Event listener for multiple draggable rectangles using imrect in matlab

function computeButton_Callback(hObject, eventdata, handles)
% hObject    handle to computeButton (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
PImage=handles.ProcessedImage;
Image=handles.Image;
imshow(Image);
hold on;
bw=bwconncomp(PImage,4);
regions=regionprops(bw,'BoundingBox','PixelIdxList');
for i=1:length(regions)
    box=regions(i).BoundingBox;
    h=imrect(gca,box);
    %rectangle('Position',box,'EdgeColor','black','LineWidth',2);
end

 %I want to make an event listener that would record the 
 %properties(height,width) of all the rectangles and even 
 %the changes on each of the draggable rectangles on the Image.

Answer 1

[Guillaume]

Do you really want to listen to all the changes in position of the rectangles? There will be a lot of event recorded when the user drag a handle. If you only need to know what the current position of the rectangles is at some point in the code, you can just query it. For that, you obviously need to save the rectangle handles somewhere:

%...  
hrectangles = gobjects(numel(regions), 1);  %allocate storage for rectangle handles
for i = 1:numel(regions)
   box = regions(i).BoundingBox;
   hrectangles(i) = imrect(gca, box);    %store rectangle handles
end
%...
%code where you need to know what the coordinates of the rectangles are:
rectpos = cell2mat(arrayfun(@getPosition, hrectangles, 'UniformOutput', false));

If you really want to listen to all the changes, you can add a new position callback to each rectangle and do whatever you need to do in that callback:

for i = 1:numel(regions)
    box = regions(i).BoundingBox;
    h = imrect(gca, box);
    h.addNewPositionCallback(@(newpos) RectPositionChangedCallback(h, newpos)
end

with

function RectPositionChangedCallback(recthandle, newposition)
    %do something with new position
end

[Naseer Khan]

I am interested in all the the rectangles and after a user have adjusted the size of the rectangle(s) using a mouse then finally I would save all the rectangles parameters in the file.How would I do that?

[Guillaume]

I would add a button to the GUI for the user to tell you when they're done adjusting all the rectangles. In the button callback, you then query each rectangle location and save that to file.

So, in your computeButton callback, you simply store all the rectangle handles, as per my first example:

%...
handles.hrectangles = gobjects(numel(regions), 1);  %allocate storage for rectangle handles
for i = 1:numel(regions)
   box = regions(i).BoundingBox;
   handles.hrectangles(i) = imrect(gca, box);    %store rectangle handles
end
guidata(hObject, handles);  %save handles structure

In you new button callback:

function SaveRectButton_Callback(hObject, eventdata, handles)
   rectpos = cell2mat(arrayfun(@getPosition, handles.hrectangles, 'UniformOutput', false));  
   dlmwrite('somefile.txt', rectpos); %or however you want to save the rectangles
end

[Naseer Khan]

I am getting following error.

While converting from 'imrect':
 The conversion returned a value of class 'matlab.mixin.Heterogeneous', which cannot be inserted into an array of class
 'matlab.graphics.GraphicsPlaceholder'.

Error in draggable>computeButton_Callback (line 107)
    handles.hrectangles(i) = imrect(gca, box);    %store rectangle handles

Error in gui_mainfcn (line 95)
        feval(varargin{:});

Error in draggable (line 42)
    gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)draggable('computeButton_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating UIControl Callback

[Guillaume]

For some reason, I thought the rectangles created by imrect were graphics objects (which can be stored in a gobject array), but no, they're their own class.

I don't think you can preinitialise an array of rectangle objects, so the workaround is to either grow the array or use a cell array:

handles.hrectangles = cell(numel(regions), 1);    %allocate cell array for storing for rectangle handles
for i = 1:numel(regions)
   box = regions(i).BoundingBox;
   handles.hrectangles{i} = imrect(gca, box);    %store rectangle handles in cell array
end
guidata(hObject, handles);  %save handles structure

In the event handler, just replace arrayfun by cellfun:

function SaveRectButton_Callback(hObject, eventdata, handles)
   rectpos = cell2mat(cellfun(@getPosition, handles.hrectangles, 'UniformOutput', false));  
   dlmwrite('somefile.txt', rectpos); %or however you want to save the rectangles
end

[Naseer Khan]

This is working well Now but only with one issue now.if I delete or add a rectangle(s) then it shows following error.

Invalid or deleted object.

Error in imrect/getPosition (line 110)
            pos = obj.api.getPosition();

Error in draggable>saveButton_Callback (line 144)
rectpos = cell2mat(cellfun(@getPosition, handles.hrectangles, 'UniformOutput', false));

Error in gui_mainfcn (line 95)
        feval(varargin{:});

Error in draggable (line 42)
    gui_mainfcn(gui_State, varargin{:});

Error in @(hObject,eventdata)draggable('saveButton_Callback',hObject,eventdata,guidata(hObject))

[Guillaume]

Indeed, if a rectangle is deleted, then its handle is no longer valid, and getPosition is not going to be happy. The simplest thing is to remove these deleted handles from the cell array:

hvalidrects = handles.hrectangles(cellfun(@isvalid, handles.hrectangles));  %remove deleted handles
rectpos =  cell2mat(cellfun(@getPosition, hvalidrects, 'UniformOutput', false));  %get position of valid rectangles

If a rectangle is added, you need to add it to cell array in whichever function is creating this new rectangle.

Answer 2

[Rajeev Yadav]

Can we add ARROW KEYS (UP ARROW, RIGHT ARROW, LEFT ARROW,DOWN ARROW) with fixed step size control to move the rectangle?