Why can a stretched drawing look right in a curved mirror?

A drawing spread across a flat surface can look bent or stretched until a cylindrical mirror reflects it. The mirror does not recognize the subject and correct it. The artist has placed marks so that reflected light reaches a chosen viewer in the arrangement needed for a recognizable image. This deliberately transformed drawing is called an anamorphosis.

Reflection follows a local rule: the incoming and outgoing rays make equal angles with the surface normal, a line perpendicular to the mirror at the reflection point. On a cylinder, the normal changes direction around the curved wall. Consequently, marks at different places on the paper need different paths to reach the same viewer.

Design can work backward from the desired view. For each point of an intended arrow, a ray is traced back toward the cylinder. Reflecting that direction at the surface and continuing to the paper identifies where the corresponding mark belongs. Repeating this operation produces the spread-out drawing. The strange shape on the paper is therefore compensation for a specific optical transformation.

The illustration uses an ideal cylindrical mirror and a distant viewing direction, making outgoing viewing rays parallel. The intended upright arrow and its mapped paper outline correspond under that assumption. A different viewing elevation needs a different drawing, and a nearby viewer requires perspective geometry instead. The result depends on the mirror and viewpoint together; a curved mirror does not restore every distorted image from every position.