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  #101  
Old 03-31-2005, 03:13 PM
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realaqu realaqu is offline
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Hi, Ro,

That is a great trick to repair the red chanel. but I still have a question here, I cropped part of your picture and read the histograms in photoshop. seems the curve still not that good. especially the blue chanel. you can take a look on my work. the RGB curves are smooth, almost start or end at the same positon. which means the exposure of that picture get corrected. maybe you need some more work on blue chanel as well.
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File Type: jpg Picture-20001.jpg (80.4 KB, 10 views)

Last edited by realaqu; 03-31-2005 at 03:39 PM.
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  #102  
Old 03-31-2005, 05:48 PM
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byRo byRo is offline
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Quote:
Originally Posted by realaqu
..the RGB curves are smooth, almost start or end at the same positon. which means the exposure of that picture get corrected...
I think you are tending towards an auto-level adjustment <ctrl><L>, this will indeed improve the exposure but at the cost of colour. If the R / G / B histograms all start and end at the same position then things are getting pretty grey. It is perfectly normal, with skin tones, for the Red to be concentrated at high levels, green in the middle and blue only at low levels.


As it is pretty common to have to deal with blown Red channels, and as the method I posted above looks pretty complicated - I made a action toolkit to make this easy (well easier):
1) Channels to layers - makes 3 layers, Red / Green / Blue;
2) Combine channels - recombines the channel information;
3) Greyscale channels - makes the 3 layers greyscale;
4) Red fix - does all the work.
Run Channels to layers, then Red fix.
(and then curves to get the colours right)

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File Type: zip byRo Channel Stuff.zip (1.0 KB, 8 views)
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  #103  
Old 03-31-2005, 07:26 PM
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realaqu realaqu is offline
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Hi, Ro
I don't meant the RGB curves have to be at the same starting or ending point. but for a good exposure, they have to cover the film dynamic range as wide as they could. that is the reason I said they almost start or end at the same position. I got this from

http://luminous-landscape.com/tutori...stograms.shtml

from your histograms, the area under the green curve are the total amout of geen lights of this shot. which means the picture is a kind of greenish. I worked on a similar case before, the photo is from another forum for discussion usage.
you might wanna have a try

http://www2.photoshopcn.com/attach/m...GgI7Z9ymXk.jpg

I am not sure if there is a copyright issue because somebody asked for help on that forum. so I didn't attach the original picture. but I uploaded my own work on that.
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File Type: jpg carafter.jpg (65.6 KB, 14 views)

Last edited by realaqu; 03-31-2005 at 07:45 PM.
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  #104  
Old 03-31-2005, 09:52 PM
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Duv Duv is offline
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Quote:
Originally Posted by byRo
Dave,
Luminosity is made by mixing the three channels together. The weights are Green 59%, Red 30%, Blue 11%.
I'm sure I'm missing something but this is precisely why I didn't use the green channel. By Blending in the green with the red and/or blue channel, you have weakened the green channel. How can it be otherwise. With your corrected version, LAB brightness (no mixing here I think) is now stronger than green channel so it might make sense to blend LAB back in to get rid of the apparent red cast to your image and put some strength back in the green channel. Also, when I blend the LAB "brightness not Luminosity" channel into the red channel, I think i get a better result. Also, here I paraphrase someone else
"Lets call RGB, LAB a color models as color spaces are used in the context of color management.

Sorry to spoil the party: RGB values only do not define any color. WOW?? RGB inside a color space does define an absolute color. Same RGB can mean very different colors in different color spaces. Any LAB color always defines an absolute color.

To me RGB is the least intuitive color model. The best for my taste is HSB as used in the color picker. But the mathematics seem to be against HSB and LAB models seem to be better.

If you think in terms of colors HSB rules:

1. Brightness (mostly responsible for detail in our vision system)
2. Hue (color base tone)
3. Saturation

In many tools and in layers it is good to think in brightness (or Luminosity not really the same but for me it is) and color (Hue, Sat).
Uwe

____________________________
Uwe Steinmueller
Editor Digital Outback Photo

Comments please Roland. I'm not as eloquent as you but still need to understand.

