The structfun documentation clearly states that the input structure must be a scalar structure. Your structure has size 1x660, so it is definitely not a scalar structure.
However there are other simple ways to apply an operation to each element of your non-scalar structure. Lets start by defining some sample data:
% fake data in a non-scalar structure:
S(1).cdata = randi([0,255],10,20,3,'uint8');
S(2).cdata = randi([0,255],10,20,3,'uint8');
S(3).cdata = randi([0,255],10,20,3,'uint8');
S(4).cdata = randi([0,255],10,20,3,'uint8');
tmp = cellfun(@rgb2gray,{S.cdata},'Uni',0);
out = cat(3,tmp{:});
Or you could use arrayfun to perform some operation on each element of the structure (remember that each element is itself a scalar structure):
tmp = arrayfun(@(s)rgb2gray(s.cdata),S,'UniformOutput',false);
out = cat(3,tmp{:});
Note that if you don't have rgb2gray, then use this:
vec = reshape([0.2989,0.5870,0.1140],1,1,[]);
rgb2gs = @(rgb)sum(bsxfun(@times,vec,double(rgb)),3);
Also note these might be considered to be elegant solutions, but a loop is likely to be faster.
