Setting tolerance for pixel perfect collision (Sprite)

Now it seems to be arbitrarily decided that if alpha of pixel is less than 0.5 (or 128), then there is no collision and if it is greater than 0.5 (or 128) there is collision. However, in some situations (like halo around sprite or other ghostly effects) it would be nice to adjust alpha value above which collision is considered happened. So e.g. collision would be detected if alpha of pixel is above 230 or if it is fully opaque (255).