Eric Stevens
And I have pointed out that you cannot reverse a change which has not actually been made. Even if it is reversible, you can't reverse something before you have done it.

nospam
the change *has* been made, just not to the pixels themselves.

Eric Stevens
And to what has the change been made?

Sandman
To the pixels. nospam is incorrect here. All LR adjustments are applied to a preview file and saved to disk.

nospam
the changes are rendered on the fly and may be cached to disk (which is the preview file you're talking about). the latter is optional.

Sandman
It's not optional. All images in LR are always rendered as previews. They are kept inside your LR library.

Eric Stevens
That's true, but all the preview images I have looked at are between about 50% and 20% of the size of the original.

They clearly can't contain as much data or be reconstructed to form a final export image.

In otherwords, what you have in these files is not an edited version of the original image.

Nor do you have to rely on the LR database to store the changes you make in an edit.

You can set up LR to store them in the sidecar files as well.

Most other software does not rely on a database but stores the edit data in a sidecar file.

nospam
it is not a pixel editor.

Sandman
Sure it is, only in another sense than old Photoshop. Every single adjustment you make in LR are applied to the pixels and saved to disk as a preview image. Difference is that the original file is always kept intact so every step is fully reversible.

Eric Stevens
It's not made to the original image: it's made to what I have described as a (reduced size) simulacrum of the original image.

Sandman
In fact - using smart filters in Photoshop is *less* of a pixel editor than LR these days, because the pixels are never touched, it's all kept in RAM and the resulting image is never saved to disk until you export/save it. Also, fully reversible of course. :)

Eric Stevens
With few exceptions, once you have made the changes and saved/exported the image, you cannot reverse the changes *in*the*exported/saved*image*.

That's what all this argument has been about. The reason you cannot reverse the changes is that most of them will not be fully reversible.

One of the few exceptions is Gaussian blur and HPS.