How can I plot line iteratively on an existing movie frame?

Hi,

I want to visualize the trajectory of a moving ball in a movie. I obtain the centroid of the ball in each frame. I want to show a line path on the movie while the movie continues playing. Please suggest me a way to perform this?

My Code:

AviFrames; % This creates the movie as a struct in the workspace.
numF = evalin('base','numberOfFrames'); % Getting the number of frames in the movie
film = evalin('base','mov'); % Assigning the movie to "film" variable

initial_xy = [57.7411 35.5395]; % intitial cooridinates of the ball
p1b = [initial_xy(1,1),initial_xy(1,2)];

xs = zeros(numF, 2); % x coorinates of ball in the consecutive frames
ys = zeros(numF, 2); % y coorinates of ball in the consecutive frames

for i=1:numF

      image = Struct2Image(i,film); % Converting struct to image (ith frame)
      imshow(image);
      [cx, cy, r] = CemberPoz(image); % Getting regionprops of the ball in the current image

      cpos = [cx cy];
      p2b = [cpos(1,1),cpos(1,2)];

      pb = [p1b p2b];

      bgx(i,:) = pb(1:2:3);
      bgy(i,:) = pb(2:2:4);

      p1b = p2b;

      line([bgx(i,1) bgx(i,2)],[bgy(i,1) bgy(i,2)],'color','b','LineWidth',2);
      hold on
      axis([0 320 0 240])
      hold off
  end

Here is one screen capture:

Thanks in advance and kind regards, Tugrul Tasci

Answer 1

[Image Analyst]

Your screenshot didn't come through. And you need to not take hold off or else your first line will blow away all others.

[Walter Roberson]

"hold" has no effect on line().

The poster is continually drawing new images and drawing new lines, so it is fine (other than from a performance point of view) to have the old contents continually blown away.

It is, however, not at all clear to me why the axis() command is there: in that context, it is going to create a new axis rather than resize the existing axis.

Answer 2

[Tugrul TASCI]

Thanks for your answers. I changed my code as below and now I succeed what I want.

AviFrames;
numF = evalin('base','numberOfFrames');
film = evalin('base','mov');

initial_xy = [57.7411 35.5395];
p1b = [initial_xy(1,1),initial_xy(1,2)];

fig = figure('Position',[600 400 640 480]);
set(fig,'DoubleBuffer','on');    
set(gca,'xdir','normal');
set(gca,'ydir','reverse');

axis([0 320 0 240]);
axis on

bgx = [];
bgy = [];

for i=1:numF
  imge = Struct2Image(i,film);
  imshow(imge);
  hold on

    [cx, cy, r] = CemberPoz(imge);  

    cpos = [cx cy];
    p2b = [cpos(1,1),cpos(1,2)];

    pb = [p1b p2b];

    bgx = [bgx pb(1:2:3)];
    bgy = [bgy pb(2:2:4)];

    p1b = p2b;

    plot(bgx,bgy);
    drawnow;
    hold off

end

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(I am not sure if this URL works) You can preview the first 10 seconds of the output movie.

https://docs.google.com/file/d/0B_MKQZgTJqehN21EdGRpcG1uTXc/preview MathWorks