Down Sample Image by Deleting Alternating Rows & Columns

I am trying to down sample an image to 1/4 and 1/16 its original size, and the instructions are to "use simple decimation to remove every other row and column to do so".

I have been reading through the MATLAB documentation for imresize, downsample, and decimate, as well as various threads here and on other sites, and I cannot seem to figure out how to implement this task?

I = imread('filename');

J = downsample(I,4);

which gives me the correct resize vertically but doesn't change the horizontal dimension. How do I apply this downsample to both rows AND columns?

Answer 1

[Matt J]

J=I(1:4:end,1:4:end);

[Jason Almeida]

Thanks Matt! I actually figured that out after posting, by looking through the provided code later on in the task, but now I am wondering what the

(1:2:end, 1:2:end,:)

part of the command is representing? That is the portion that was provded (but not explained), so I just want to understand what "1","2", and "end" represent, as well as the ":" as the last term in parenthesis. Even if you (or anyone) could point me to where in the MATLAB documentation I would find this command/info to learn more about its use and syntax?

[Matt J]

You're welcome. Please Accept-click the answer if it solves your problem.

To read about the 'end' operator and matrix indexing in general, see for example,

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html