When do you use a "Difference Mask'?

[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.

[mistermonday]

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

[albatrosss]

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

Thanks

[Kraellin]

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

[CJ Swartz]

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.

[Jiger]

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

[CJ Swartz]

Very well done, Jiger!! Good stuff!

[Kraellin]

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.

[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.

[Jiger]

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 !

[CJ Swartz]

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")?

[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.

[Kraellin]

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.

[albatrosss]

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.

[Kraellin]

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