RetouchPRO

Go Back   RetouchPRO > Technique > RP Tutorials
Register Blogs FAQ Site Nav Search Today's Posts Mark Forums Read


RP Tutorials Discussion for tutorials published via our automated system, and about the tutorial publishing system itself.

Removing Color Cast By the Numbers Possible?

Reply
 
Thread Tools
  #1  
Old 09-27-2007, 09:59 AM
jojaspi jojaspi is offline
Junior Member
 
Join Date: Oct 2004
Location: Amsterdam
Posts: 4
Question Removing Color Cast By the Numbers Possible?

Several times I stumbled upon a tutorial that states that it is simple to remove color cast by the numbers (see for instance here on this forum).
The method in short: use treshold to determine darkest and lightest point; use 50% gray layer in difference blend mode to determine a 50 % grey point; use these points to set the three eyedroppers in a levels adjustment and voila: color cast gone!

But it never worked for me.
And I think it can't work like this because this method translates a 50 % gray to print as ... a 50 % grey! And precisely because of the color cast that point shouldn't print to 50% grey at all, it should print to the opposite of the (unknown) color cast!
I think you can't really do without a subjective decision as to which point should be neutral grey...

Anybody there with a brilliant method to prove me wrong ??

Actually I hope so, because I shurely would like to have a simple tool to make the decisions for me...
Reply With Quote top
  #2  
Old 09-27-2007, 10:19 AM
des151 des151 is offline
Senior Member
 
Join Date: Jun 2006
Posts: 222
Re: Removing Color Cast By the Numbers Possible?

Hi jojaspi,
This video tutorial will help you. www.Layersmagazine.com/color-correcting-images.html
Ray
Reply With Quote top
  #3  
Old 09-27-2007, 10:43 AM
Britsdad's Avatar
Britsdad Britsdad is offline
Senior Member
 
Join Date: May 2006
Location: South Yorkshire, England
Posts: 536
Re: Removing Color Cast By the Numbers Possible?

Hi jojaspi.....are you sure you're getting the tutorial right? it always works well for me and Dave Cross (one of the "photoshop guys") did a demonstration of it on Photoshop TV some time ago. Here is the same method from a different site, http://www.onlyphotoshop.com/photosh...adjustment.php ....Oh and welcome to RTP.
Regards....John
Reply With Quote top
  #4  
Old 09-27-2007, 04:36 PM
Charlene5 Charlene5 is offline
Member
 
Join Date: Jul 2007
Location: Deep in the Heart of Texas
Posts: 75
Re: Removing Color Cast By the Numbers Possible?

If we're talking color cast here I swear by a Katrin Eismann method. I've never found anything that works better or as fast. Make two duplicate layers of your image. In the first duplicate do a blur/average. Take your little eyedropper tool from the toolbox and click on the resulting blur mess in your photo. Go up to the next layer and open either a curves or levels adjustment. Using the middle eyedropper ind the dialog box - the set gray point one - click on the averaged color in your over in your tool box. The color cast will vanish instantly.
Reply With Quote top
  #5  
Old 09-28-2007, 03:19 AM
jojaspi jojaspi is offline
Junior Member
 
Join Date: Oct 2004
Location: Amsterdam
Posts: 4
Re: Removing Color Cast By the Numbers Possible?

Quote:
Originally Posted by Charlene5 View Post
If we're talking color cast here I swear by a Katrin Eismann method.
Charlene,

I will try this method, I can imagine this one works well and fast. Doesn't this in essention do the same as autocolor (with more flexibility because of the curves)?

John,

I think the flaw in my "proof" that the method can't work is that I focused on the Grey point only, without thinking about the effect of the black and white point.
The reason my try with this method in practice didn't work out very well lies - I guess - in the fact that my black and white point were BOTH blown out. In other words neither of those two points had a color cast!
I think the method only works if both the black point as the white point show signs of the color cast.

But still, the only thing a 'method by the numbers' can do, is make the image neutral.

Correcting a close-up of a red rose will - by whichever method - deliver disappointing results. Unless you like a grey rose of course.

So removing a color cast will always remain a subjective decision...
Reply With Quote top
  #6  
Old 09-28-2007, 06:23 AM
jojaspi jojaspi is offline
Junior Member
 
Join Date: Oct 2004
Location: Amsterdam
Posts: 4
Angry Re: No no no, this method MUST be wrong!!

Quote:
Originally Posted by jojaspi View Post
I think the flaw in my "proof" ...
No, No, NO!

Theoretically it CAN'T be right after all.

If we use the 50%grey-difference-blending method to determine the grey-point, we will map a point that is grey in the color-casted original to grey in the resulting print. That's not right.

Example: if we have a heavy blue cast, the "grey-point" we will find is a point in the image that's yellow in the reality; yellow-in-reality plus blue cast = neutrally grey in the image. Color-cast-correction should bring this back to yellow-in-the-print. Not to neutrally grey.

Do I make sense, or am I wrong??

Last edited by jojaspi; 09-28-2007 at 06:30 AM.
Reply With Quote top
  #7  
Old 09-28-2007, 07:50 AM
mistermonday's Avatar
mistermonday mistermonday is offline
Moderator
Patron
 
Join Date: Dec 2005
Posts: 3,028
Re: Removing Color Cast By the Numbers Possible?

The "gray" point is not necessarily equal to the neutral point and is not necessarily gray. In real images, things whose color are gray are often tinted by reflections from surrounding light sources, objects, and reflections. Moreover, it is rare that a cast is linear across the spectrum of highlight to shadow.
Examination of image 1 below clearly indicates a blue cast. If you create a gray difference layer and set the neutral points, you find that they are not yellow but actually blue as seen by the eye dropper samplers 3 and 4.
If you proceed to make a levels correction and just set the shadow and highlight (samplers 1 and 2), the image changes contrast a little but still sports a heavy blue cast. It isn't until you set the gray point 3 that the green of the trees returns. The water becomes gray-ish in the shadows and bluish where is has a lot of reflected sky on it.
Is this now correct? Well it looks more natural, although it is probably not perfect. The reality is that images which have heavy color casts have the relationships between the pixels of the various channels shifted non linearly. Three correction points in a Levels adjustment can not restore the entire balance. Finding the neutral point (at leastin luminosity) in conjunction with B&W point adjustment brings the image back into a range from which it can be fine tuned to what is pleasing to the eyes of the beholder.
Regards, Murray
Attached Images
File Type: jpg Blue Cast.jpg (98.5 KB, 55 views)
File Type: jpg Gray Difference.jpg (167.6 KB, 44 views)
File Type: jpg Blue Cast corrected.jpg (168.0 KB, 50 views)
Reply With Quote top
Reply

  RetouchPRO > Technique > RP Tutorials


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Practice eye-hand coordination - chase the numbers (Flash) CJ Swartz Salon 11 02-04-2007 02:32 PM
Help removing complex hair from crazy background renato17 Image Help 16 11-24-2006 10:30 PM
Problem Removing Canon 550 Flash From Eye Glasses gho64 Image Help 10 05-03-2006 05:23 AM
How to keep the definition of a person when removing blemishes Gerald McClaren Photo Restoration 1 12-08-2005 11:29 AM
removing harsh shadows RedAlert Image Help 30 09-05-2002 08:41 AM


All times are GMT -6. The time now is 04:11 AM.


Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
SEO by vBSEO ©2011, Crawlability, Inc.
Copyright © 2016 Doug Nelson. All Rights Reserved