reduce number of points in activecontour?

I am using the activecontour function and getting a mask back. Then, I want to use bwboundaries to extract the coordinates, and that works well too. However, I'm getting too many points, and would like to have less (it's ok if the contour will be less smooth as a result). But I cannot find how do I do that, assuming I use the Chan-Vese method, and not 'Edge' (meaning, I can't use SmoothFactor property).

Answer 1

[Walter Roberson]

You might be able to make use of reducepatch()

It might also be suitable to do spline interpolation. Or you might be able to use the interparc File Exchange contribution (which uses spline interpolation in some cases.)

[Guy Tsror]

Neither of those help here - I have a binary 2D image, so reducepatch() won't help; Spline interpolation also won't really help - I'm trying to remove points rather than add; And interparc - same, plus it seems to be talking about equal-distance points, which I would not like.

[Walter Roberson]

After you get the boundary you are not working with a binary image: You have a something you can create a (2D) patch from that can be passed to reducepatch. (Or you could read the source to reducepatch and see that it calls a mex file reducep() with the information from the patch, so you could call that directly without creating the patch.)

"I'm trying to remove points rather than add"

Interpolate at larger distances than your existing points and throw away the existing points.

"plus it seems to be talking about equal-distance points, which I would not like."

Your existing procedure gets a list of adjacent pixels; those are all either distance 1 or sqrt(2) apart. You are starting from pretty much fixed distances. And you said "it's ok if the contour will be less smooth as a result".

[Guy Tsror]

Okay I see what you mean with the patching, thanks. However, when using it, I'm getting the coordinates back sorted by a specific axis, and was wondering do you know any method of keeping them organized in the shape of the polygon?

[Walter Roberson]

I suggest trying interparc:

NumPointsToUse = 100;

bounds = bwboundaries(...);

smoothed_bounds = cellfun(@(C) interparc(NumPointsToUse, C(:,2), C(:,1), 'spline'), bounds, 'uniform', 0);

You could modify the NumPointsToUse to something like min(NumPointsToUse, floor(size(C,1)/3)) if you were concerned about regions which had fewer than NumPointsToUse to start with.

[Guy Tsror]

Thanks. I ended up using the patch methods you suggested, converting it to a patch and then using reducepatch to the point needed.

[Walter Roberson]

I tried reducepatch but I found that it generated a lot of triangles. Perhaps if I had used boundary() (an active contour) on the vertices after that...

[Guy Tsror]

Well yeah that's what happened to me too. I just re-organized the vertices using their arctan, compared to the centroid of the polygon