int A, B, C, D, x, y, gray;
float x_ratio = ((float)(width-1))/nw;
float y_ratio = ((float)(height-1))/nh;
float x_diff, y_diff, ya, yb;

for (int u = 0; u < nw; u++) {
    for (int v = 0; v < nh; v++) {
        x = (int)(x_ratio * u) ;
        y = (int)(y_ratio * v) ;
        x_diff = (x_ratio * u) - x;
        y_diff = (y_ratio * v) - y;

        A = getPixel((int)x, (int)y) & 0xff;
        B = getPixel((int)x+1, (int)y) & 0xff;
        C = getPixel((int)x, (int)y+1) & 0xff;
        D = getPixel((int)x+1, (int)y+1) & 0xff;

        gray = (int)(A*(1-x_diff)*(1-y_diff) +  B*(x_diff)*(1-y_diff) + C*(y_diff)*(1-x_diff)   +  D*(x_diff*y_diff));
        result.setPixel(u, v, (byte)gray);
    }
}