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When do you use a "Difference Mask'?

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  #11  
Old 02-03-2007, 05:40 PM
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Re: When do you use a "Difference Mask'?

Quote:
Originally Posted by albatrosss
Thank you for the timely responses. I really don't want to make a mountain out of a molehill but I just wanted to see if I could find another tool for making difficult extractions. Quite frankly this seems to simply add an additional step or two and I wanted to ascertain what, if anything, that I was missing by not using this technique.

I don't see any mountains around here! We're always interested in learning new ways of doing things, and it's great that you're searching for a better way of doing things.

So, how would YOU select the girl and take her out of the one picture and put her in the other (without "adding the additional step")?
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  #12  
Old 02-03-2007, 11:54 PM
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Re: When do you use a "Difference Mask'?

Extract worked beautifully and in a fraction of the time.

As I stated earlier I just wanted to be sure that I was not shunting aside a technique that would be valuable at a later time. From what I gathered from your responses, this technique is not the most popular approach on this subject.

Once again, thanks to all, for the timely and thoughtful responses.
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  #13  
Old 02-04-2007, 12:47 AM
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Re: When do you use a "Difference Mask'?

albatross,

glad you found what you needed.

and just to carry this a bit forward, and i'm not directing this at albatross now, luminosity masks can be very useful, especially if you have a difficult extraction you're contemplating. with a luminosity mask you may not need to extract at all and end up doing a better job than if you had extracted.

masking is more or less non-destructive. extractions can be very destructive. detail is often lost. by masking, all of your detail remains intact.

by adding blend modes like difference and luminance and others to the mask, you can enhance the area that needs treatment even further and leave the other areas alone. and dont forget that you may need to invert the mask, depending on how you're tackling the job.
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Old 02-04-2007, 09:01 AM
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Re: When do you use a "Difference Mask'?

Thanks Kraellin for the explanation.

An older thread on Retouch Pro explains it beautifully.

http://www.retouchpro.com/tutorials/lum-mask-sepia.html

I'm not sure of how to use this new found information but I am now really curious about its applications. Will continue to see if I can understand when I should use it in place of, or as a supplement to, other techniques.

Never expected this to go this far but I guess it's an area that many of us have never taken the time to explore.
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Old 02-04-2007, 09:33 AM
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Re: When do you use a "Difference Mask'?

albatross,

you're welcome.

as to when and where to use it, i can give you one quick example, landscapes. you'll often have a fairly bright sky and a darker land area. and let's also say the land area has a number of trees and shrubs where the bright sky is poking through, making extractions difficult. and let's say also that you ONLY want to treat the land somehow, but not the sky. this is a good place for a luminosity mask. the mask is based on the luminance of the overall image. it is created based on the lights and darks (and all shades inbetween) of the image itself. you can then further enhance the differences in the mask to get even more contrast, thus giving you better separation between the bright sky and the darker land and making whatever you're doing to the image that much easier and clearly defined. and, you never have to move or extract any pixels to work on separately.

here's a thread where i used a luminance mask on an image: luminance mask
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  #16  
Old 02-04-2007, 12:32 PM
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Re: When do you use a "Difference Mask'?

Quote:
Originally Posted by albatrosss
Extract worked beautifully and in a fraction of the time.

As I stated earlier I just wanted to be sure that I was not shunting aside a technique that would be valuable at a later time. From what I gathered from your responses, this technique is not the most popular approach on this subject.

Once again, thanks to all, for the timely and thoughtful responses.
Just want to suggest that you not "shunt aside" any technique -- the Extract tool will work on parts of many images, but learning how to create your own selections and masks will be a VERY important skill as you continue to work with Photoshop. Creating selections and masks from the image's own channels will often give you the best answer -- each image will be different, but we can learn to look at the channels and see which ones will help us create our selections. Often there will be a need to combine information from more than one channel, and you will learn to use "Calculations" and "Apply Image" to add information from one channel to that of another to help you create a particular mask so that you can perform a function on a specially selected portion of the image. This is not something you need to know now, but it's good to know NOW that there are many more ways of making selections than the tools palette will perform. Be ready for more fun as you learn many new skills, and you will be the one who decides which to use when.

There are chapters in many books that explain different techniques; you will know when you are ready to start tackling them. Just don't close any doors on any of them -- there may just be one image in your future where a particular method will work best for you...
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  #17  
Old 02-04-2007, 01:25 PM
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Thumbs down Re: When do you use a "Difference Mask'?

Kraellin,

That is what I was looking for..............a practical use for the difference mask. I tried it and it works nicely with some old images I had. I wonder however, if it's any more efficient, faster, better etc. than the more conventional masking.

In any event I am pleased that I see a practical application and I am sure that I will find use for it in the future.

Thanks again to all of you. Each day I learn a bit more and each day I learn more of how little I know. Photoshop is certainly a very humbling program.
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  #18  
Old 02-04-2007, 02:00 PM
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Re: When do you use a "Difference Mask'?

CJSwartz,

Interesting that you should say that since I was just reading McClelland's chapter dealing with the Calculations command.

Thanks for the input.
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  #19  
Old 02-04-2007, 03:44 PM
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Re: When do you use a "Difference Mask'?

Quote:
Originally Posted by albatrosss
CJSwartz,

Interesting that you should say that since I was just reading McClelland's chapter dealing with the Calculations command.
Thanks for the input.
Well, you're ahead of me there! I haven't read his chapter. I've read other folks explanations, and will keep reading and practicing because I understand how to do Calculations, but don't yet know when to do it or how to do it for a particular image situation.
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Old 02-04-2007, 11:53 PM
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Smile Re: When do you use a "Difference Mask'?

Just completed the section dealing with Calculations. The results are just beautiful. I just followed his instructions step by step. Now I have to go back (not tonight obviously) and see if I can see the logic in what he has done.

As usual in Photoshop, it takes a while to understand the technique. This one seems to be intuitive rather than logical but will not know until I try it a few times.

I'm glad that I asked my original question which has taken me to this point.

Thanks again to all of you for your input.

George
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