How to lower resolution of a picture?

Write a program to upload an image in Matlab and reduce its size averaging a number (NxN) of pixels specified by the user. For example, if the image is 6x12 pixels and the user specifies N = 3, the new image will be 2x4 pixels and it will get averaging squares 3x3 pixels as shown in the figure. If the user specifies a value of N that is not a common divisor of In the 2 dimensions of the figure, the program should approximate N to the closest common divisor. In the title of his figure specify what was the value of N specified by the user and what was the value that was finally used.

[Edgar Rivera]

Sorry. Here are the pictures

Answer 1

[Image Analyst]

There is no figure attached.

Use imresize to specify the final output image size

outputImage = imresize(inputImage, [2, 4]);

or

[rows, columns, numColorChannels] = size(inputImage)
numOutputRows = round(rows/N);
numOutputColumns = round(columns/N);
outputImage = imresize(inputImage, [numOutputRows, numOutputColumns]);

if you have the N value from your user.

[Casey]

imresize doesn't perform the averaging as requested. For example,

A =

0 0 0 0 0 1

0 0 0 0 0 0

0 0 0 0 0 0

0 1 1 1 0 0

B = imresize(A,[3,2])

Yields

B =

-0.0218 -0.0716 0.2965

0.2542 0.5384 0.0043

Whereas by simple averaging it should be

0 0 .25

.25 .5 0

[Casey]

erfan's answer on this site seems to perform as requested:

https://stackoverflow.com/questions/39975720/matlab-scale-down-an-image-using-an-average-of-four-pixels

Answer 2

[Image Analyst]

You can use blockproc to move in "jumps" of 2 pixels:

A = [
    0   0   0   0   0   1
    0   0   0   0   0   0
    0   0   0   0   0   0
    0   1   1   1   0   0];
%===============================================================================================================================
% MEAN of 2 pixel by 2 pixel block
% Block process the image to replace every pixel in the 
% 2 pixel by 2 pixel block by the mean of the pixels in the block.
% Image will be smaller since we are not using ones() and so for each block
% there will be just one output pixel, not a block of 2 by 2 output pixels.
% Define a filter function that will give 1 pixel as output for every 2-D block of input.
meanFilterFunction = @(theBlockStructure) mean(theBlockStructure.data(:));
blockSize = [2, 2]; % Whatever window size you want.
B = blockproc(A, blockSize, meanFilterFunction)

B = 2×3

0 0 0.2500 0.2500 0.5000 0

Attached are a bunch of my other blockproc demos.