Today’s Question: You mentioned saving selections as alpha channels in Photoshop. Is this intended to save the work to return to that photograph for continued editing, or can you save the selection and use it in a new image?
Tim’s Quick Answer: The main reason for saving a selection in Photoshop is so you can return to that selection either to refine it or to use it as the basis of some other work. That said, it is actually possible to transfer a selection to a different image as well.
More Detail: Generally speaking, I don’t actually find it necessary to use the Save Selection command (found on the Select menu), which will preserve the selection as an alpha channel on the Channels panel. Note that with a selection saved as an alpha channel you need to save the image in a format that supports alpha channels, such as TIFF or PSD.
The reason I don’t generally need to save a selection is that the selection will most often be used as the basis of a layer mask, either for an adjustment layer or with an image layer in a composite image. That layer mask in effect represents a saved selection, even though it isn’t saved in the same way as with the Save Selection command.
Once a selection has been used as the basis of a layer mask, you can always load a selection based on that layer mask by holding the Ctrl key on Windows or the Command key on Macintosh while clicking on the thumbnail for the layer mask on the Layers panel. This is similar to loading a saved selection with the Load Selection command.
Of course, there may be situations where you’re working on a selection that is quite complex, and you want to save your work in stages along the way. In that type of situation, you can use the Select > Save Selection command to preserve the selection in the current state. You can then return to the image later to load the selection and continue refining it. Of course, in many cases I will actually create a layer mask based on the selection even if it is incomplete, and then continue working to refine the layer mask rather than a saved selection.
It is also possible to effectively transfer a selection to a different image, though this isn’t an especially useful feature since in many cases the destination image might be at least slightly different, requiring a different selection. However, you can save a selection to a different document.
With both images open, you can start by creating a selection in the first image. Then choose Select > Save Selection from the menu. In the Save Selection dialog you can select from the currently open images on the Document popup. Type a name for the new selection in the Name field and click the OK button. The selection from the current document will then be saved as an alpha channel in the document you selected in the Save Selection dialog. While there aren’t a large number of situations where most photographers would put this feature to use, it certainly can be helpful in some cases.