Is there anyway to write an algorithm for wavelet without dwt2 command ?

Hi

im trying to write down dwt2 command algorithm , i try to gain wavelet transform of an image with haar filter as i realize i should use high and low pass filter but my problem is down sampling when i down sample the image the result is cut in half and its not correct answer is there nay way to implement this idea at all ? how can i get wavelet transform of an image with haar filter at all ???

thanks