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When do you use a "Difference Mask'?

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  #1  
Old 02-03-2007, 08:48 AM
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albatrosss albatrosss is offline
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When do you use a "Difference Mask'?

Yesterday I did a tutorial from Katrin Eismann's "Restoring and Retouching". It was the first time I had attempted a Difference Mask. When I had completed the tutorial I wondered why that method was recommended since an additional selection was required using another selection tool.

My questions is: When should one use a Difference Mask? What are the circumstances when this method would be superior to another masking method? I tried "googling" this question but the responses were not too helpful.

Thank you.
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Old 02-03-2007, 09:20 AM
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Re: When do you use a "Difference Mask'?

Albatross, if you could point out exactly where in her book you are referring to, it would be easier to understand what you mean by a difference mask.
Regards, Murray
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Old 02-03-2007, 09:29 AM
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Re: When do you use a "Difference Mask'?

Sure. Chapter 7 "Rebuilding, Rearranging and Re-creating Portraits." My text pp146-148.

Thanks
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Old 02-03-2007, 11:38 AM
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Re: When do you use a "Difference Mask'?

i cant answer your question specifically because paint shop pro doesnt have a 'difference mask'. but, it does have more than just a 'show all' and 'hide all'. there is also one based on the luminance of the source image, one based on everything but a 0 value in the source and one based on the opacity of the source image.

'show all' and 'hide all' are the most commonly used masks. show all lets everything through the mask and hide all lets nothing through. the 'luminance' mask reads the source image for lightness and darkness and makes the mask according to that, such that parts of the mask will allow stuff through and other parts not. the other masks work in the same way according to their parameters as set by the program.

i can only speculate what a 'difference mask' is, but i would guess it does something similar based on other parameters. if this is in photoshop, you shld be able to call up the manual and figure out what those parameters are or go to adobe's site and find something there.

..... ok, i just went to adobe.com and typed in 'difference mask' in their search feature and got this page of results: http://www.adobe.com/cfusion/search/...ifference+mask
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Old 02-03-2007, 01:27 PM
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Re: When do you use a "Difference Mask'?

Quote:
Originally Posted by albatrosss
Yesterday I did a tutorial from Katrin Eismann's "Restoring and Retouching". It was the first time I had attempted a Difference Mask. When I had completed the tutorial I wondered why that method was recommended since an additional selection was required using another selection tool.

My questions is: When should one use a Difference Mask? What are the circumstances when this method would be superior to another masking method? I tried "googling" this question but the responses were not too helpful.

Thank you.
It is great that you are asking questions instead of simply following a procedure for one image and not thinking about how you can use the technique elsewhere. But it's hard to give a "list" of where you will use it next.

She used the "Difference" layer blend to help her create a layer mask -- the mask helped her select the woman's hair -- a woman's hair is frequently difficult to select because it is often not "slicked down" like men's hair, and there may be one or more strands sticking out here and there. She was trying to select the woman's image out of another photo so she could add her into another photo to combine the two into a new portrait.

The "Difference" layer blend mode is very useful when you want to see the difference between two layers -- if you ever take two images of the same subject from the same location but with different exposures and want to make sure that each layer is perfectly "registered" or perfectly aligned with the other, you can use the "Difference" layer blend to see if there is total blackness (perfectly aligned so that there is NO difference) or some light shining thru where there is some difference. (This is NOT creating a mask, just showing how the "Difference" layer blend works). Believe me, I'm no expert in using masks, but Katrin Eismann chose to make that mask using the Difference layer blend because hair is not blue and the background was blue-- therefore, the "Difference" layer blend would show off the hair that she wanted to select.
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Old 02-03-2007, 02:15 PM
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Re: When do you use a "Difference Mask'?

Kraellin's search on Adobe.com was a brilliant idea, but unfortunately the hits all concerned After Effects.
Did a similar search and added "+ photoshop" and got this:

http://www.adobe.com/cfusion/search/...+%2B+photoshop

Last edited by Jiger; 02-03-2007 at 02:20 PM.
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Old 02-03-2007, 02:23 PM
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Re: When do you use a "Difference Mask'?

Very well done, Jiger!! Good stuff!
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Old 02-03-2007, 03:21 PM
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Re: When do you use a "Difference Mask'?

ah, good work, jiger. so the 'difference mask' is one based on the differences between the luminosity of two channels... at least that's what i get from briefly reading that stuff you found.

in psp, in case someone wants this for that, you wouldnt be able to do the same thing, since individual channels arent displayed in the layer palette, but you could make a duplicate of the background, make a mask based on image\source image luminance and set the new group mask layer to difference. that might get you something similar.

or, you might highlight the mask within the group and add a brightness/contrast adjustment layer OVER the mask layer and set the bright/contrast layer to difference. that might be closer.
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Old 02-03-2007, 03:55 PM
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Re: When do you use a "Difference Mask'?

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.
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Old 02-03-2007, 04:30 PM
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Re: When do you use a "Difference Mask'?

Now when I saw it I remember having seen this explanation by Deke before.
A bit "scary" at first casual look with the Calculations command and all, but shouldn't bee to hard to get some grip over as a base for experimentation.
As Deke says: "Experiment with Invert and Blending".

Just watch Kraellin picking up the concept and do an emediate adaptation of it to PSP. Good to see it got his attention and focus on possible solutions.
Be inspired, people !
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