How to think beyond the picture frame?

What I really like about this painting from Arthur Melville is that one of the main characters is not in the scene, but has a major influence on the whole piece anyway.

It is like a crime movie where you feel the presence of the killer, but you do not see him. 

It is a giant tree on the right side of the frame (you see a few green leafs on the right side) that casts this big shadow onto the scene. The shadow gives the piece it’s mid value range and therefore lowers the really hard contrast between the shadow under the gate and the bright sunlit areas.

Because the tree is not visible, we don’t know if the shadow is cast “right”. So Melville could fully control its appearance and use it for compositional purposes.

So it is a great idea to think about the light source and objects (that maybe cast shadows) outside the picture frame. 

Another thing I wonder about is the bright triangular shape behind the procession. I think it shows curtains pulled aside so that the procession can walk through. Because of the sharp edges and the highest value contrast in the whole painting, it grabs our attention even more than the flag bearer. This extends the story, and gives us the impression that the painting is not about the procession only, but where the people come from too.