Here is Carpaccio’s painting of Saint George and the Dragon.
Add red lines from the left corner at 75, 60, 45, 30, and 15 degrees off horizontal. Note that the steepest one, at left, is an axis around which the dragon’s tail oscillates. The vertical dropped from the top of the 60-degree line passes through the right corner of the square-planned tower in the middle ground. Crucially, the vertical dropped from the top of the 30-degree line passes through the point where the lance intersects its crosspiece.
Take the lower left corner as the center of a very large octagon whose right-hand facet coincides with the red line dropped from the top of the 45-degree line. A line of slope 22.5 degrees from the lower left corner of the painting intersects the red vertical dropped from the 30-degree red line, thus locating the precise point of the lance/crosspiece intersection.
Taking that intersection point on the lance as a center, create two dodecagons; one whose lower facet aligns with the bottom of the painting, and a smaller one whose upper facet aligns with the top of the painting. The lance, with its 15-degree slope, passes through corners of these dodecagons. The tree foliage at the upper right also lies along this axis.
The right margin of the painting may be found by dropping a 45-degree diagonal down from the uppermost left corner of the small dodecagon down to the bottom margin of the painting; this operation thus sets the overall aspect ratio of the composition. A line of 15-degree slope descending to the left from this same upper point passes along the shadows on the rocky hills about a third of the way across the composition. A horizontal through the lance intersection point aligns with the ground level of the hills on the far right.
This painting is the property of the Scuola di San Giorgio degli Schiavoni, Venice.
This analysis was based on a color-enhanced version of the following image: