Animate a MATLAB figure by pressing left and right arrow keys - matlab

Question

I am trying to change the radius of a sphere by using left and right arrow keys, plotted in a MATLAB figure. For example by pressing right arrow, the value of radius increases by one and then plot the new sphere with the updated radius. Similarly, pressing left arrow key decreases the radius by one and then plots a smaller sphere. However, I want this change to be limited between 1 and rmax.
I got some ideas of how I can approach after reading this post but still it is not what I am looking for. Therefore, I used two global variables to achieve this task in order to somehow pass the information by reference to KeyPressFcn so when a key is pressed KeyPressFcn know what those limits are. In the example below, the code does increase and decrease the radius by one but it is it does not restrict the change of radius within the specified range after left and right arrows are hit.
Is there a better way of approaching this? How can I pass the value of radius and its limits to KeyPressFcn? I want KeyPressFcn to know how much it can change the radius when left and right arrows are pressed.
function animateme()
fig_h = figure;
set(fig_h,'KeyPressFcn', #key_pressed_fcn);
global r rmax
p0 = [0 0 0];
[x,y,z] = sphere;
rmax = 10;
r = 1;
while 1==1
h = surf(x*r+p0(1), y*r+p0(2), z*r+p0(3));
set(h, 'FaceAlpha', 0.5, 'FaceColor', rand([1 3]))
axis equal;
pause
end
function key_pressed_fcn(fig_obj, eventDat)
global r rmax
if strcmpi(eventDat.Key, 'rightarrow')
r = r + 1;
if r < 1
r = 1;
end
elseif strcmpi(eventDat.Key, 'leftarrow')
r = r - 1;
if r > rmax
r = rmax;
end
end
disp(r)

Answer 1

First of all, don't use global variables as there is (almost) always a better way of accomplishing the same thing.
Here is an example by using nested functions which automatically have access to the variables within the parent function's workspace.
function animateme()
fig = figure();
hax = axes('Parent', fig);
set(fig, 'KeyPressFcn', #keypress)
p0 = [0,0,0];
[x,y,z] = sphere();
% Specify limits here which are accessible to nested functions
rmax = 10;
r = 1;
h = surf(x,y,z, 'Parent', hax);
% Subfunction for re-plotting the data
% This prevents you from needing a while loop
function redraw()
set(h, 'XData', x * r + p0(1), ...
'YData', y * r + p0(2), ...)
'ZData', z * r + p0(3));
set(h, 'FaceAlpha', 0.5, ...
'FaceColor', rand([1 3]))
axis(hax, 'equal')
drawnow
end
% Go ahead and do the first redraw
redraw();
% Callback to process keypress events
function keypress(~, evnt)
switch lower(evnt.Key)
case 'rightarrow'
r = min(r + 1, rmax);
case 'leftarrow'
r = max(1, r - 1);
otherwise
return
end
% Always do a redraw
redraw();
end
end
Another option is to store the current value of r within the graphics objects themselves using the UserData field. So you could put it in the surf plot itself. This is actually my preferred method because then your callback function can live anywhere and still have access to the data it needs.
function animateme()
fig = figure();
% Data to store for plotting
data.p = [0,0,0];
data.r = 1;
data.rmax = 10;
% Create a blank surface for starters
h = surf(nan(2), nan(2), nan(2));
set(h, 'UserData', data);
% Update the display of the surface
redraw(h);
% Set the callback and pass the surf handle
set(fig, 'KeyPressFcn', #(fig, evnt)keypress(h, evnt))
end
function redraw(h)
% Get the stored data from the graphics object
userdata = get(h, 'Userdata');
[x,y,z] = sphere();
set(h, 'XData', x * userdata.r + userdata.p(1), ...
'YData', y * userdata.r + userdata.p(2), ...
'ZData', z * userdata.r + userdata.p(3));
set(h, 'FaceAlpha', 0.5, ...
'FaceColor', rand([1 3]))
axis equal
drawnow;
end
function keypress(h, evnt)
% Get the stored data
userdata = get(h, 'Userdata');
switch lower(evnt.Key)
case 'rightarrow'
userdata.r = min(userdata.r + 1, userdata.rmax);
case 'leftarrow'
userdata.r = max(1, userdata.r - 1);
otherwise
return;
end
% Update the stored value
set(h, 'UserData', userdata);
redraw(h);
end