word segmentation using horizontal projection

i followed word segmentation algorithm, according on histogram as steps: 1-construct the vertical histogram for the image. 2-count white pixel in each column. 3- use histogram to find the columns containing number of white pixel . i tried in this code , but i don't have any idea about how count white pixel in each histogram column, could you please help me.

   x=imread('doc4.jpg');
xg=rgb2gray(x);
xbw=im2bw(xg);
imshow(xbw);
numWhitePixels = sum(xbw(:));
numBlackPixels = numel(xbw) - numWhitePixels;
sum(xbw,1);
sum(xbw,2);

px=sum(xbw,1); py=sum(xbw,2);

figure(1),plot(px); figure(2),plot(py);

imhist(px);

imhist(py);