the saturation so it gets more grey. If R=G=B the Saturation is 0. If a Pixel is eg. Red only the Saturation is 100%.

I did a quick test with my gradation curves filter (beta with HSV support) - Loaded the filter 2 times - first extracted the blue channel only then in the second I played around with in the HSV space. The S curve will show only an effect if you change the input value 255 (100%) of the curve - set it to 0 and you get grey only (blue converted to grey).

If you want to stay in YUV you could manipulate the U(cb) curve - the upper half of the curve (128-255) if you are in a range of 0-255 and you will probably get what you want.

Try and play arround a bit with my VirtualDub Gradation curvs filter (beta) to see the effects. There is a avisynth filter that can read the curves as well and supports some of these other color spaces as well.

[LigH]

If someone knows the behaviour of the "Flaming Pear" Photoshop filter "Vitriol":

http://www.flamingpear.com/dl/freebies.zip

Select the color you want to change as background color. Select the "direction" (the color you want the selected hue to be changed towards) as foreground color. Apply the filter. Get amazed. :D

Such a kind of behaviour would be great. -- Is there an AviSynth filter that applies 8BF filters? :D Ah, nonsense, would require RGB.

[mg262]

Well we have an RGB colour space -- or two! Or is it that you're worried about loss of accuracy? I would have said that given that the colours are being changed anyway, the loss of accuracy wouldn't be a serious problem...but I might be wrong.

But sadly...In 2002 Adobe restricted access to the Photoshop SDK, which contains the specifications for Photoshop pluginsfrom http://thepluginsite.com/knowhow/tutorials/introduction/introduction.htm

The look of the vitriol filter is very impressive, but I doubt that he does anything we can't replicate using gradated masks... but it might take a bit of work reverse engineering the filter to figure out what exactly it does. (For example, you could create 256x256 bitmaps with e.g. R on one axis and G on the other (B fixed), try the filter with a lot of different values, and plot the results in Excel to get a feel for what it was doing. Is it certain that it works in the RGB colour space rather than Lab?

[LigH]

Vitriol works well in any (more or less) Photoshop compatible graphic program, e.g. Paint Shop Pro. I don't know how it works internally, but it works on RGB images.
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It is not really my problem, I'm asking for a member in the german doom9/Gleitz board.

In my opinion, there are already a few ways to try, maybe his problem is solved already...