Image reconstruction with linear interpolation

I am attempting to reconstruct a down sampled gray scale image using interp1. I have this code but I am confused to why it is not working. I am getting an error on line 4. Basically, what do I need to put in place of the image name in line 4?

[r_L,col_L]=size(image); %generalize so it works for any image
r3i = 1:(1/3):r_L; % locations between the row indices are created
c3i = 1:(1/3):col_L; %-- locations between the column indices are created
%Error occurs here
xxrowi=interp1(1:1:r_L,image,r3i);%interpolation on image width
xxlinear=interp1(1:1:col_L,xxrowi',c3i);
%transposing allows one to interpolate on a length
xxlinear=xxlinear'; %retransposing image vector
show_img(xxlinear)

Its supposed to work with any image but we first need to down sample the image and convert it to gray scale with the following:

image_bw=rgb2gray(image);
image_DS=downsample(image,3);