The feather makes the selection fade.
IE: Make a document 600X600 72 dpi. Now make a selection with the marquee tool and fill it with a color. now, make a new document and make the same selection but in the options bar type in 25 px for "Feather". Fill it with a color and the box/circle will have a fade to it.
When you make a selection that does not have a sharp edge, it isnt really possible for the marquee (or marching ants) to clearly define the edges of that selection since there are no clearly definable edges. Instead, it makes a sort of approximation of what is selected so that you have some kind of visual reference as a clue to what you are doing. If you actually want to see exactly what is selected and to what degree you can always look at the selection in quick mask mode which will show the entire selected and unselected areas in shades of red by default.
Adding a feather to a marquee selection is the same as blurring the selection (a dip into quickmask mode before and after feathering will verify this)…and since the marching ants, or the selection edge follows the 50%-selected boundary, it will appear rounded just as the 50% boundary of a blurred rectangle will curve around the corners.
what do you mean by:
"the selection edge follows the 50%-selected boundary"
Meaning that the marching ants (selection edge) follow the pixels that are selected at 50% opacity. That is why you get that warning when your selection does not select any pixel more than 50%… you do not see the marching ants, but you still have pixels selected.
The reason you get rounded corners with feathering is that on the corners the 50% feather of the top and the 50% feather of the side are combined (for example) making the total 25% (half of a half). Thus the actual 50% point on a corner is closer to the center than either top or side.
This relates very closely to a question I have too.
It is my understanding (and this is what I would like confirmed) that when a selection is feathered it is feathered either side of the selection. ie if you set a feather size of say 12 px (refered to as a radius of 12px) Photoshop would in effect create a selection gradient from 100% at 12 px inside the current selection (marching ants) to 0% at 12 px outside the current selection. Leaving the current selection exactly 50% selected.
If this is the case then the marching ants shouldn’t move (except perhaps in the corner as mentioned above).
Is this correct?
Yes, I believe it is, Barry.
Thanks.
So the question still remains as to why the selection varies (in my experience it is quite common.)
I just had a thought – many of my selections are done by the magic wand or by selecting the shadows or highlights – selections with a tolerance setting in the selection process. If PS partially selects (rather than fully selects) pixels that are captured within the tolerance then that would explain everything. If the selection marquee is already only 50% selected rather than fully selected, then when the feather is done the 50% selected boundary will effectively shrink.
Is this what is happening do you think. Are the magic wand, shadow, midtone and highlight selections already effectively feathered by PS, unless the tolerance is set to 0?
