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BWFILL is a grandfathered function that has been replaced by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

or to remove the previously selected

point. A shift-click, right-click, or double-click selects

a final point and then starts the fill; pressing

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

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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

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The input image BW1 must be a numeric or logical matrix. The output

image BW2 is logical.

See also bwselect, imfill, roifill.