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

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Old 04-10-2005, 09:59 AM
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Stroker Stroker is offline
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Duv, it's not so much the difference between L in Lab and PS's own Lum.
L in Lab = RGB > Y > L
PS's Lum = R*0.30 + G*0.59 + B*0.11
While there is difference, it's negligable when it comes to contrast (negligable when it comes to a lot of things, but it might matter to a number freak).

Contrast in two main flavors:
1. Manipulating RGB either together or in separate channels. This has a direct effect on PS's flavor of Saturation.
2. Manipulating your favorite flavor of Luminosity or Lightness or what have you. Depending on several things, this may or may not affect PS's Saturation.

Hopefully later today I'll have some material ready that gets into #2 and some related things.

Sorry, Duv. I totally missed a large chunk of your little post.
If it's not much trouble, try both methods for a given task and see which one you like more. No big deal.
If you want to know more about the differences, look into Cie xyz as it kind of a hub between a lot of colour related things.
It's a big can of worms and I don't feel anywhere near qualified to talk about such things.

Now, if contrasting, or other manipulations, lum in RGB mode, I would prefer to use PS's Lum because it's 'true to PS's RGB internals' so to speak.
Personally, it doesn't make much sense to me to RGB > Lab > RGB (even in part) when a 'pure' method is right there in RGB.
Ah, kinda like that.

If working in Lab, then I would use the L in Lab.

Last edited by Stroker; 04-10-2005 at 10:38 AM.
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Old 04-10-2005, 10:59 AM
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byRo byRo is offline
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Originally Posted by Stroker
I would prefer to use PS's Lum because it's 'true to PS's RGB internals' so to speak.
Right on, Stroker.
This is the main reason why I stick with Luminosity. Not because I understand all of the aspects of colour -> "brightness" conversion. But because it requires less PS math manipulation and hassle. It is easy to separate the luminosity into a layer (easier still, thanks to Stroker's post above) and easy to blend back in (as Luminosity, of course)

So, a shorter answer to Dave's question.
1) There is no real Luminosity. There are various depending basically on what you're going to use it for (TV, monitors, printing, Photoshop / JPEG).
2) Luminosity (R*0.30 + G*0.59 + B*0.11) is more natural to Photoshop. Easy. So why fight it?

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Old 04-10-2005, 02:58 PM
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Okay, Curves and Saturation.

If you are not sure how Curves work, read the link previously posted.

Photoshop's flavor of Saturation is:
Max(RGB) - Min(RGB).
That is, find the lowest value amoung R, G, and B.
Find the highest value amoung R, G, and B.
Take the difference and you have Saturation.

*There are other flavors of Saturation, but PS's flavor is what we'll be dealing with.

When dealing with Sat, it's not really necesarry to think in terms of RGB. Rather, you have to twist your brain around a bit and learn to think in Max(RGB) and Min(RGB). Might be hard at first, but learning to think like that is good.

When you understand Curves and Sat, it's not that hard to put them together when dealing with things RGB.

See attachment.

On the left is a crappy drawing of an S curve and what happens to sat when Min and Max are centered around the middle of the S curve. Min is lowered on the lower half of the curve. Max is raised on the upper half of the curve. The net result is an increase in saturation for those values - the difference becomes greater.

On the right is what happens to different set of Min and Max on a curve. In this example, Min is raised and Max is raised. However, Min is raised more than Max. The net result is lower saturation.

Now, you could think of the second curve as the upper half of the first curve. This means that the higher Min Max values on the S curve will actually have lowered saturation. Oddly enough, the Lum will be raised and have the same effect of lowering saturation at the extremes. Coincidence? I think not! We just ran in a circle with and S curve. Heh.

If you Curve RGB, chances are very good that you will be lowering/raising Sat in various places. And that's not getting into how sat is affected when you do curving in seperate RGB channels. If you take the time to understand it, you can avoid it, negate it, or even take advantage of it. Or know when not to worry about it.

That's RGB, S curve, and Saturation in a very quick nutshell.
More coming about tossing Lum into the mix and the circle we ran around in.
Attached Images
File Type: gif scurve1.gif (8.5 KB, 27 views)

Last edited by Stroker; 04-10-2005 at 03:22 PM.
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Old 04-10-2005, 08:55 PM
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Min + n and Max + n
Will change RGB, but not Sat.
Same for Min - n and Max - n.

