which type of correlation to use?

i've a grayscale image and i've divided it into discrete blocks of 8x8

Then, i've calculated the mean of each block and i want to check how one block is correlated to the other block

which type of correlation should i use?

 xcorr2

 corr2

 or something else???

i want the correlation of two image blocks in the range -1, 0, 1

can anybody tell me how to do so?

i've used corrcoef

    Error using bsxfun
Mixed integer class inputs are not supported.

 Error in cov (line 93)
  xc = bsxfun(@minus,x,sum(x,1)/m);  % Remove mean

 Error in corrcoef>correl (line 209)
r = cov(x);

 Error in corrcoef (line 92)
   r = correl(x);

 93    xc = bsxfun(@minus,x,sum(x,1)/m);  % Remove mean

but the above error occurs.