Play multiple videos in one subplot

Hi,

I'm trying to play multiple avi videos in one subplot, and borrowed online code but always showing me the following error:

Error using VideoReader/read (line 134)
Cannot call READ method after using READFRAME or HASFRAME methods or setting the CURRENTTIME property. Recreate the object to read a frame at a
specific frame index.

Error in videoplay (line 18)
              v1frame = v1.read(i1);

And here is the code:

clc;clear all;
v1= VideoReader('\\cab-fs07.mae.virginia.edu\NewData\NHTSA\2016-AutomatedVehicle\1Simulation\Project\RESULTV3.5\M50-OS_Standard_Reclined_0_Int_0_RMDB\1_cut.avi');
v1.CurrentTime = 1.5;
v1_numFrames = ceil(v1.FrameRate*v1.Duration);
v2= VideoReader('\\cab-fs07.mae.virginia.edu\NewData\NHTSA\2016-AutomatedVehicle\1Simulation\Project\RESULTV3.5\M50-OS_Standard_Reclined_0_Int_0_RMDB\2_top.avi');
v2.CurrentTime = 1.5;
v2_numFrames = ceil(v2.FrameRate*v2.Duration);
i1 = 0;
i2 = 0;
fig = gcf;
ax1 = subplot(2,2,1, 'Parent', fig);
ax2 = subplot(2,2,2, 'Parent', fig);
currAxes = axes;

while i1 < v1_numFrames && i2 < v2_numFrames
          if i1 < v1_numFrames
              i1 = i1+1;
              v1frame = v1.read(i1);
              if isempty(v1frame)   %unexpected!
                warning(sprintf('skipped v1 empty frame #%d', i1));
              elseif ishandle(ax1)
                image(v1frame, 'Parent', ax1);
              else
                break;    %axes is gone, figure is probably gone too
             end
          end
          if i2 < v2_numFrames
              i2 = i2+1;
              v2frame = v2.read(i2);
              if isempty(v2frame)   %unexpected!
                warning(sprintf('skipped v2 empty frame #%d', i2));
              elseif ishandle(ax2)
                image(v2frame, 'Parent', ax2);
              end
          end
      drawnow
      end

Thanks

Answer 1

[Walter Roberson]

You use v1.read(i1) after v1.CurrentTime = 1.5 . You can only use read() if you have never set the CurrentTime. You will need to switch to readFrame()

Warning: multiplying framerate by duration has round-off error even for fixed framerates, and is just plain wrong for variable frame rates. The only way to accurately know the number of frames is to loop reading frames and counting them.

I recommend that you do not bother to try to estimate frames and instead use a structure like

while true
  got_one = false;
  had_problem = false;
  if hasFrame(v1)
    v1frame = readFrame(v1);
    if isvalid(ax1)
      image(v1frame, 'Parent', ax1);
      got_one = true;
    else
      had_problem = true;
    end
  end
  if had_problem; break; end

    if hasFrame(v2)
      v2frame = readFrame(v2);
      if isvalid(ax2)
        image(v2frame, 'Parent', ax2);
        got_one = true;
      else
        had_problem = true;
      end
    end
    if had_problem; break; end

    if ~got_one; break; end
    drawnow();
  end

Except that I would recommend changing the CData of the image objects instead of using image() calls, for performance reasons.

[Hongnan Lin]

Thanks Walter! It's really helpful. However, I have another problem which is if I don't Ctrl+C, the code will never stop running and were displayed. Is there any way to solve it? Thanks!

[Walter Roberson]

The got_one logic is designed to end as soon as both objects run out of frames. Could you do a bit of debugging to see which of the two objects is thinking it has remaining frames? The code I posted should end as soon as the longer video ends.

[Hongnan Lin]

Thanks for the reply Walter. Actually the videos I want to play are of the same frame numbers.

[Walter Roberson]

When you use the debugger, is hasFrame(v1) or hasFrame(v1) returning true?

Answer 2

[Hongnan Lin]

Thanks Walter!

Here is another problem:

As attached is the figure to show, I'd rather hide all the axis however, there are only axis in the last plot hidden.

Following is the code:

while true 
  thisFrame = getframe(gcf);
     i1 = i1+1
     got_one = false ; 
     had_problem = false ; 
     if hasFrame(v1) 
        v1frame = readFrame(v1); 
        if isvalid(ax1) 
           image(v1frame, 'Parent', ax1); 
           axis off;
           got_one = true ; 
        else 
           had_problem = true ; 
       end 
     end 
     if had_problem; 
         break; 
     end

    if hasFrame(v2) 
       v2frame = readFrame(v2); 
       if isvalid(ax2) 
       image(v2frame, 'Parent', ax2); 
       axis off;
       got_one = true ; 
       else 
       had_problem = true ; 
       end 
    end 
    if had_problem; 
        break;
    end;
    if ~got_one;
        break; 
    end

    image(v3,'Parent', ax3);%for .png
    axis off;
  %   set(gcf,'PaperUnits','inches','Position',[1 1 1500 400]);

    writeVideo(v, thisFrame);

Thanks

Lin

[Walter Roberson]

   axis(ax1, 'off');

and

axis(ax2, 'off');

and

axis(ax3, 'off')

Note: it is more efficient to update the CData property of an existing image than to image() and axis()