As long as the difference stays the same, Sat will stay the same.

Is there a way of doing this, and thus boosting contrast without affecting Sat? Absolutely. One of my favorite ways is Linear Light.

Are you familiar with colour cast blasting? One technique goes like this:
1. Pick the colour that you want to blast.
2. New Layer and fill.
3. Invert.
4. Set to Colour blending mode.
5. Reduce opacity to taste.

That is a good technique, but reduces Sat across the whole spectrum. That reduction in Sat has to do with the Hue <> RGB paradigm and reduced opacity.

If you think about it, you can do a variation on that technique with Linear Light that will reduce Sat for the given colour, but will actually boost contrast at the opposite hue. Neat, eh?

Another way of boosting contrast while leaving Sat alone is through Luminosity. This will also move Min and Max around by the same values and essentially leave Sat alone.

However - and this is the part that really cool - Luminosity will actually limit and modify Saturation if need be.


See attachment.
*I made it in a hurry and didn't bother with some particulars. As such, some aspects are wrong. However, should be good enough to get the point across.

On the left is a slice of HSL.
The vertical center is Lightness and has Sat=0.
On the outides are yellow and blue with Sat=255/100%.

That little diagram on the left tells us quite a bit about how Lightness limits Saturation.

For example, Sat can be 255/100% *only* when Lightness = 128/50%.
Anything above or below Lightness=128/50% will result in a lesser maximum of Saturation.

When Lightness = 191/75%, Sat has a maximum of 128/50%.
When Lightness = 64/25%, Sat has a maximum of 128/50%.

That's SL in HSL in a nutshell.

What about Photoshop's priority method of Luminosity? That's a bit tricky.
Lum = R*0.30 + G*0.59 + B*0.11
(Getting tired of that yet?)
Since Lum weights the RGB channels differently, the Lum is greatly affected by Hue. This means that Sat is limited by Lum differently depending on Hue.

Lum of pure yellow = 227
Lum of pure blue = 28
* Notice 227 + 28 = 255
* Coincidence? I think not!

Using 227 and 28, a similiar slice for Photoshop's space can be constructed. In the attachment, this is that funky parallelogram on the right.

In a similiar vein, this means:
Yellow can only have Sat=255/100% when Lum = 227/89%.
And so on.

This same idea can also be seen in Lab space. Even though Lab doesn't have Sat persay, it can easily be derived and it is limited by L in the same manner.

Boost contrast while keeping saturation?
Luminosity is your best bet - even if it does limit saturation in funny ways at the top and bottom.

There is still so much more to this.
All of these bits-n-pieces can be put back together is so many cool ways.
Even though RGB and HS/Lum are a far cry from each other, you can even mix them together to do some amazing acrobatics. Some of you already do.
I love this stuff.
Attached Images
File Type: jpg lumlimitssat.jpg (12.9 KB, 11 views)

Last edited by Stroker; 04-10-2005 at 09:19 PM.
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Old 04-10-2005, 11:53 PM
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While there is difference, it's negligable when it comes to contrast (negligable when it comes to a lot of things, but it might matter to a number freak).
Originally Posted by Ro
Luminosity (R*0.30 + G*0.59 + B*0.11) is more natural to Photoshop. Easy. So why fight it?
Here's why. Heron's crest. Image 1: Original Image 2: HighPass 10 pixel LAB Lightness Image 3: HighPass 10 pixel RGB Luminosity.

Most images I have changed are perhaps not as dramatic as this, sometimes impossible to see the difference but I personally cannot recall ever having an RGB Luminosity correction turn out "better" than a LAB luminance correction. If you can provide a sample I'd love to see it. It does exist somewhere, I'm sure. These esoteric discussions are great and very interesting but at some point you have to show why your ramblings result in a better image. I'm open to see what you can produce guys.

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Old 04-11-2005, 06:10 AM
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My flow is Lum for various reasons. Ro's flow is Lum for his reasons.
If you dig Lightness in Lab, go for it.
Personal preference and all that jazz.

In the example you posted, the difference is negligable to me. That's fine.
Dramatic to you and perhaps others. That's fine, too.

I think the major difference between Lum and Light is largely gamma.
Gets into the hardware aspect of Cie xyz.
Theoretically, you should be able to get one to match the other with small % error with the mid-tone slider in Levels.

Whichever you prefer.
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