Calculate a widget's canvas and its children's bounding boxes
PhRect_t *PtChildBoundingBox( PtWidget_t *widget, PhRect_t *canvas, PhRect_t *render );
This function calculates the canvas of widget and finds the bounding box of widget's children relative to the origin of the parent's position.
The same pointer as the render argument.
PhRect_t, PtCalcCanvas() in the Photon Library Reference