Layer placement

Layers are positioned by aligning the layer origin (origin=) with the background layer origin at an offset specified by pos=.

If the layer origin is not specified explicitly for an image layer, it is calculated as follows:

1. Determine the image anchor. Use anchor=, or, if not specified, catalog::Anchor.
2. If the image anchor is defined, apply the layer transforms and extend= to convert it to an origin= value.
3. If no image anchor is defined, the layer origin is placed at the center of the layer rectangle (after applying extend=).