Use perspective correction when a planar source was not captured straight-on and lines that should be parallel converge in the image.
Four-point method
Mark the four corners of a planar area in the order shown by the application. This method is suitable when the corners or another rectangular reference are clearly visible.
Parallel-line method
Mark several lines that should be mutually parallel in reality. This method is useful when corners are not clearly available but reliable horizontal and vertical lines are present in the drawing.
Checking the result
Always check the preview before applying the correction. A mathematically valid transformation may still be incorrect in practice if unsuitable references were selected.