How can I toggle a transparent image overlaid on another image?

Within my GUI, I have an image on a set of axes. I need to be able to toggle a 3x3 grid of points on and off, so that they display overtop of the image when a checkbox is checked, and disappear when the checkbox is unchecked.

In order to achieve this I tried to set the AlphaData matrix to the appropriate alpha matrix when the checkbox is ticked, and to a zero matrix of the same size when unchecked.

 function targetGridCheck_Callback(hObject, eventdata, handles)
    [a,map,alpha] = imread('dot-grid-3x3_768x768.png');
    I = imshow(a,'Parent',handles.axes2);
    if (get(hObject,'Value') == get(hObject,'Max'))
      set(I,'AlphaData',alpha);
    else
       set(I,'AlphaData',zeros(size(I)));
 end

With this code, ticking the checkbox shows the grid, but also vanishes the background image. When the checkbox is unticked the background image stays gone (the axes are completely blank).

How do I keep the background image in all cases?

Answer 1

[Image Analyst]

Why not just clear the lines using a function like this:

%=====================================================================
% Erases all lines from the image axes h (which could be gca).
function ClearLinesFromAxes(h)
axesHandlesToChildObjects = findobj(h, 'Type', 'line');
if ~isempty(axesHandlesToChildObjects)
  delete(axesHandlesToChildObjects);
end
return; % from ClearLinesFromAxes

Then just draw your 3x3 grid again with line() when you want to show it again. That's what I'd do.

[Daphne Walford]

Thanks for the answer. That's definitely tempting. The grid I need is a grid of nine points, for which I've been using a PNG with a transparent background (my attempts at plotting the points all failed horrifically). May I ask if line() can be used to draw points? My impression was that it cannot.

[Image Analyst]

plot() can draw points. Just don't specify a line style

plot(x, y, 'b.', 'MarkerSize', 20); % plot blue dots