VIDEO WRITER and imagesc coding

hello guys  I am running into problem to create this video sequence of the str images as mp4 file.L = 

1×6 Image array:

      Image    Image    Image    Image    Image    Image

F = 

    1×501 struct array with fields:

      cdata
      colormap

Warning: No video frames were written to this file. The file may be invalid. 
> In VideoWriter/close (line 267)
  In VideoWriter/delete (line 202)
  In patientdata (line 35) 
Error using VideoWriter/writeVideo (line 369)
IMG must be of one of the following classes: double, single, uint8

Error in patientdata (line 37)
writeVideo(v,L)

x=double(RPread('08-33-19.rf'));

t=1

% for i=1:size(x,3)
for i=1:3
%     for j=1:size(x,3)
       for j=1:3
        if(i~=j)

              b(k)=a(i);
              c(l)=a(j);
              rf1=x(:,:,i);
              rf2=x(:,:,j);
              wsize1=128;
              wsize2=192;
              wshift=32;
              app_strn=0.01;
              min_strn=0;
              max_strn=2*app_strn;
              [str,dsp,ccf] = EstStrn(rf1,rf2,wsize1,wsize2,wshift,app_strn,min_strn,max_strn,'lsus');
              L(t)=imagesc(str)
               pause(20)
              title(['pre frame=', num2str(i),'post frame',num2str(j)]);
              F(t)=getframe(gcf)
              pause(1)
              t=t+1;
          else
              continue
          end
      end

end

v = VideoWriter('newfile2.mp4');
open(v)
writeVideo(v,L)
close(v)