Savageduck
On 2014-09-17 03:48:48 +0000, Eric Stevens <eric.stevens@sum.co.nz>said:

Eric Stevens
On Tue, 16 Sep 2014 09:12:12 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk>wrote:

Martin Brown
On 16/09/2014 05:37, John McWilliams wrote:

John McWilliams
On 9/15/14 PDT, 7:07 PM, Floyd L. Davidson wrote:

Floyd L. Davidson
Savageduck <savageduck1@{REMOVESPAM}me.com>wrote:

Savageduck
I got what Floyd was talking about when he was talking of high pass sharpening, and reversing it by applying the corresponding reverse parameter blur. However, he also stated above, "UnSharpMask is not reversible". My point addressed the fact that for some of us, that is not an entirely valid statement.

Martin Brown
IN an environment that supports a saved original copy so that all edits are non-destructive then that is true.

Eric Stevens
You can't reverse a process if you have never executed it. If you make a copy of an image and edit it, you cant reverse the process of editing by just hauling out your original image. The edited version of the image remains edited and in most cases there is nothing you can do to reverse it.

Savageduck
You have Photoshop installed on your computer don't you? The time has come for you to actually learn how to use it. We will leave Lightroom for later.

Eric Stevens
The water is muddied by the several applications which make use of a sidecar file of some kind to preserve a list of edits which are only executed when the image file is exported from the editing environment. Modifying a sidecar file by deleting an editing process from it does not make that process reversible. It merely makes that process asthough it never was.

Savageduck
If you make the adjustments in Photoshop with a non-destructive workflow there is no use of sidecar files or catalog entries as in Lightroom.

Martin Brown
But the mathematics of blurring and of unsharp masking make it irreversible if you are only given just the processed image. (and not some hybrid Photoshop workflow encapsulated format)

Floyd L. Davidson
That is in fact a valid statement. The USM function is not reversible.

That isn't a opinion, it's a fact.

John McWilliams
For one definition of the word!

Martin Brown
This seems to have degenerated into a heated and utterly pointless semantic argument over the meaning of "reversible" that is in conflict with normal image processing and mathematical parlance.

A mathematical operation is reversible if a strict inverse function exists that can exactly get you back to where you started.

Any operation can be made non-destructive simply by saving a copy of the original before applying the irreversible filter but that isn't very interesting. Some packages do support this sort of safe workflow.

Eric Stevens
--

Regards,

Eric Stevens