Foreshortening is a technique used by artists to create realism in their work. By foreshortening the arm, the painter made it look as though it was reaching out of the canvas.
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Visual Deception
When foreshortening, objects are distorted to give the impression that they are receding in space. The knight's lance was foreshortened to look like it extended back into the distance.
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3D Illusion
Foreshortening makes two-dimensional art look three-dimensional by showing depth. Foreshorten the road on paper, and it appears to wind into the picture.