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Color and luminance separation comparison results

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  #1  
Old 06-17-2011, 01:22 AM
ISO ISO is offline
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Color and luminance separation comparison results

Hello folks

With reference to the earlier thread...

http://www.retouchpro.com/forums/pho...nance-rgb.html

I have done comparison of method suggested by John with Lobster droplet from free gamma (www.freegamma.com/).

The method of John is working very well for me but when it comes to saturated yellow color images there is very slight luminosity variation. Lobster give near perfect results even for such type of images.

Here are results of screen shots..

1. Splitted image (John method) original layer off.
http://i187.photobucket.com/albums/x.../untitled2.jpg

2. Splitted image (John method) original layer ON. Note diff.
http://i187.photobucket.com/albums/x.../untitled3.jpg

3. Lobster splits original layer off
http://i187.photobucket.com/albums/x...s/untitled.jpg

4. Lobster splits original layer ON
http://i187.photobucket.com/albums/x.../untitled1.jpg

5. John method color layer
http://i187.photobucket.com/albums/x.../untitled4.jpg

6.Lobster generated color layer
http://i187.photobucket.com/albums/x.../untitled5.jpg

It seems Lobster droplet employ different method of splitting color from RGB images.

Any thoughts or suggestion are most welcome.

Regards,
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  #2  
Old 06-17-2011, 07:21 AM
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Der_W Der_W is offline
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Re: Color and luminance separation comparison resu

I just played with a test file that someone over at Luminous Landscape sent me and it seems like Lobster is blending the original layer back into their chromaticity layers with some kind of mask (I guess it's a chromaticity mask).

The results are therefore perfect, but not what was originally planned, because the separation gets lost.
You can try this yourself when creating a 50% gray layer above the chromaticity set and blending it in "Color" mode. If the result is anything but a 100% solid gray (no variation at all), then it's a mix of luminosity and color (which is the case in my test file).
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Old 06-17-2011, 07:45 AM
ISO ISO is offline
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Re: Color and luminance separation comparison resu

Hello Jonas

Things have become interesting and mysterious after your reply.

Thank you very much for your input.

I will try as per your guidelines.

Pl. note. Luminosity layer is the same what we achieved. I deleted lobster luma layer and placed our luma layer above colour and it works.

Regards,
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  #4  
Old 06-17-2011, 02:51 PM
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John Wheeler John Wheeler is offline
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Re: Color and luminance separation comparison resu

Hi Mukund

Jonas is correct.

I took a closer look at the differences going on between Lobster and the blends in PS. It was not quite what I thought before yet close.

Warning to others, this is more of a technical post of how PS blends vs Lobster blends work and their differences. If you don't need or like to know those differences, save yourself some time and skip this post. You've been warned

They both use the same definitions for Hue, Saturation, and Luminosity as per the PS blend modes:

Hue: Standard mathematical definition (Range 0 to 360 degrees)
Saturation: Max (R,G,B) - Min (R,G,B) (Range 0 to 1)
Luminosity = .3R + .59G + .11B (Range 0 to 1)

Note that in PS, Color is meant to mean combination of Hue and Saturation as per the definitions above. Also note that the above Blend definition of Saturation is different than the Saturation numbers you see in PS Color Pickers.

Unlike RGB where each color channel can independently have any value from 0 to 255, Hue, Saturation, and Luminosity as defined above have dependencies on their Range (e.g. for a given Hue, not all combinations of Luminosity and Saturation are possible).

In the PS blends of Hue, Sat, Color, Luminosity,, that means there has to be priority given to one of the channels when there is a conflict. In PS: Hue, and Luminosity take precedence over Saturation as defined above. E.G. Solid Yellow (R=255,G=255,B=0) Hue=60degrees, L=.89, Sat=1.

In extracting Color (Hue and Sat) in Photoshop with a gray layer at 50%, with priority given to Hue and Luminosity, Saturation must be reduced with a result of R=144,G=144,B=1 Hue=60 degrees, L=.5, Sat=.56.

