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Did you know this about the S curve? I didn't!

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  #1  
Old 04-10-2005, 02:42 AM
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Did you know this about the S curve? I didn't!

I had a shattering "out-of-Photoshop" experience today! The S curve works completely different from what I thought. If you're interested, you may have a look at the complete thread here DP Review thread about S curve but I can give you the summary:

Andrea writes
Quote:
How can I apply the famous "S curve" to improve contrast without affecting color balance, i.e. without changing skin color?
Then I answer, completely wrong!!!
Quote:
The S curve only changes the luminosity, the greyscale appearance of an image. Colour, that is the balance between R G and B, will not change.
Mapril corrects me
Quote:
Effectively the S curve may change the tonality. I will illustrate this with one example: suppose that one pixel have values of 200 for red and 40 for blue. When you apply the S curve, the value of the red will increase for that pixel, while the value of the blue will decrease. This may be perceived as an increase in saturation, but is not quit so. To avoid changing the tonality, you can do this way: if you use an adjustment layer, change the blending mode from Normal to Luminosity; if you don’t use an adjustment layer, select menu Edit/Fade Curves after applying the curve, and change mode from Normal to Luminosity.
And finally I test this!
Quote:
I tried exactly this. I created a colour consisting of
R=200 G=128 B=50

Then I applied a strong, but not unrealistic S curve to it. Result?
R=231 G=128 B=34

WOW!

Since I was testing more, what I was really testing, was a gradient. This was just my middle point. The initial colour is a light brownish hue. After the S curve, the darker portion turns strongly red and the lighter portion strongly yellow. But there's more. I get banding. The former gradient becomes four bands of black, red, yellow and white, with the original colour as an unnoticeable transition between read and yellow.

My artificial colour is not far from some Asian flesh tones and very light negroes or white/black offspring. Oops! Disaster lurks in the S curve...

The S curve doesn't at all work the way I thought it did.

Thanks Andrea and Mapril! I've learnt a lot today!!!
Any brown people around here? You knew this all the time, didn't you?

What an eye-opener!
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Old 04-10-2005, 09:12 AM
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Wow. Talk about timing. Yesterday I just finished up some work for somethinge extremely similiar that hits on most of those things.

An S Curve in Normal mode will increase saturation for values centered around the middle of the S curve.
PS's Sat=Max(RGB)-Min(RGB)
If you increase Max(RGB) and decrease Min(RGB), you get more saturation.

If you S Curve Luminosity, Sat will largely be left alone in the wide middles. However, Luminosity will cause Sat to be decreased the closer you get to 0 or 255 depending on Hue. For example, increasing the Lum of pure yellow will desaturate faster than decreasing the lum of pure yellow. The opposite is true for pure blue.

Let's say I want to desaturate a photo with lots of red <> orange. In a case like that, I will manually boost contrast in the seperate RGB channels with an S curve in each channel. (I also do this when trying to cull things like scar tissue.)

If a photo is kind of washed out and needs some 'regular' contrast, I will S curve the Luminosity. If a photo is too intense and needs some washing out, Luminosity again.

When you get into the RGB <> HS/Lum paradigm, things get really cool.

The work I was doing? A plug-in I call Uber Contrast.
My next one is going to be called Soft Contrast.
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Old 04-10-2005, 09:15 AM
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BTW. Levels works the same way!

Quote:
Originally Posted by Rexx
....if you use an adjustment layer, change the blending mode from Normal to Luminosity;
Or, as I would prefer - separate out the Luminosity of the image and work on just that. When you're satisfied with the greyscale, then add the colours back in and fix them.

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Old 04-10-2005, 09:30 AM
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The same situation applies to sharpening images. And changing to a Luminosity adjustment layer doesn't work either. Luminosity is a blending mode that combines the lightness component of the active layer with the hue and saturation components of the underlying layer. So even fading is not going to eliminate the color shift. So Roland is right in that you need to isolate Luminosity.

I guess that why I do virtually all my contrast building and sharpening to the lightness channel in LAB because it totally isolates the lightness component from the color. Try your adjustments in Luminosity Blend mode then try it with the Lightness in LAB and see what you get.

Dave
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Old 04-10-2005, 09:38 AM
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To understand Curves a little deeper, I recommend Steve's tutorial:
http://www.gurusnetwork.com/tutorial/curves/

Quick-n-sleazy method for extracting Luminosity in RGB mode in the Layer palette:
1. Copy photo.
2. Edit > Fill
- Use: Black, White, or 50% Grey (any Sat=0 will do)
- Mode: Saturation
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Old 04-10-2005, 09:46 AM
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Quote:
Originally Posted by Stroker
If you S Curve Luminosity, Sat will largely be left alone in the wide middles. However, Luminosity will cause Sat to be decreased the closer you get to 0 or 255 depending on Hue.
Couldn't be any other way. As you increase the Luminosity the channels get max'ed out, when they all get to 255 you have no colour at all - Saturation 0. So, in the middle there's a lot of room for colours , at the ends Sat=0 .

Quote:
Originally Posted by Stroker
For example, increasing the Lum of pure yellow will desaturate faster than decreasing the lum of pure yellow. The opposite is true for pure blue.
Most images, and especially ones with skin tones have low values in the Blue channel and, besides that, the Blue channel by itself has a very low Luminosity component - so Blue luminosity starts out at the very low end. A lot of room to go up, not much to go down.

Quote:
Originally Posted by Stroker
When you get into the RGB <> HS/Lum paradigm, things get really cool.
Shame we don't see in RGB, would make this a lot easier.

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Old 04-10-2005, 09:52 AM
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Stroker Stroker is offline
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Stop it, Ro. You are encouraging me to make graphics and explain all of this stuff on different levels in different colour spaces and paradigms.
Gets me all giddy.
I *love* this stuff! Ahhh!
Must resist... urge... to ramble...!
~grits teeth~
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Old 04-10-2005, 10:16 AM
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Thank you, Stroker, learned two new things in one post.
Quote:
Originally Posted by Stroker
Quick-n-sleazy method for extracting Luminosity in RGB mode in the Layer palette:
1. Copy photo.
2. Edit > Fill
- Use: Black, White, or 50% Grey (any Sat=0 will do)
- Mode: Saturation
Nice one, Quicker-n-sleazier than my way!

Quote:
Originally Posted by Stroker
To understand Curves a little deeper, I recommend Steve's tutorial:
http://www.gurusnetwork.com/tutorial/curves/
<ctrl><alt><M> repeats the last curve setting. (BTW <ctrl><alt><L> works for Levels)

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Old 04-10-2005, 10:19 AM
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Quote:
Originally Posted by Stroker
Stop it, Ro.
It's Rexx's fault - He started it
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  #10  
Old 04-10-2005, 10:45 AM
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Will the real Luminosity please step forward. The left is the Lightness channel in LAB. The right is Stroker's method. Why are they different and does one perform better than the other whether it be curves, sharpening or anything else?

Dave
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File Type: jpg Luminosity.jpg (90.5 KB, 42 views)

Last edited by Duv; 04-10-2005 at 11:06 AM.
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