3 ideas
Every painting works on two levels at once. It is a two-dimensional arrangement of shapes, lines and colors on a flat surface, and it is also an illusion of a three-dimensional scene. Many of an artist's key decisions are about balancing the design across the surface against the believability of the depicted space, so reading a picture means noticing both levels and how they help or pull against each other.
Artists painting the same Last Supper faced one fixed problem: how to single out the traitor. Castagno isolates Judas on the near side of the table, which makes him easy to identify but stiff as a drama. Leonardo keeps him on the same side as the others and marks him instead through posture, shadow and gesture within groups of three, so the story is told through reaction rather than seating.
In early Netherlandish painting, ordinary objects in a convincingly realistic room carry religious or moral meanings. Examples include a candle, a clean towel, a lily or a small dog. Because the objects look natural in the setting, the symbolism stays hidden until the viewer knows the conventions, so a picture can read as a plain domestic scene and a coded statement at the same time.