A super-fast mini-tutorial on defining previews
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To define a preview interactively, select the New Preview mode (Image > Mode > New Preview, or Alt+N, or click on the corresponding tool button). Then click at any point on the image and drag:
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Once you release the mouse, the new preview is created covering the selected rectangle. Then the Edit Preview mode (Alt+E) is automatically selected, in case you want to refine preview's coordinates:
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Finally, if you want to modify an existing preview, select the Edit Preview mode (Alt+E) manually and drag any corner or side of the preview's rectangle:
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It may happen that a preview entirely covers a smaller preview. If this happens, to select the smaller preview in Edit Preview mode, press the Ctrl key while you click on the image.
Hope this clarifies.