This is the approach that I would take. First, beginning in 18b the Image Processing Toolbox has a new suite of ROI tools. I would suggest using those. The basic work flow to do this would be as follows:
1. Draw an ellipse and turn off all interactions so it is static after being drawn
2. Create a line ROI object
3. Add a listener to listen for any line ROI motion
4. In the listener callback, check if the line is outside the ellipse and set the previous position if true.
Here is an example to accomplish this task:
function lineInsideEllipse
% Draw an ellipse and then turn off all interactions
hEllipse = drawellipse('InteractionsAllowed','none');
% Create an empty line ROI
hLine = images.roi.Line('Color','red');
% Listen to any line movement
addlistener(hLine,'MovingROI',@(src,evt) movingCallback(src,evt,hEllipse));
% Begin interactive placement of the line starting from the ellipse center
beginDrawingFromPoint(hLine,hEllipse.Center);
end
function movingCallback(hLine,evt,hEllipse)
xCandidates = evt.CurrentPosition(:,1);
yCandidates = evt.CurrentPosition(:,2);
% Check if the line current position is now outside the ellipse
TF = inROI(hEllipse, xCandidates, yCandidates);
if ~all(TF)
% Line is at least partially outside the ellipse, reset the line
% position to be the previous position
hLine.Position = evt.PreviousPosition;
end
end
