The neighboring picture by Hans Holbein is called “The Ambassadors” (1533, from Wikimedia Commons). Every thing is painted very realistic, in trompe-l’œil style. But what is the strange shape in the bottom foreground?
What to do
You could try a slanting look on the image from top right. Easier: further down I have cut out this shape; with the slider at the right you can change its angle, with the slider at the bottom its horizontal scale. This allows you to make the shape recognisable. Two tips: (1) set both sliders to rather low values, (2) memento mori.
The shape is rendered with a strong perspective distortion, an example of an anamorphotic display. Now in the National Gallery in London, the painting originally hung at the bottom of a staircase, so when descending the angle was just so.
Anamorphotic images can also involve mirroring on curved surfaces, cylinders or cones.