Sometimes you cannot choose the optimum angle to take a picture. If the object is essentially planar (e.g. a painting on a wall) or the angle is not off by much then there is hope that you can correct the perspective afterwards. The opposite may also be of interest if you want to make a composition and place objects at different angles, e.g. pep up screenshots for a brochure. The Perspective plugin lets you carry out both active ("Modify") and passive ("Rectify") transformations. You can rotate the image around three axes, then reposition and resize it. Grid lines can be displayed both before and after transformation.