Add the ability to draw shapes outside of the image

[minewilliam]

Proposed in the rectangle rotate function feature, follow-up from discussion #1374 (comment).

Discussion about drawing polygons outside of the canvas for the purpose of adding image padding: #1075
Question about drawing points outside of the canvas: #1404

Description

It makes sense for some shapes to be drawn partially outside of the image (circle, rectangle, rotated rectangle), since some models can make predictions on occluded/partial objects.

Requirement

Add a user setting to allow/prevent this behavior.
I would opt for a setting entry for each shape type, to set whether it can be drawn outside the canvas.

canvas:
  ...
  allow_drawing_outside:
    polygon: false
    rectangle: false
    rotated_rectangle: true
    line: false
    point: false
    linestrip: false
    circle: true
    points: false

Note: rotated_rectangle is a work in progress, which benefits from this feature.

Behavior

Change to circle shape

By default, I would opt to change the current behavior of the circle shape, since I believe it does not benefit to being restricted to the boundaries of the canvas.

Loading annotations that are outside the image

We must handle the loading of previously annotated shapes, which may reside outside of the canvas. The expected behavior is to do nothing and leave the shapes as they are; Only apply the restriction when the user draws/drags a shape/vertex.

• A shape dragged from the outside of the canvas must "snap" inside the canvas. If a shape is too big for the canvas, this is considered an edge-case. The user is expected to modify/redo their annotation.
• A vertex dragged from outside of the canvas must "snap" inside the canvas.
• The undo (ctrl + z) action should correctly revert a modified shape/vertex and place it back outside of the canvas, if such was the case.

Drag group of shapes

While dragging a group of shapes, only the shapes with a restriction are tested against the boundaries of the image.

Acknowledgements

• The mask shape type would be quite difficult to implement this way, thus it is excluded from this setting. If such a feature is necessary, a separate, more complex feature should be proposed.

Questions

• Suggestions for a better setting name than allow_drawing_outside?