Extracting the luminosity as indicated before works fine since it takes priority.

The downside the blending prioritization approach in PS is that once the Luminosity and Color (Hue + Sat) are extracted, since the Sat is not always maintained, if you combined the Luminosity and Color back together, you would not end up with the original image. That is why in the comparison Mukund provided when trying to combine the luminosity and color back together, the saturation was reduced.

In Lobster (I do not have a copy to verify), I believe that the key difference is the when extracting the Color/Chroma (Hue and Sat) priority is given to Hue and Saturation over Luminosity set at .5. This will create an extraction with the same Hue and Saturation as the original yet the result will not be a constant Luminosity. The advantage of this approach is that you can combine the extracted Luminosity and the Lobster extracted Color/Chroma and end up with the same original image.

The extraction as per Lobster can also be done in PS yet with quite a few more pushups than what I had provided before. This could all be automated in a PS action. I question the value though if one already has Lobster software in their possession.

It was an interesting puzzle and I learned a little more about PS blends in the process so thanks for posting the thread.

Sorry for the long technical post
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Old 06-17-2011, 05:50 PM
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Der_W Der_W is offline
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Re: Color and luminance separation comparison resu

Out of pure technical curiosity (I haven't found any of my typical images on which the easy workflow with the luminosity at 0.5 wouldn't work perfectly) but how would you go about recreating what Lobster does in Photoshop?

I'm actually a little stuck in my own methods so I'm probably running against a wall trying to replicate their result the whole time and so it'd be great if you could give me a hint :-).
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Old 06-17-2011, 07:23 PM
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John Wheeler John Wheeler is offline
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Re: Color and luminance separation comparison resu

Hi Jonas

(and all others not interested in technical details save some time and skip the post)

I too initially thought that separating out the luminosity and then the color at gray/L at 50% could be recombined quite successfully. I was aware that the Saturation as defined in previous post was at risk yet also incorrectly thought that pulling out Color at 50% would cover that. For images with lower saturation it is not as obvious. With colors with high saturation it is quite a problem. The following example shows this:

- I started with a fully saturated color wheel
- Extracted Color at gray/L at 50%
- Extracted Luminosity
- Combined Luminosity (lower layer) with Color (upper layer) with upper layer Blend set to Color
- Grouped passthrough and masked to show result on top portion of image and original in bottom portion of image.

You can see that for many colors at max saturation the color appear similar while others there is a more significant shift. In reality, the saturation is lower for most all of the result colors. Again this is using a worst case starting point of only fully saturated color wheel yet it demonstrates quite well the flaw in my original thinking that Color extracted at gray/L=50% was OK for recombining back with the Luminosity to get the Original Image.

Screen shot 2011-06-17 at 5.30.42 PM.jpg

The efficacy of accurately reproducing the Lobster extracted colors I believe is in question. The reason being is that it (as well as PS) is based on a definition of Saturation which IMO was based on computational efficiency rather than meaningfulness from a color model. You may see why below.

I do not know any specifics about Lobster extraction yet I would guess it follows something like my approach outlined below per your request/question.

First let me give a definition based on RGB values with
Cmax being Max (R,G,B) Range 0 to 1
Cmin being Min (R, G, B) Range 0 to 1
Cmid being the remaining color channel other than Cmax and Cmin Range 0 to 1

So we have 3 color values in the range of 0 to 1 typically all 3 somewhere in the middle of that range for most images.

Sat = Cmax-Cmin

I have not double checked my math yet it appears that if you want to shift the Luminosity by Delta L yet hold the Hue and the Saturation, all you have to do is subtract that Delta L value equally from all the Color Channels. This is computationally efficient and only works when:
- If delta L is postitive that Delta L <= Cmin
- If delta L is negative that |Delta L| <= 1-Cmax

Visually, what this means is that the relative spacing among the 3 color values remains fixed and you slide the group down or up with a limit that Cmin can't go below 0 and Cmax can't go above 1. This keeps the the Blend definition of Sat whole as long as this is maintained.

