matlab视频处理代码理解小问题

[沧海msa]

想要问一下，if里面的pixels= double(cat(4,avi(1:frameSkip:end).cdata))是干啥的，frameSkip=1.
if ischar(video)
% Load the video from an avi file.
avi = VideoReader(video);
matrix
pixels = double(cat(4,avi(1:frameSkip:end).cdata))/255;
clear avi
else
% Compile the pixel data into a single array
pixels = double(cat(4,video{1:frameSkip:end}))/255;
clear video
end;