How can I change movie to gray (frame by frame)?

I know there are probably easier methods but it must be frame by frame

this is what I have:

movi = VideoReader('xylophone.mpg');

nFrames = movi.NumberOfFrames;

vidHeight = movi.Height;

vidWidth = movi.Width;

mov(1:nFrames) = ... struct('cdata', zeros(vidHeight, vidWidth, 3, 'uint8'),... 'colormap', []);

for k = 1 : nFrames

   poj_ramka=mov(k).cdata;

   funkcja=zad(poj_ramka);

   kraw=uint8(funkcja)*255;

   kraw=cat(3,kraw,kraw,kraw);

   mov(k)=struct('cdata',kraw,'colormap',[]);

end

okno=figure;

set(okno,'position',[150 150 vidWidth vidHeight]);

movie(okno,mov,1,25);

and zad.m file:

function[obraz]=zad(poj_ramka)

    obraz=rgb2gray(poj_ramka);

end

what i get is a black window :/

Answer 1

[Image Analyst]

Why not just use rgb2gray() and im2frame():

kraw = rgb2gray(poj_ramka);
mov(k)= im2frame(kraw);