least destructive method of re-saving jpeg files

[mbalensiefer]

In PS:

 The "Resample Image:" dialogue is present when saving or resaving a file.
Under this, as options, you have:

Nearest Neighbor (preserve hard edges)
Bilinear
Bicubic (best for smooth gradients)--also default
Bicubic (best for enlargement)
Bicubic (best for reduction)

If I am saving the file without resizing, then is there a preferred method of resampling? --OR--do I not select the Resample box at all (so that this box remains empty when selecting "OK"?

Thank you!
Michael

[Mark D Segal]

In PS:

 The "Resample Image:" dialogue is present when saving or resaving a file.
Under this, as options, you have:

Nearest Neighbor (preserve hard edges)
Bilinear
Bicubic (best for smooth gradients)--also default
Bicubic (best for enlargement)
Bicubic (best for reduction)

If I am saving the file without resizing, then is there a preferred method of resampling? --OR--do I not select the Resample box at all (so that this box remains empty when selecting "OK"?

Thank you!
Michael

Michael, there is simply TONS of information all over the web on this question. Anyhow let us get the contexts and purposes in order. Resampling means changing the pixel dimensions of the image. You resample and therefore check that box if you want a certain combination of PPI and linear length and width which differs from what you would get without resampling. If you do not resample, pixel dimensions remain the same whatever the changes to PPI and linear dimensions - PPI and linear dimensions simply adjust to eachother proportionately to maintain pixel dimensions whnever you change one of them. It is only when you resample that you have a choice of resampling method, and the advice Adobe provides in the pull-down pane is the advice to follow. If your resampling changes result in increased total pixel count, use Bi-Cubic smoother. If your resampling changes result in reduced total pixel count use Bi-Cubic Sharper.

An example of the choice between resampling and not resampling: Let us say your image starts life at 360PPI and has linear dimensions (LD) of 15.6 inches by 10.4 inches. For this image Pixel dimension is 120.3 millilons. Now let us say you DO NOT resample, but you reduce the LD to 10 by 6.667 inches. In this case the resolution (PPI) goes up from 360 to 561.6 PPI preserving the total pixel dimension of 120.3 millions. But that's too much resolution relative to what your printer optimally handles - let us say it likes 360PPI. So to get these reduced LD at 360, you need to check the Resample box, and change the Resolution back down to 360 PPI. When you do that, Pixel dimensions reduces from 120.3 millions down to 49.4 millions. So you would select Bicubic Sharper. Hope this helps.

[mbalensiefer]

Thank you, Mark.

 If printing is not in the equation, and all that I would be doing with a given .jpeg image would be retouching it (and subsequently saving at the same H/W dimensions), then would I want to resample it at all?

V/R
~Mike

[LoisWakeman]

If printing is not in the equation, and all that I would be doing with a given .jpeg image would be retouching it (and subsequently saving at the same H/W dimensions), then would I want to resample it at all?

Not usually, no.

As for re-saving, the compression/lossiness is only embedded in the file as soon as you close it - you can do lots of editing on the version in memory without losing anything during a single session.

[Mark D Segal]

Lois, If the file is being re-saved at the same dimensions, my understanding is that no data loss occurs.

Mike - you don't need to resample UNLESS you need to change the PPI while retaining the same linear dimensions. The final PPI you need depends on the ultimate usage you will make of the image, and if you are not sure, you want to save with the highest existing resolution you have in the file. It's much better to shed pixels than to "re-invent" them.

[John.Murray]

Mark, I think Lois is referring to the specific example of saving a .jpeg - since the file format uses lossy compression, you will lose information - even if saving at same pixel dimensions......

[mbalensiefer]

...Mike - you don't need to resample UNLESS you need to change the PPI while retaining the same linear dimensions. The final PPI you need depends on the ultimate usage you will make of the image, and if you are not sure, you want to save with the highest existing resolution you have in the file. It's much better to shed pixels than to "re-invent" them.

I don't need to input any PPI at all, however, unless I eventually want to print my file, correct?

[Mark D Segal]

I don't need to input any PPI at all, however, unless I eventually want to print my file, correct?

You need to input PPI if the current value is not what you want for a particular purpose or specification.

[mbalensiefer]

Alright:
If I am saving the file without resizing, then is there a preferred method of resampling? --OR--do I NOT select the Resample box at all (so that this box remains empty when selecting "OK"?

[Mark D Segal]

If you do not need to change the pixel dimensions of the image (i.e. do not need to change PPI at fixed linear dimensions or change linear dimensions at a fixed PPI), you leave that resample box unchecked and there are no resizing options because no resizing happens.

[fike]

what everyone else has said...don't resample unless you are changing the images pixel dimensions.

What no one has said yet is that you should rarely be resaving JPGs as JPGs.  I always save working files as a TIF or PSD.  The only time you should resave as JPG is when you are publishing to the web.

[schrodingerscat]

what everyone else has said...don't resample unless you are changing the images pixel dimensions.

What no one has said yet is that you should rarely be resaving JPGs as JPGs.  I always save working files as a TIF or PSD.  The only time you should resave as JPG is when you are publishing to the web.

Exactly. Try and save jpgs as tiffs as early in the process as possible. You can always go back and mage a jpg copy later on.

[Mark D Segal]

Mark, I think Lois is referring to the specific example of saving a .jpeg - since the file format uses lossy compression, you will lose information - even if saving at same pixel dimensions......

Theoretically yes, but in practice with no change in pixel dimensions or compression ("quality" factor in the save operation), the loss is likely to be undetectable in prints or web images. In general the less resaving the better. Gnerally speaking, it is best to avoid editing images in JPEG format as much as possible, especially if they are lower-res JPEGs.