Non wokring checkerboard

for (pptrx = 0 ; pptrx <= image_w-1; pptrx = pptrx + 1)
for (pptry = 0 ; pptry <= image_h-1; pptry = pptry + 1)
{
{

pptr=pptrx+pptry*image_w*bytesperpixel;

pixcol=((pptrx + pptry)% 2)*255;
*(image+pptr)=0;
*(image+pptr+1)=pixcol;
*(image+pptr+2)=pixcol;
*(image+pptr+3)=pixcol;
};
};