First, objects.spritebank covers both objects.png and people.png files. Here's an image depicting how the images are arranged and their x,y co-ordinates:
Why they couldn't just create a people.spritebank like for the other .png sprite files I have no idea.
The next confusing thing was how a marker in the file relates to a sprite? Trixi did some excellent investigative work:
Each sprite covers an area in the file. If the marker co-ordinate (which is offset from top-left of objects.png, even if the sprite/marker are in people.png) falls within the area of a sprite, then that marker is associated with the sprite.
So, hope that helps anyone that's been struggling with objects.spritebank
And huge kudos to Trixi for working out how it all fit together.
