How does the adjustment layer Hue/Saturation change saturation?

[Guy Burns]

This question is really for Premiere Pro, but I've posted over there, and no luck so far. I may end up having to use Photoshop, because it appears that Premiere does not have the ability to alter colour the way that Hue/Saturation does. I thought that if I can find out how Hue/Saturation works, I may be able to find a workaround in Premiere.

How does Hue/Saturation desaturate?

What does it actually do to the RGB numbers?

Say I'm desaturating only the reds, what range of reds does it work on, and what does it do to them?

Background

I'm running CS6 and I'm trying to deal with 8mm colour footage from the 1940s. The reds are very strong and can't really be dealt with via Premiere's RGB curves, the usual way I deal with colour problems. In Photoshop, I'd do this:

• apply a Hue/Saturation adjustment layer;

• select the red channel (from the available options of RGBCMY); and

• desaturate the reds by 10-30%

Works well on a still from the film in Photoshop, but I can't find any colour effect inside Premiere that allows me to desaturate just the reds. So I thought I should find out exactly how Hue/Saturation works, and go from there.

[Trevor.Dennis]

Hi Guy.  I have just had a look at your thread on the Prem Pro forum, and you have not had any expert replies as I post this.

Is there a Premiere analogue of Photoshop's Hue/Saturation adjustment layer?

You should wait for people like John Smith to respond.  I am sure there will be more,  but look for people with a ton of points and an orange ACR badge.  There will be others I am sure, but John is the only one I know for sure.

My experience with Prem Pro is occasional, and I will say that I found colour became  more difficult for mew with recent versions, because Adobe made it a lot more powerful, and I have not invested the time to learn the new tools, but John and the lads will point you in right direction.  I do feel that you would be better to persevere with PP than make do with Photoshop for video.

Good luck

[Guy Burns]

Thanks for the reply. I'd still like to know how PS desaturates colours. What does it do to the RGB numbers when it is desaturating just the reds? That's the part I can't figure out.

[D Fosse]

I assume it selects/targets pixels where the R value is significantly higher than the average G/B values. Then that difference is reduced, again by averaging.

[davescm]

Hi Guy

I've dropped you a reply in the Premiere forum as you can target and reduce saturation of specific colours using Lumetri HSL secondary.

For your initial question the impact of reducing saturation in say Red is to reduce the Red value  and increase the Blue and Green values so the Lightness remains the same but the colour moves closer to grey.

Dave

[D Fosse]

Whoops, I seem to have accidentally marked my own reply as "helpful" - didn't know you could even do that.

Anyway, there's a slight distinction here, which may or may not matter in this case. Hue/Sat works in RGB mode, which means the desaturation is a simple averaging out of RGB values. It does not take into account the inherent lightness of the original color, and as a result tonality may shift.

Splitting hairs, maybe, but with a bright yellow you would really see it becoming darker.

Desaturating in Lab works differently, by preserving the inherent lightness of the color.

[G_Hoffmann]

I don't explain Photoshop (Adobe doesn't publish algorithms), but I can provide a

generic algorithm how to manipulate colors selectively for sectors R,G,B,C,M,Y.

Color spaces with polar coordinates Hue, Lightness and Saturation:

First of all one needs procedures for the transformations HLS-->RGB and RGB-->HLS.

Here this color space is called by its original name HLS=Hue, Lightness, Saturation;

other identical or similar systems are HSL, HSV, HSB, with V for Value and B for Brightness.

These are formalistic substitutes for RGB, without any psychophysical color appearance

aspect and without references to color management.

As an example: Definition of Red:

All colors in the core sector within the hue angles -15° and +15° belong to Red. But any

change has to be applied seamlessly. Therefore we define -30° to -15° and +15° to +30°

as transition sectors. The authority of any manipulation is 100% in the core sector and

washed out from there in the transition sectors down to 0%. These transitions are smooth.

Linear functions could be used, but S-like curves are better (S is here the letter S...).

Degree numbers may be varied slightly.

Application to a single pixel:

Convert RGB-->HLS. Check whether H is in the total sector -30° to +30°. If so:

Reduce S by e.g.20% by S:= 0.8*S . Apply the washout function in transition sectors,

which can be executed by one table function for the total sector. Then Convert HLS-->RGB.

This will not change numerical values for Hue and Lightness, while color appearance

deviations may happen, which doesn`t matter, because these manipulations are

normally applied interactively (and can be corrected by appearance).

Some illustrations and procedures for RGB<-->HLS conversions are shown here:

http://docs-hoffmann.de/hlscone03052001.pdf

Best regards --Gernot Hoffmann