Historical methods for drawing anaglyphs in geometry teaching

International audienceThe historical anaglyphic method was in use for more than hundred years to create spatial illusions of mathematical objects and for technical constructions. While algebraisation is predominant at school, students lack experience in understanding the causalities of technical tools. Modern technical devices rarely allow for direct investigations on underlying technical principles. Here we use the historic anaglyphic method to enable the students to produce high-standard 3D illusions just by using coloured pencils, a ruler and glasses with colour filters. We developed an approach to the anaglyphic method that uses nothing but similarity and especially surpasses projective geometry. The presented approach relates plane and spatial geometry, and can be grasped by all students that have some understanding of the similarity of triangles