- tx, ty are the origin of the parent's coordinate system relative to the origin for its layer. When a layer paints, it establishes a CTM such that its own origin is 0, 0 (I think).

The more obvious way, given the variable names, would be to make x(), y() an IntPoint, with a name like originInParentCoordinates()

(hopefully less verbose, but you get the idea). tx, ty could be named parentOffsetFromLayerCoordinates or something. This seems to be the intent of the names - that x,y is a point and tx, ty is a translation. But this doesn't work in point/size logic. You repeatedly add x(), y() to tx, ty to get a new tx, ty. But that means you're adding a point to a size and expecting to get a new size - but that's not how it works.