认真研读一下MATLAB的help文档吧，解释最权威：
BWFILL Fill background regions in binary image.
BWFILL is a grandfathered function that has been replaced by
IMFILL.
BW2 = BWFILL(BW1,C,R,N) performs a flood-fill operation on
the input binary image BW1, starting from the pixel (R,C). If
R and C are equal-length vectors, the fill is performed in
parallel from the starting locations (R(k),C(k)). N can have
a value of either 4 or 8 (the default), where 4 specifies
4-connected foreground and 8 specifies 8-connected
foreground. The foreground of BW1 comprises the "on" pixels
(i.e., having value of 1).
BW2 = BWFILL(BW1,N) displays the image BW1 on the screen and
lets you select the starting points using the mouse. If you
omit BW1, BWFILL operates on the image in the current
axes. Use normal button clicks to add points. Press
<BACKSPACE> or <DELETE> to remove the previously selected
point. A shift-click, right-click, or double-click selects
a final point and then starts the fill; pressing <RETURN>
finishes the selection without adding a point.
[BW2,IDX] = BWFILL(...) returns the linear indices of all
pixels filled by BWFILL.
BW2 = BWFILL(X,Y,BW1,Xi,Yi,N) uses the vectors X and Y to
establish a nondefault spatial coordinate system for BW1. Xi
and Yi are scalars or equal-length vectors that specify
locations in this coordinate system.
[X,Y,BW2,IDX,Xi,Yi] = BWFILL(...) returns the XData and YData
in X and Y; the output image in BW2; linear indices of all
filled pixels in IDX; and the fill starting points in Xi and
Yi.
BW2 = BWFILL(BW1,'holes',N) fills the holes in the binary
image BW1. BWFILL automatically determines which pixels are
in object holes, and then changes the value of those pixels
from 0 to 1. N defaults to 8 if you omit the argument.
[BW2,IDX] = BWFILL(BW1,'holes',N) returns the linear indices
of all pixels filled in by BWFILL.
If BWFILL is used with no output arguments, the resulting
image is displayed in a new figure.
Remarks
-------
BWFILL differs from many other binary image operations in
that it operates on background pixels, rather than foreground
pixels. If the foreground is 8-connected, the background is
4-connected, and vice versa. Note, however, that you specify
the connectedness of the foreground when you call BWFILL.
Class Support
-------------
The input image BW1 must be a numeric or logical matrix. The output
image BW2 is logical.
See also bwselect, imfill, roifill，
