User input during animation

I have created an animation that graphs the motion of a test particle under the influence of a force field in 2D and I want to add a user interface. I'd like the user to be able to nudge the test particle by using the keyboard, e.g. the arrow keys. I'd like the user input to be interpreted as a variable in the program, so it can be added to the internal variables that are being graphed. I'd like this to happen without interrupting the animation. How might this be possible to accomplish in Matlab?

Ted

[Jan]

It depends on how you have implemented the animation yet. Is it an animated GIF, a figure or uifigure with a loop? Is it a simulink application?

[Ted Theodosopoulos]

My animation is based on a simple for-loop with short pauses between the plotting of individual points of a trajectory. I'd like the user to be able to influence this trajectory, as they are watching it play out. Does this make sense?

Ted

Answer 1

[Walter Roberson]

Use a WIndowsKeyPressFcn callback, and check the event information to figure out which key was pressed in order to figure out what changes to make to your variables. Use one of the methods of sharing variables to get the data to the compute loop.

Your compute loop will need to periodically pause() or drawnow() to receive the interrupts, but it has to use one of those to make the animation visible anyhow.

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

[Ted Theodosopoulos]

Thank you for this information. Unfortunately, I am not familiar with the callback functionality in Matlab. Could you possibly send me a short example code that shows how to use it? You may assume that we are in a loop, we've just plotted a point in a numbered figure (say figure(1)) and that we are in a timed pause (e.g. pause(0.05)) as part of the animation. What command can I use that will return the button, if any, that the user has just pressed? Do I need to write a subroutine that accomplishes that? If so, what should it look like? Thanks so much for your continued support with this inquiry.

Ted

[Walter Roberson]

function fig1_key_callback(hObject, event, handles)
   xv = handles.particle_xv;
   yv = handles.particle_yv;
   switch event.Key
       case 'leftarrow'
           xv = xv - 5;
       case 'rightarrow'
           xv = xv + 5;
       case 'uparrow'
           yv = yv + 5;
       case 'downarrow'
           yv = yv - 5;
       case 'q'
           handles.quit = true;
       otherwise
           %do nothing
   end
   handles.particle_xv = xv;
   handles.particle_yv = yv;
   guidata(hObject, handles)
end

function processing_loop(hObject, event, handles)
   %stuff
   handles.x = x0; handles.y = y0; handles.quit = false;
   handles.xv = randi([-10 10]); handles.yv = randi([-10 10]);
   guidata(hObject, handles);
   
   h = animatedline(handles.x, handles.y);
   while true
      handles = guidata(hObject);   %pull back updates to handles
      if handles.quit
         handles.quit = false;
         break;
      end
      handles.x = handles.x + handles.xv;
      handles.y = handles.y + handles.yv;
      addpoints(h, handles.x, handles.y);
      guidata(hObject, handles);
      pause(0.05);
   end
end

In this demonstration, the user does not control the x or y position directly, and instead affects the velocity, nudging it higher or lower (including negative).