If the Delta L is larger then these limits then the Sat is not held constant while the Hue is held.

For the PS Luminosity and Color to be properly combined, the Max Delta L allowed for a pixel should not exceed the limits above and therefore Saturation is given priority and Luminosity is sacrificed.

-We already have what starting L value is. Delta L is just is a subtracting .5 when L is above .5 and is subtract from .5 when less than 1.
- Take the min with Cmin or 1-Cmax respectively and then use Subtract Blend or Linear Dodge (Add) respectively.

I have not double checked the above approach nor looked form more layer efficient ways to proceed (I am sure there would be) yet have tested the concept and it seems to work. This should give you an idea of what would be needed to get you going. Like I said, the efficacy of doing this is limited by the value and reason behind PS definition of Saturation for these Blend modes. As you can see from my above explanation, it appears to be more computationally easy than necessarily color model meaningful.

I don't know if this is the Lobster approach so am guessing based on the images that they and Mukund have shared.

Hope this was of some use to you.
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  #7  
Old 06-18-2011, 12:58 AM
ISO ISO is offline
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Re: Color and luminance separation comparison resu

Thanks John for the detail reply.

You might want to check your approach with actual Lobster droplet. Following are links for free evaluation copy.

For Mac

http://www.freegamma.com/LobsterDemo.dmg

For win

http://www.freegamma.com/LobsterDemoInstaller.zip

It works with CS3 & CS4
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Old 06-18-2011, 10:10 AM
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Der_W Der_W is offline
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Re: Color and luminance separation comparison resu

Hi John,

thank you for your valuable input :-)!
It turns out that I tried something quite similar (subtracting lower/higher luminosity values to keep the channels from clipping and combining the results), but your way seems more efficient than my earlier tries :-)!

I'll try your method and see if it works for me as well!
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Old 06-18-2011, 09:29 PM
Flashtones Flashtones is offline
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Re: Color and luminance separation comparison resu

Can someone explain how this separation would be used in day to day retouching?

Best I got from a quick look at the Lobster site is it's a better way of making luminosity adjustments. But what else?
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Old 06-18-2011, 10:07 PM
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John Wheeler John Wheeler is offline
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Re: Color and luminance separation comparison resu

Good question Flashtone

I don't know all that Lobster has to offer vs the normal blend options of Color, Hue, Sat, and Luminosity.

Sometimes the term "separating out" the Luminosity from Color (Hue and Sat) makes it sound more complicated that it really is. Here are a couple examples

If I have an image and put a Levels (or Curves) adjustment Layer above it, if I use the middle slider on Levels on all 3 RGB color channels simultaneously (or non linear curve with Curves), one will introduce a Hue shift for some parts of the image. By setting the blending mode to Luminosity on the Adj Layer, the Hue of the original image is maintained, the adjustment Layer only changes Luminosity (no Hue shift). Note that the blend attempts to keep the Sat of the original image yet will give priority to the Luminosity shift of the Adj Layer

Another simple example same as above yet I use a Hue/Sat adjustment Layer. If I want to shift the colors yet maintain the Luminosity of the original image, I set the Blend mode of the Adj Layer to Color. This will respect the Hue shift from the Adj Layer yet maintain the Luminosity of the original image. Note that this blend tries to use the Sat value from the Adj Layer yet gives priority to the Luminosity of the original image.

So in both of the above cases, the Luminosity and Color are effectively split and modified (or not) separately.

There are many other options/techniques. I like to think of it as a complete additional color mode (Luminosity, Hue, Sat) sitting right inside RGB color mode (except you don't have the downside of switching color spaces and associated issues e.g. possibly having to flatten layers etc etc)

Hope that makes sense and is more helpful than what can come across as technobabble of my other posts
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