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Avoid Updating Static Data

If only a small portion of the data defining a graphics scene changes with each update of the screen, you can improve performance by updating only the data that changes. The following example illustrates this technique.

Code with Poor PerformanceCode with Better Performance

In this example, a marker moves along the surface by creating both objects with each pass through the loop.

[sx,sy,sz] = peaks(500);
nframes = 490;

for t = 1:nframes
   surf(sx,sy,sz,'EdgeColor','none')
   hold on
   plot3(sx(t+10,t),sy(t,t+10),...
         sz(t+10,t+10)+0.5,'o',...
        'MarkerFaceColor','red',...
        'MarkerSize',14)
   hold off
   drawnow
end

Create the surface, then update the XData, YData, and ZData of the marker in the loop. Only the marker data changes in each iteration.

[sx,sy,sz] = peaks(500);
nframes = 490;

surf(sx,sy,sz,'EdgeColor','none')
hold on
h = plot3(sx(1,1),sy(1,1),sz(1,1),'o',...
   'MarkerFaceColor','red',...
   'MarkerSize',14);
hold off

for t = 1:nframes
   set(h,'XData',sx(t+10,t),...
      'YData',sy(t,t+10),...
      'ZData',sz(t+10,t+10)+0.5)
   drawnow
end

Segmenting Data to Reduce Update Times

Consider the case where an object’s data grows very large while code executes in a loop, such as a line tracing a signal over time.

With each call to drawnow, the updates are passed to the renderer. The performance decreases as the data arrays grow in size. If you are using this pattern, adopt the segmentation approach described in the example on the right.

Code with Poor PerformanceCode with Better Performance
% Grow data
figure('Position',[10,10,1500,400])
n = 5000;


h = stairs(1,1);
ax = gca;
ax.XLim = [1,n];
ax.YLim = [0,1];
ax.ZLim = [0,1];
ax.NextPlot = 'add';

xd = 1:n;
yd = rand(1,n);

tic
for ix = 1:n
   set(h,'XData',xd(1:ix),'YData',yd(1:ix));
   drawnow;
end
toc
% Segment data
figure('Position',[10,10,1500,400])
n = 5000;
seg_size = 500;
xd = 1:n;
yd = rand(1,n);

h = stairs(1,1);
ax = gca;
ax.XLim = [1,n];
ax.YLim = [0,1];
ax.ZLim = [0,1];
ax.NextPlot = 'add';

tic
start = 1;
for ix=1:n
   % Limit object size
   if (ix-start > seg_size)
      start = ix-1;
      h = stairs(1,1);
   end
   set(h,'XData',xd(start:ix),...
      'YData',yd(start:ix));
   % Update display in 50 point chunks
   if mod(ix,50) == 0
      drawnow;
   end
end
toc

The performance of this code is better because the limiting factor is the amount of data sent during updates.