if (view_yview[0] <= 0)
{
    image_alpha = 0;
    oalpha = 0;
}
if (view_yview[0] > 0)
{
    a = oalpha - ((top / view_yview[0]) * oalpha);
    if (a > 0)
        image_alpha = a;
    else
        image_alpha = 0;
}