Why does updating a simple graphics object take more time if there is another graphics object in the same figure with a large number of points in MATLAB?

I use the code below to move a red line across a waveform. If I increase tmax from 1e3 to 1e6, it take an extremely long time for the red line to move across the plot. It is gets slower even though the number of times I update the red line is constant.

Changing the renderer does not help. Setting the NextPlot property for the figure and axes to 'add' also does not help.

tmax = 1e3;

t = 1:1:tmax;

%%Frequency Components

f1 = 2000;

f2 = 3500;

f3 = 5000;

f4 = 6500;

f5 = 9000;

%%Input Signal

y = sin(2*pi*f1*t)+...

      sin(2*pi*f2*t)+...

      sin(2*pi*f3*t)+...

      sin(2*pi*f4*t)+...

      sin(2*pi*f5*t);

%%Draw the Signal

haxes1 = axes;

set(gcf,...

      'Numbertitle', 'off',...

      'Name', 'Sound Player',...

      'CurrentAxes',haxes1)

plot(t,y)

drawnow;

set(haxes1,'NextPlot','add');

%%Draw the moving line marker.

ind_res = 500;

step = (max(t)-min(t))/ind_res;

axis manual;

htemp = line([1 1], [-1 1], 'Color', 'r');

drawnow;

for ii = step:step:max(t)

      set(htemp,'XData',[ii ii]);

      drawnow;

end

Answer 1

[MathWorks Support Team]

This is expected behavior. As the number of points increases for a graphics objects in the figure, updating another graphics object in the figure will slow down.

One workaround is to set the EraseMode property to 'xor' for the object that is being updated. This should speed the operations up significantly because it operates on pixels instead of coordinates. For example, use the following command to define the red line:

htemp = line([1 1], [-1 1], 'Color', 'r', 'erasemode','xor');