Beginner issue with GCO logic

Hello, I am a student new with Matlab and I am learning to work with mouse input.

My objective is to make it so that when the figure is clicked, a new shape is created,

however when an already existing shape is clicked, no new shape is created but instead the

existing one is deleted. I wrote the following code below and I am really unsure as to why the

else statement executes regardless. I can seemingly only delete the previously created shape instead of

any on the scene. I would really appreciate if you could point me in the right direction as I have

been stuck on this for a few days now.

Kindest regards!

Here is the code:

clear, clc, close all

fig = figure('color','k');

ax = axes('Position',[0 0 1 1])
hold on
axis([0 5 0 5],'off')
h = 0;

while true
    waitforbuttonpress
    if ~isempty(h) & gco == h
      delete(h);
    else
      xy = get(gca,'CurrentPoint'); 
      h = fill(xy(1,1) + rand(1,8)-0.5, xy(1,2) + rand(1,8)-0.5,
      rand(1,3),'EdgeColor','w'); % The random colours
    end
end

Answer 1

[Geoff Hayes]

Jakub - look closely at the code

if ~isempty(h) & gco == h
    delete(h);
else
    xy = get(gca,'CurrentPoint'); 
    h = fill(xy(1,1) + rand(1,8)-0.5, xy(1,2) + rand(1,8)-0.5, ...
        rand(1,3),'EdgeColor','w'); % The random colours
end

Since h is being used to delete the graphics object (the fill) and it assigned the last created fill object, then the only time that you can potentially delete an object is if it were just added (I think that I was able to do this once or twice in a dozen or more tries). What you really want to do is delete the object when the mouse pointer hovers over it while you press the mouse button. This can be accomplished using the hit test which determines if the object can respond to the mouse click it captures or passes the click to its closest ancestor. It seems that this property is enabled (on) by default so you just need to add a callback to the fill objects and the axes to do what you want.

function HitTestExample

    close all
    fig = figure;
    ax = axes('Position', [0 0 1 1], 'color', 'k');  % <-- added colour to axes
    hold on
    axis([0 5 0 5])                                  % <-- removed 'off' so visible

    set(fig, 'HitTest', 'off');
    set(ax, 'ButtonDownFcn', @OnAxesButtonDown,'HitTest','on')

    % Graphics Object (GO) button down callback
    function OnGOButtonDown(hObject, eventdata)
        delete(hObject);
    end

    % Axes button down callback
    function OnAxesButtonDown(hObject, eventdata)
        xy = get(hObject,'CurrentPoint'); 
        fill(xy(1,1) + rand(1,8)-0.5, xy(1,2) + rand(1,8)-0.5,rand(1,3), ...
          'EdgeColor', 'w', 'ButtonDownFcn', @OnGOButtonDown); % The random colours
    end

end

We ensure that the 'HitTest' is 'on' for the axes and 'off' for the figure. When the mouse button is pressed over the axes (but not over a child of the axes), then the OnAxesButtonDown is called and a new shape is created. When the mouse button is pressed over a graphics object, then the OnGOButtonDown is called and we delete the object.

[Jakub Klimiuk]

Thank you for this, it is very helpful to me and I understand where my mistake is, however do you know if there is any way of achieving this without functions and callbacks to the functions? I am currently using Octave rather than Matlab as we only have Matlab in university and I cannot access it due to Covid. Octave currently hasn't implemented callbacks to nested functions I believe. The code gives me an error.

[Geoff Hayes]

What is the error?

[Jakub Klimiuk]

This is the error when I remove the main function:

error: @OnAxesButtonDown: no function and no method found
error: called from
    Part2 at line 9 column 5
error: evaluating argument list element number 3
error: called from
    Part2 at line 9 column 5
>>

This is the code when I leave your code as it is and try to run:

error: handles to nested functions are not yet supported
error: called from
    Part2 at line 11 column 5
error: evaluating argument list element number 3
error: called from
    Part2 at line 11 column 5
>>

[Geoff Hayes]

The nested functions aren't necessary (just convenient). Try putting all three functions in the same file as

function HitTestExample
    close all
    fig = figure;
    ax = axes('Position', [0 0 1 1], 'color', 'k');  % <-- added colour to axes
    hold on
    axis([0 5 0 5])                                  % <-- removed 'off' so visible
    set(fig, 'HitTest', 'off');
    set(ax, 'ButtonDownFcn', @OnAxesButtonDown,'HitTest','on')
end

% Graphics Object (GO) button down callback
function OnGOButtonDown(hObject, eventdata)
    delete(hObject);
end

% Axes button down callback
function OnAxesButtonDown(hObject, eventdata)
    xy = get(hObject,'CurrentPoint'); 
    fill(xy(1,1) + rand(1,8)-0.5, xy(1,2) + rand(1,8)-0.5,rand(1,3), ...
      'EdgeColor', 'w', 'ButtonDownFcn', @OnGOButtonDown); % The random colours
end

[Jakub Klimiuk]

This works perfectly, thank you tons for your time and effort helping me with this! I can finally progress with my assignment. Im gonna have a further read on this and where the eventdata actually comes from. I had no idea this could be so simple.

[Geoff Hayes]

Glad to have been able to help, Jakub!