Try this:
% Uses blockproc() to get RMS of image blocks.
% Demo code to divide the image up into 8 pixel by 8 pixel blocks
% and replace each pixel in the block by the RMS
% of all the gray levels of the pixels in the block.
%
clc;
clearvars;
close all;
workspace;
fontSize = 16;
% Read in a standard MATLAB gray scale demo image.
folder = fileparts(which('cameraman.tif'));
if ~exist(folder, 'dir')
% If that folder does not exist, don't use a folder
% and hope it can find the image on the search path.
folder = [];
end
baseFileName = 'cameraman.tif';
fullFileName = fullfile(folder, baseFileName);
grayImage = imread(fullFileName);
% Get the dimensions of the image. numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(grayImage)
% Display the original gray scale image.
subplot(1, 2, 1);
imshow(grayImage, []);
caption = sprintf('Original Grayscale Image\n%d rows by %d columns', rows, columns);
title(caption, 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Position', get(0,'Screensize'));
set(gcf,'name','Image Analysis Demo','numbertitle','off')
% Define the function that we will apply to each block.
% First in this demo we will take the rms gray value in the block
% and create an equal size block where all pixels have the RMS value.
% Image will be the same size since we are using ones() and so for each block
% there will be a block of 8 by 8 output pixels.
% rmsFilterFunction = @(theBlockStructure) rms(theBlockStructure.data(:)) * ones(size(theBlockStructure.data), class(theBlockStructure.data));
rmsFilterFunction = @(theBlockStructure) rms(theBlockStructure.data(:));
% Block process the image to replace every pixel in the
% 8 pixel by 8 pixel block by the RMS of the pixels in the block.
blockSize = [8 8];
blockyImage8 = blockproc(single(grayImage), blockSize, rmsFilterFunction); % Works.
[rows, columns] = size(blockyImage8);
% Display the block image.
subplot(1, 2, 2);
imshow(blockyImage8, []);
caption = sprintf('Block RMS Image\n32 blocks. Input block size = 8, output block size = 8\n%d rows by %d columns', rows, columns);
title(caption, 'FontSize', fontSize);
