On 05/02/12 13:37, Noel Stoutenburg wrote:
Erik P. Olsen wrote:

Why can't I rotate images similarly with Gimp?

and I concur with the answer that BK gave, as far as it went. But to expand a
bit, there are several ways to rotate an image. For the purpose Erik mentioned,
I'd use the method BK mentioned:

Image > Transform > Rotate...

This is the equivalent of rotating the substrate upon which the image lies.

There is a second way, useful when one wants to modify some part (or all) of the
image with respect to the substrate.

Layer > Transform > Rotate...

acts similarly to the method BK described, except that it rotates the layer
containing the image, or some part of it, with respect to the rest. If one has
an image that has unequal dimensions, like a rectangle or oval, and rotates the
layer relative to the substrate (canvas is the term GIMP uses), then part of the
layer is no longer over the substrate. This gives a result similar to what the
OP describes: part of the image is no longer over substrate, and gets lost.

A third method works similarly to the method of the layer transform: the Rotate
tool in the toolbox. The rotate tool will rotate a selection of the image (which
may, but need not necessarily coincide with an image) and rotate that selection
relative to the remainder of the image. And just like the layer transform I
describe above, there is a risk that a selection with unequal dimensions will
lose part of the information.

It turns out, though, that even if one seems to have lost the information by
choosing layer or selection rotation instead of image rotation, that besides
undo, there is a convenient way to recover the information no longer over
substrate. One can resize the canvas. Once the canvas is resized with the
dimensions of the image again matching the dimensions of the rotated part, the
full image again becomes visible.

Thanks for this detailed description. Yes, I've rotated the layer only, so now at least I know how to rotate correctly. But please enlighten me why would you want to rotate a layer and not the entire image?

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