About the kernel size of Gaussian blur by downsampling for SISR

[wenchen4321]

Could you please tell me why the kernel size is even when the scale factor is 2 or 4 in Blurring by Gaussian kernel followed by downsampling for SISR?

kernelsize = ceil(sigma*3)*2+2;
kernel     = fspecial('gaussian',kernelsize,sigma);
blur_HR    = imfilter(im, kernel,'replicate');
LR         = blur_HR(scale/2:scale:end-scale/2,scale/2:scale:end-scale/2,:);

[cszn]

Check the bicubic kernel as a reference, see https://github.com/cszn/USRNet