When do you use a "Difference Mask'?

[CJ Swartz]

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...

[albatrosss]

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.

[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.

[CJ Swartz]

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.

[albatrosss]

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

[JayNads]

Generally speaking, a difference mask is useful when you want to make an adjustment layer but the adjustment isn't supported by that function. For example - shadow and highlights. Once you run it, that's it, you can't ever go back and tweak it. So . . . create a new layer (paste visible), run shadow and highlights but only for either shadows or highlights, not both, and then create the difference mask as shown in the book. Using that as a mask, run a curves adjustment layer, adjusting for either shadows or highlights as per your original S&H run. Do this a second time for the opposite, and now you have two curves adjustment layers that effect only those places where the original S&H would have effected your image. Now you can go back and adjust it at any step in the future.