降采样就是隔行或隔列对图像进行采样呀！

clc

clear

close all

data=imread('X.bmp');

[row,line]=size(data);

data_new=data(1:2:row);%隔两行采样

�ta_new=data(1:2:line);%隔两列采样

imwrite(data_new,['directory\','filename']);