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Color dodge eliminate yellowing. Why?

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  #1  
Old 03-08-2006, 02:48 AM
dario dario is offline
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Color dodge eliminates yellowing. Why?

Hi, it's me again with my comics restauration hobby.
I just discovered a nice trick to eliminate automatically the yellowing of the old pages, but i do not understand why it works. Can you explain it ?
Here is the trick
When you have a yellowed comic page (first image) create a new layer filled with the color of the paper (that supposedly was almost white at the origin).
Now invert the layer, and use Color Dodge as blending mode. The result is a restored page (second image)

I would like to understand why it works. What EXACTLY the color dodge blending mode does? I really would like to understand the math behind it (i'm good with math so don't worry )

thank Dario
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File Type: jpg colordodge2.jpg (94.5 KB, 72 views)

Last edited by dario; 03-08-2006 at 03:43 AM.
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Old 03-08-2006, 05:29 AM
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byRo byRo is offline
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Dario,

If that was Linear Dodge, and not Color Dodge, the formula would have been very simple. Linear Dodge is the simplest blend of all - just add the values of each channel.

In your "trick", sampling the (supposedly) white border gives a reading of around 251/241/184.
Inverting that, gives you the difference from 255 in each channel - or 004/014/071.
Adding that (if it were Linear Dodge) (not surprisingly) gets you back to 255 in each channel. The rest of the image will be brighter (because we're adding) and bluer, or less yellow (because the blue value is higher).

There is a very much simpler techique that gives the same result as the Color Dodge trick. Make a levels adjustment layer, select the White eye-dropper and click on the (supposedly) white border.
This gives exactly the same result as the Color Dodge. (and after some exploration should explain the math(s) behind it).

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Old 03-08-2006, 08:48 AM
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mistermonday mistermonday is offline
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Color Dodge

Dario, a good explanation by Ro.
ColorDodge brightens the backdrop color to reflect the source color. Painting with black produces no changes.
FYI, the math formula for Color Dodge is as follows:

B(cb,cs) = { Min (1, cb/(1-cs)) if cs<1
1 if cs=1

Where cb is the background color and cs is the source color
Regards,
Murray
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Old 03-08-2006, 10:44 AM
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byRo byRo is offline
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Thanks Murray.

I searched (and searched) and didn't discover that! Where did you get that from? Must be interesting reading (for Geeks ).

Re-writing for (normal) channel values gets...

Min(255, b * 255/(255-a))

(adding or subtracting some 1's here and there because channel values start at 0)


By symmetry, I also got the Color Burn formula...

Max(0, 255*(1 - (255- b)/a)))
(0 for a=0)

Now I am one happy Geek!

Murray, now where did you find that?

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Old 03-08-2006, 11:22 AM
dario dario is offline
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Quote:
Originally Posted by mistermonday

B(cb,cs) = { Min (1, cb/(1-cs)) if cs<1
1 if cs=1
Thanks!
Do you have the formula for all the other blending modes too ?

Dario
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Old 03-08-2006, 11:32 AM
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Cameraken Cameraken is offline
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Here is a link.

Look under 'Burn Modes'

http://www.pegtop.net/delphi/blendmodes/

Ken
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Old 03-08-2006, 12:33 PM
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byRo byRo is offline
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Thanks Ken,

update for the link - http://www.pegtop.net/delphi/articles/blendmodes/

Seems that a bigger problem than understanding the formula is trying to describe it.......
Quote:
This mode is some kind of inverted multiply mode (the multiplication is replaced by a division of the "inverse").


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Old 03-08-2006, 12:41 PM
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Cameraken Cameraken is offline
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You are Welcome.

Here is another.

http://www.vanderlee.com/tut_fm_mixingmodes.html

Ken.
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Old 03-08-2006, 01:27 PM
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Flora Flora is offline
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Hi dario!

Welcome to RP!

For all the others....

...that's really IT! .... I've had it .... I'm going to pick up knitting as a hobby!!!!
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Old 03-08-2006, 02:18 PM
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Craig Walters Craig Walters is offline
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now flora, dont do that or we'd all have to take up knitting as well just to try and do as well as you do

craig
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