Cheers
Dave
Attached Images
File Type: png OriginalRoDave.png (93.4 KB, 12 views)
File Type: png GreenChannels.png (13.8 KB, 5 views)
File Type: png RedChannels.png (12.9 KB, 4 views)

Last edited by Duv; 04-01-2005 at 08:51 AM.
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  #105  
Old 04-01-2005, 10:44 AM
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byRo byRo is offline
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Quote:
Originally Posted by Duv
I'm sure I'm missing something but this is precisely why I didn't use the green channel. By Blending in the green with the red and/or blue channel, you have weakened the green channel.
Dave, couldn't follow you here. Seems that this is what I was saying. The Luminosity will have some of the distortion from the Red channel and some of the noise from the Blue channel mixed in - and that's why I'd use the Green channel by itself.
Quote:
Originally Posted by Duv
To me RGB is the least intuitive color model. The best for my taste is HSB as used in the color picker. But the mathematics seem to be against HSB and LAB models seem to be better.

If you think in terms of colors HSB rules:

1. Brightness (mostly responsible for detail in our vision system)
2. Hue (color base tone)
3. Saturation

In many tools and in layers it is good to think in brightness (or Luminosity not really the same but for me it is) and color (Hue, Sat).
Uwe

____________________________
Uwe Steinmueller
Editor Digital Outback Photo
Fully ageed! I was lucky enough to cut my Retouching baby teeth on PSP and not PS. In PSP the HSB is (or was, anyway) a front-line option and not something relegated to the off-line "Goodies" folder of the CD. I will always prefer treating Luminosity and Colour separately.
Quote:
Originally Posted by Duv
Comments please Roland. I'm not as eloquent as you but still need to understand.
Dave, it's content and not the packaging that counts! ah... and don't follow me - I'm not sure where I'm going

Any how, the point that I was trying to make os that once you've fixed the blown-out Red channel the rest of the correction is pretty straightforward.
I have a little collection of skin Gradient Maps that I use for corrections / colourizing - here are three examples of correction: Blushing red, normal(?), Latin yellow.



(PS the numbers on the attachments are Hue and Saturation at 60% Luminosity)
Attached Images
File Type: jpg Lene_byRo_13-37.jpg (97.4 KB, 6 views)
File Type: jpg Lene_byRo_19-41.jpg (97.7 KB, 10 views)
File Type: jpg Lene_byRo_26-46.jpg (95.1 KB, 10 views)
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  #106  
Old 04-01-2005, 08:22 PM
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Duv Duv is offline
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Thanks for your reply Roland. It's because of my communication skills that I don't do tutorials. On the other hand your's are excellent. I think the only point I was trying to make, and perhaps incorrectly, is that the LAB Brightness channel might be better in this instance over the Green Channel. Correct me if I'm wrong but I think Luminosity and Brightness are different..Luminosity as you suggest a blend of colors and Brightness which has no such values. There have been several references to the Red Channel being blown out. Certainly 255 is not a good thing but many of the images I have produced show Red at 235 to 245..not a long way from 255. The brightness channel in LAB does look weaker than the green channel but I still think it gives better results when you blend back into Red over the Green channel without the color shifts.
I guess what I'm saying is that I don't see Realaqu's and your preference in using Green Channel blending over LAB brightness, but I don't want to flog the poor old horse to death. You did get good results..even though I think mine is better..HA!

I hope one of these days we can get into a discussion about the value of working in different Synthetic Color Spaces to achieve brightness and color corrections.

Cheers
Dave
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  #107  
Old 04-01-2005, 09:00 PM
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Stroker Stroker is offline
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PS's Lum = R*0.30 + G*0.59 + B*0.11
HSB:Brightness = Max(R,G,B)
HSL:Lightness = (Max(R,G,B)+Min(R,G,B))/2

To get L in Lab, you have to convert RGB > XYZ > Lab. For just L, only Y is needed. However, to get Y, you need R, G, and B. So, RGB > Y > L. I think that's right. The weights are different depending on various things.

Something like that for the curious.

I just finished a plug-in and am putting it through some paces. So far, it seems to be doing extremely well for blasting colour cast and related things. Not sure how intuitive it will be for others, though. Few more touches and I'll let ya'll take a whack at it.

Also got a few other plugs up my sleave that could be helpful with these things.

Hopefully this weekend I'll have the time for another marathon and get things ready.
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  #108  
Old 04-04-2005, 11:52 PM
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realaqu realaqu is offline
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Hi, this is realaqu

I just learned a new way to handle a case like this. maybe pretty old to all of you. just change the image mode to LAB, and create a levels adjustment layer. play with A chanel. and make another curve adjustment layer for fine tune. the result seems not bad as well.
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  #109  
Old 05-11-2005, 07:44 PM
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Nanls Nanls is offline
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Red face may be too late... just joined

may be too late... just joined
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  #110  
Old 05-11-2005, 07:51 PM
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Nanls Nanls is offline
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having trouble uploading

having trouble uploading

Last edited by Nanls; 05-11-2005 at 07:52 PM. Reason: upload problems
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