'struct'类型的输入参数的未定义函数'preview'

Question

我在框架上使用Back round减法方法。现在我想查看这些框架的预览。但是出现错误“未定义函数'预览'用于'struct'类型的输入参数这是代码：

for i = 2:numFrames

  %fr = vid(i).cdata;
  % read in frame 
  fr= read(vid, i);

  fr_bw = rgb2gray(fr);       % convert frame to grayscale 
  fr_diff = abs(double(fr_bw) - double(bg_bw));  % cast operands as double to avoid negative overflow 
  threshHold=mean(fr_diff);%Experimetn
  for j=1:width                 % if fr_diff > thresh pixel in foreground 
      for k=1:height 
          if ((fr_diff(k,j) > threshHold)) 
              fg(k,j) = fr_bw(k,j); 
          else 
              fg(k,j) = 0; 
          end 
      end 
  end
  bg_bw = fr_bw; %store old frame to bg_bw variable

  imshow(fr_bw);
  figure(1),subplot(3,1,1),imshow(fr) 
  subplot(3,1,2),imshow(fr_bw) 
  subplot(3,1,3),imshow(uint8(fg)) ;
  z=i-1;

  q(z) = im2frame(uint8(fg),gray(256));            % put frames into movie
  preview((q(z))); 

end