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how to separate colour and luminance from RGB

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Old 06-09-2011, 03:31 PM
Tony W's Avatar
Tony W Tony W is offline
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Re: how to separate colour and luminance from RGB

Hi John,
Thanks for the examples and your explanation. From my own limited experiments I have not experienced any banding converting from RGB to LAB and back again to RGB. Therefore I am having difficulty accepting that this is a real problem.

I have seen banding issues when I foolishly decided to edit 8 bit jpegs in ProPhoto .

I am not surprised though that your examples of a computer generated gradient exhibits problems - although I am at a loss to explain it technically, I accept that this can happen and therefore such images should only be made in the final output space RGB or CMYK.

So I have to say that so far as I have not experienced issues converting to LAB and back and the fact that Dan Margulis experiments seem to back this up, I am yet to be convinced that there is a real problem that would seriously degrade many images.
Of course I reserve the right to change my mind in the future should my new experiences prove these thoughts flawed
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Old 06-09-2011, 08:11 PM
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John Wheeler John Wheeler is offline
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Re: how to separate colour and luminance from RGB

Hi Tony
Your approach/thinking is not flawed. Independent of technical specs, actual experience is much more powerful. The effect I am talking about is not large and usually only comes up when you have very low noise images (noise in the image masks the effect).

What I did to provide a more real life example is to show a sunset with a very low noise background where the color shifts from yellow sun to darker oranges as you move out. This is an RGB image.
Sunset at 800px.jpg

What I did was duplicate this layer; made duplicate Layer a Smart Object; went into the Smart Object and changed it to Lab mode; Saved Smart Object; Clicked back to original Layer Stack; Smart Object goes back to RGB mode in original stack; Mask to reveal only right hand side of Smart Object so left hand side is original RGB image. Here is the image at 200%. The banding is subtle yet is there - higher for going to Lab and back:

Screen shot 2011-06-09 at 6.41.13 PM.png

Depending on monitor this can be hard to see so increasing the contrast and brightness the comparison becomes more clear with smoother step gradient on the left original image and not as even for going to Lab and Back on the right.

Screen shot 2011-06-09 at 6.41.52 PM.png

Now if the image already had some noise or you add some noise/dither afterwards, this does not become noticeable.

What is interesting you mentioned editing an 8 bit sRGB image in ProPhoto. Turns out, taking this 8 bit sRGB image to Lab and back has a very similar effect. All of the colors in most images only occupy a very small fraction of the A and B Lab channels (the A and B channel are both represented by 8 bit values). Therefore the color component/channels that were spread out over a larger number of RGB bits are compressed down into a smaller number of A and B channel bits (check you typical image and see what range of A and B are actually used - it is relatively small compare to the entire Lab range). When you go from Lab back to sRGB that compression comes back as a very small amount of color posterization/banding.

So even though it is subtle, so are the differences in many things we do in our the workflow. All added up, each of those subtle differences can make a difference. So I just use the general rule of thumb to stay in 16 bit until the very end to avoid such issues. Hope that brings the discussion back from the technical (I have a tendency to do that ) to a more real life example.

Image leveraged from stock exchange - modified for use in this example
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Old 06-10-2011, 12:57 AM
ISO ISO is offline
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Re: how to separate colour and luminance from RGB

Hi John

Thanks for sharing your way of doing it. Now next thing is I'm going to test this setup for the examples given in the lobster documentation. If the results are identical then we are successful.

If there is deviation I will report back.


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Old 06-10-2011, 03:43 AM
Tony W's Avatar
Tony W Tony W is offline
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Re: how to separate colour and luminance from RGB

John, many thanks for going to the trouble of finding an example of the effect.

For me I think your early statement "Independent of technical specs, actual experience is much more powerful" really nailed it! Thinking back to where I have used LAB there are generally two scenarios a. working with my own digital files from camera or scanner - always in 16 bit and b. forcing colour apart in poor images for restoration. So rarely do I work in 8 bit unless I have to.

Your images clearly indicate the potential problem and one that I experienced when I decided to edit 8 bit in ProPhoto albeit with blue sky. So it does look like 8 bit RGB to LAB moves and 8 bit editing in 16 bit work space are open to this type of effect and therefore should generally be avoided.

As you say the problem may be small but further work can/will compound the effect
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Old 06-21-2011, 01:08 PM
davidlicious davidlicious is offline
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Re: how to separate colour and luminance from RGB

I feel like I'm jumping into the color & luminance separation topic a little too late.

I understand how to do the separation but my question is... why? Why would you do this and how would you use it?

Please enlighten?

Thanks... David
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Old 06-21-2011, 02:08 PM
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Chain Chain is offline
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Re: how to separate colour and luminance from RGB

Using the Color/Luminosity blend modes, or separating the image like discussed, you can:

* Edit the Luminosity ("brightness") without affecting the color (hue+saturation).
* Edit the Color without changing the Luminosity.

This can be useful on many occasions when retouching an image. One example is if you want to change the color of an object without making it brighter or darker, or if you wanted to even out skin color without affecting the Luminosity/brightness of the skin.
There are many other applications as well (only limited by the imagination of the retoucher I guess).

Ps: I recommend learning how to use the blending modes first before you consider if separating the image (or even converting to Lab) would provide you with a more useful workflow.
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