The simplest way is the following.
Let's start with a really simple example:
[x y z v] = flow;
q = z./x.*y.^3;
p = patch(isosurface(x, y, z, q, -.08, v));
p.FaceColor = 'yellow';
view(3)
I want to keep everything between X=2 and X=4.
minx = 2;
maxx = 4;
inverts = p.Vertices(:,1) >= minx & p.Vertices(:,1) <= maxx;
The variable inverts is now a mask that is the size of my Vertices array and has a true for each vertex that I want to keep. Now I need to turn that into a mask of the faces I want to keep. To do that, I need to look at each column of the Faces array:
infaces = inverts(p.Faces(:,1)) & inverts(p.Faces(:,2)) & inverts(p.Faces(:,3));
Now infaces is a mask that is the size of my Faces array and has a true for every face where all three vertices have trues in inverts. Now we can use that to shrink the Faces array.
p.Faces = p.Faces(infaces,:);
The one problem with this approach is that the patch's Vertices array still contains all of those vertices that have been excluded. That's generally OK, but you might want to get rid of them. The problem is that to do that, you need to translate all of the indices in the Faces array from indices in the old, big Vertices array into indices into the new, small Vertices array. That's a messy bit of bookkeeping.
