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Re: Lenses and sharpening

Martin Brown
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SubjectRe: Lenses and sharpening
FromMartin Brown
Date09/16/2014 10:12 (09/16/2014 09:12)
Message-ID<AhSRv.308247$KL.110792@fx19.am4>
Client
Newsgroupsrec.photo.digital
FollowsJohn McWilliams
FollowupsSavageduck (56m)
Eric Stevens (19h & 36m)
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.

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

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!

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.

-- Regards, Martin Brown

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