Floyd L. Davidson
nospam <nospam@nospam.invalid>wrote:

nospam
In article <8738bs2076.fld@barrow.com>, Floyd L. Davidson <floyd@apaflo.com>wrote:

Floyd L. Davidson
UnSharpMask is not reversible.

nospam
it is with a non-destructive workflow.

Floyd L. Davidson
I'm sorry that you don't understand the meaning of that.

nospam
i absolutely do know the meaning, since it's all i use.

it's you who doesn't understand what a non-destructive workflow means

Floyd L. Davidson
A non-destructive workflow means you can *undo* and then *redo*.

That is not a reversible function.

nospam
it is to the user, which is what matters.

in other words, the user sharpens today and then tomorrow or next month or whenever, they can readjust it or remove it entirely. that means to the user, it's reversible.

that's why a non-destructive workflow is so powerful.

Floyd L. Davidson
Non-destructive is wonderful. It especially impresses Chicken Little, Humpty Dumpty and nospam and probably other cartoon characters.

But no matter how you try to squirm, no matter how you squeal, *unsharp mask is a nonreversible function*.

For example, you can add sharpening with a high pass sharpen tool to an image, save it as a JPEG, send it to someone else, and they can use a blur tool to reverse the sharpen.

nospam
not perfectly. you even said 'virtually reverses' in your description.

that's another way of saying 'there is some loss.'

Floyd L. Davidson
I don't see the term "virtually reverses" in that sentence.

But the previous reference is in fact precise. The reason for saying "virtually reverses" is because if the sharpen and blur algorithms are not exactly the same and using precisely the correct parameters, the reversal isn't total. Which is to say that if it is done by inspection the result will be such that there is no visible difference.

If it is actually measured, there will be an insignificant difference.

I'm sorry that you have so much difficulty with precision use of language.

In article <87bnqh1mby.fld@barrow.com>, Floyd L. Davidson <floyd@apaflo.com>wrote:

nospam
Not the case. It is the high pass sharpen tool that is the inverse of blur. They can use the exact same algorithm with different parameters. Using one and then the other virtually reverses the results.

Floyd L. Davidson
If the sharpening is done with UnsharpMask that cannot be done. USM is not reversible.

nospam
in a destructive workflow that is true.

in a non-destructive workflow, it is not true.

Floyd L. Davidson
Obviously you didn't understand what the term "reversible" means, and thought non-destructive is the same. It isn't.

See: http://en.wikipedia.org/wiki/Reversible_computing

Then read this,

http://en.wikipedia.org/wiki/Undo

And note the distinction between reversible and a non-linear undo.