Visibility
The observer at V looks from the front (larger y = closer), at H – from above (larger z = closer).
Invisible parts are dotted. Toggled by the "Visibility" checkbox.
Save and export
"Save" / "Open" – tasks are saved in SQLite (long press in the list = delete)
"PNG" – saves the drawing with the result text in Pictures/DescriptiveGeometry
(Android 9 and older asks for write permission)
Conventions
The x-axis is horizontal, the positive direction is to the left.
Above axis: frontal projection V (x, z); below axis: horizontal projection H (x, y)
Gestures: drag = move, pinch = scale