Displaying the slope of a line created by imline in a text box - matlab

Question

This post is a continuation of the thread: "Matlab imline snapping" which was resolved. The below is the working code that snaps the imline object to a curve.
function calc_slope(handle,event)
axis_h = findobj(gcf,'Type','axes');
obj_h = get(axis_h,'Children');
obj_type = get(obj_h,'Type');
if ~iscell(obj_type),obj_type = cellstr(obj_type);end
for i=1:length(obj_type)
if strcmpi(obj_type{i},'line'),data = obj_h(i);end
end
xdata = get(data,'XData');
ydata = get(data,'YData');
on = get(handle,'State');
if strcmpi(on,'on') || strcmpi(on,'off'),
fcn_constr = #(pos) imline_snap(pos, [xdata(:) ydata(:)]);
xy = imline(axis_h, 'PositionConstraintFcn', fcn_constr);
addNewPositionCallback(xy,#(pos) disp_slope(pos));
end
function constr_pos = imline_snap(new_pos, positions)
[~, ind1] = min(sum(bsxfun(#minus, new_pos(1,:), positions).^2, 2));
[~, ind2] = min(sum(bsxfun(#minus, new_pos(2,:), positions).^2, 2));
constr_pos = [positions(ind1,:); positions(ind2,:)];
function disp_slope(pos)
delete(findobj(gca,'Type','text'));
text((pos(1)+pos(2))/2,(pos(3)+pos(4))/2,['\DeltaY/\DeltaX = ',num2str((pos(4)-pos(3))/(pos(2)-pos(1))),...
' [\DeltaX = ',num2str(pos(2)-pos(1)),', \DeltaY = ',num2str((pos(4)-pos(3))),']']);
Every time the toggle button on the toolbar of a figure is toggled (on and off), a new imline object is thrown in. There are many figures with different parameters so that the data has to be extracted from the figure. In a given figure, there can be multiple objects: imline objects, text, and/or line; thus, the first seven lines in the calc_slope function.
The imline objects snaps to the nearest data point of a curve and it's beautifully done by imline_snap function which is an answer by "Luis Mendo". Thank you so much. This has been a biggest headache.
The final problem is now to show the slope of an imline object in a text box (instead of the title or the floating box). It's attempted in the disp_slope function (and it's miserable).
I'm doing "delete(findobj(gca,'Type','text'));" only because without something like that, as the imline object is moved around, it will leave millions of text boxes. I only want to show one most current slope calculation.
There are multiple problems with "delete(findobj(gca,'Type','text'));". If I stop moving the line around, it will nicely show the last slope calculation. However, as soon as I throw in another imline object and move the new one around, the text box in the first imline object will get deleted and of course.
Another problem is that even if I delete the imline object, the associated text box will remain.
In summary,
I want to show the current calculated slope in a text box
I want the text box for each imline object to remain even if there are multiple imline objects.
Finally, I want the corresponding text box to disappear as well when a particular imline object is deleted.
Can this be done? Help please.
Thanks,
Eric

Answer 1

Don't create a new text object every time. Create one initially
ht = text(.45, .85, ''); %// modify coordinates to place it where you want
and then update its content ('String' property) when the imline changes. To do the updating, modify the imline_snap function to accept ht as a third input and add the following line at the end:
set(ht, 'String', ...
num2str((constr_pos(2,2)-constr_pos(1,2))/(constr_pos(2,1)-constr_pos(1,1))));
So the function becomes
function constr_pos = imline_snap(new_pos, positions, ht)
[~, ind1] = min(sum(bsxfun(#minus, new_pos(1,:), positions).^2, 2));
[~, ind2] = min(sum(bsxfun(#minus, new_pos(2,:), positions).^2, 2));
constr_pos = [positions(ind1,:); positions(ind2,:)];
set(ht, 'String', ...
num2str((constr_pos(2,2)-constr_pos(1,2))/(constr_pos(2,1)-constr_pos(1,1))));
Then, when defining fcn_contr, pass the reference ht to the text object:
fcn_constr = #(pos) imline_snap(pos, [xdata(:) ydata(:)], ht);
Here's an example, borrowing the curve from my previous answer:
h = plot(0:.01:1, (0:.01:1).^2); %// example curve. Get a handle to it
a = gca; %// handle to current axes
ht = text(.45, .85, ''); %// create text
xdata = get(h,'XData'); %// x values of points from the curve
ydata = get(h,'YData'); %// y values of points from the curve
fcn_constr = #(pos) imline_snap(pos, [xdata(:) ydata(:)], ht); %// particularize function
imline(a, 'PositionConstraintFcn', fcn_constr); %// create imline
You can also update the text position ('Position' property). Just change the last statement of imline_snap to include that. For example:
set(ht, 'String', ...
num2str((constr_pos(2,2)-constr_pos(1,2))/(constr_pos(2,1)-constr_pos(1,1))), ...
'Position', ...
mean(constr_pos) + [.03 -.03]); %// manually adjust offset if needed
The offset [.03 -.03] is intended to avoid the text overlapping with the line. You may need to change it. Also, it may help to create the text object with boldface. The line becomes
ht = text(.45, .85, '', 'Fontweight', 'bold'); %// create text, in boldface
Here's an example with text position updating:
To delete the associated text object when you delete the imline object you need and event listener. This is an object with three main properties: a cell array of source objects, an event, and a callback function. When the indicated event happens to one of the source objects the callback function of the event listener is executed.
To create an event listener for the imline object's deletion, use that object's addEventListener method and specify the event name and callback function. The callback function is specified by means of a function handle, and it should expect two inputs, which correspond to the source object and the event (this is how the callback function will know "why" it's being called). Even if those inputs won't actually be used, the function needs to be defined that way.
In this case, the event we want to listen to is ObjectBeingDestroyed, the source object is the imline object, and the callback function is delete(ht) (to delete the text object). So, the code in the above example becomes
h = plot(0:.01:1, (0:.01:1).^2); %// example curve. Get a handle to it
a = gca; %// handle to current axes
ht = text(.45, .85, '', 'Fontweight', 'bold'); %// create text, in boldface
xdata = get(h,'XData'); %// x values of points from the curve
ydata = get(h,'YData'); %// y values of points from the curve
fcn_constr = #(pos) imline_snap(pos, [xdata(:) ydata(:)], ht); %// particularize function
hi = imline(a, 'PositionConstraintFcn', fcn_constr); %// create imline and get a handle
addlistener(hi, 'ObjectBeingDestroyed', #(obj,event) delete(ht))
where only the last two lines are new.
Now whenever the imline object is deleted the action delete(ht) will be performed, thus deleting the text object.