Rogier van der Weyden, Saint George and the Dragon
Here is Rogier van der Weyden's painting of Saint George and the Dragon, created around 1435.
Its geometry involves decagons and pentagons, as the 72-degree slope of the saint's spear suggests. More specifically, one can begin to understand the composition by inscribing the largest possible circle within the bottom section of the panel, and then circumscribing a decagon around that circle. The rightward extension of the decagon's lower facet forms a perpendicular with the spear. The top vertex of the decagon locates the central tower of the city wall in the distance, while the vertices to the left locate the forehead of the princess, and the end point of the sweep in her dress.
To find the exact location of the spear, one must first draw 45-degree diagonals rising from the bottom center of the panel. The left one parallels the horse's left lower leg, while the right one intersects the joints just above its front hooves. Next, diagonals from the bottom corners of the panel rise to meet the first set, one passing into the horse's rear right leg, and the other passing through the dragon's eye to a point on its back. An arc swung down from the latter point locates the tip of the spear along the previously constructed decagon extension.
The tip of the spear then serves as the tip of a large pentagram whose vertical axis coincides with that of the panel. The spear aligns with the steep right facet of the pentagram, while the corresponding left facet traces the plume descending from George's shoulder, eventually passing through the bottom leg joint of the horse. The horizontal facet of the pentagram passes just over the horse's head, while the shallow facet of the pentagram passes just beneath its nose. Other details can also be seen as related to the pentagram, including the folds in the princess's dress, the crack pattern in the rocks at right, and the top of the horse's rump.
Further details can be added to the picture by constructing lines of 30-degree slope, here shown in green. The green line rising from the bottom left corner of the panel aligns with the forelegs of the horse, and its reflection back to the upper left passes directly through George's eyes and through the corner of his raised elbow. The corresponding line rising from the bottom right corner locates the bottom of the princess's dress on the left margin, and its reflection back to the upper right passes just above George's head.
Finally, in blue, a semicircle is shown in the bottom sector of the panel, describing the area occupied by the dragon at right, and by the horse's tail at left. A horizontal resting on this semicircle frames the horse's body, and the inflection point in the curve of the plume falling from George's shoulder. A higher horizontal through the tip of the large pentagram defines the horizon line. A V-shape with the 72-degree slope characteristic of the pentagram separates the space of the princess on the left, of George in the center, and of the rock outcrop at right; its intersections with the green lines in the upper corners locate the corner of the tower at left and the small tree at right, respectively.
This painting belongs to the National Gallery of Art in Washington, DC. This analysis is based on the image:https://commons.wikimedia.org/wiki/File:Saint_George_and_the_Dragon_Rogier.jpg