public void Flip()
{
for (var y = 0; y < height/2; y++) {
for (var x = 0; x < width; x++) {
var top = y * width + x;
var bottom = (height - y - 1) * width + x;
var temp = pixels [top];
pixels [top] = pixels [bottom];
pixels [bottom] = temp;
}
}
}