how i can do sub-sampling for an image with factor 4

to perform sub-sampling using matlab

Answer 1

[Youssef Khmou]

hi ,

you can use the function " downsample" twice ,

example :

 I=imread('circuit.tif');
 J=downsample(I,4);
 J=downsample(J',4)';
 whos I J
 imshow(J)
 figure, imshow(I)

Size(I)= 4² * Size(J) .

[Wayne King]

keep in mind that downsample() in the Signal Processing Toolbox only downsamples the matrix along the row dimension.

[Youssef Khmou]

yes Wayne, thanks for reminding , i edited the code .

Answer 2

[Wayne King]

Do you want to downsample along the row dimension, along the column dimension, or both?

X = randn(24,24);
% Along row only
Im = X(1:4:end,:);
% Along column dim only
Im = X(:,1:4:end);
% Along both
Im = X(1:4:end,1:4:end);

Keep in mind that the above gives only one "phase" of the downsampling. When downsampling by M, there are M such phases. To see an illustration of this for a 1-D signal, see

https://www.mathworks.com/help/signal/ug/downsampling.html