LINEINMASK determines portions of the line in and out of a mask

[inX inY] = LINEINMASK(XS,YS,BW,XV,YV) computes a the segments of the
line specified by verticies XV and YV that are inside the mask BW.
Points XS and YS specify the limits (or pixel values) of the
coordinates of the matrix BW.

[inX inY] = LINEINMASK(BW,XV,YV) assumes XS=1:N and YS=1:M where
[M,N]=SIZE(BW).

[inX inY] = LINEINMASK(...,METHOD) Uses the given method to create the
polygon from the mask.
Inner | Outer | {Exact}

[inX inY outX outY] = LINEINMASK(...) outputs the segments of the line
both in and out of the mask.

Example:
I = imread('rice.png');
level = graythresh(I);
bw = im2bw(I,level);
bw = bwareaopen(bw, 50);
[a b] = size(bw);
xv = b*rand(10,1);
yv = a*rand(10,1);
[inX inY outX outY] = lineinmask(bw,xv,yv,'exact');
figure
imagesc(bw)
colormap(bone)
hold on;
h1 = plot(inX,inY,'r');
h2 = plot(outX,outY,'b');
legend([h1 h2],'Inside Mask','Outside Mask');

By J Sullivan, August 2011