Does rotating a photo count as a destructive editing?

Question

I usually rotate all my jpg photos (using Windows Photo Viewer) after removing them from my camera's card. Is this correct? Am I losing quality for rotating them? Should I just leave them as they are and later incorporate the rotation as another retouch step - at a post-processing software like, for instance, Lightroom?

Sometimes I see websites like DPreview.com showing their camera's sample photos not rotated, thus I thought rotating them would reduce (a lit bit) their quality. It it a fair assumption?

Answer

Whether rotation loses quality or not depends on software (and its version) used and image dimensions. Images that have width or height (measured in pixels) not divisible by 8, cannot be rotated in a lossless way.

As for Windows Photo Viewer, Matt Grum has already given a stellar answer to the more specific question. For other software, you could use the sample images in his answer and try the same experiment.

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How to offset the positioning of the Preview Gridlines in GIMP?

Question

When rotating an image with GIMP, there is an option to superimpose gridlines on the image. The number of lines is settable but the offset is not. I'd expect that when changing the center of rotation, the grid will be moved as well, but it is not.

Why would I want to do so? When rotating in Corrective mode, I want to align the grid with a supposedly horizontal/vertical line in the image to get perfect leveling. However, when the reference line in the image is not at the place of a gridline, it is more difficult. I found out that increasing the number of gridlines gets me close to this, but I wonder -

Is it possible to change the offset of the grid?

While we are at it - how to automatically adjust the canvas after rotation in GIMP, when Clipping is set to Crop?

Answer

AFAIK there is no way to offset the grid. However, there is a simpler way to straighten the photo: simply drag a guide from the ruler at the edge of the image until it lies along the horizon (or whatever else you want), then rotate as normal.

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