Still dont work checkerboard

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

pptr=pptrx*bytesperpixel+pptry*image_w*bytesperpixel;

pixcol=((pptrx + pptry)% 2)*255;
*(image+pptr)=0x0;
*(image+pptr+1)=pixcol;
};
};