Hello and welcome back. Active contours, also called snakes, are
very important in image and video processing.
They appear in a number of commercial products and they are very useful for
numerous image and video segmentation. Let me illustrate the basic concept
behind active contours. The idea is that you start from one or
multiple curves. For example, I'm going to draw a curve
that surrounds the flower, which is my object of interest.
And the basic idea behind active contours is to design velocities that will move
this curve towards the flower. Hopefully, as the curve moves, it's going
to end up in the border of the flower. So, we need basically to design
velocities that will move any initial contour towards the contour of interest,
which is the boundary of the object of interest.
Now, I can write down, the formulation for this and the equations for this, but
I think it's better if we first learn the material that is coming next week, as a
back ground, and that will help us to write this in a much, much elegan, form.
So, this week, instead of writing down the exact weight
of active counters, I'm going to just illustrate you some
examples to motivate you and get you very excited to learn the background, material
that we're going to learn next week. So, let's go and watch some movies with
examples of active contour, inside the image processing online
package and inside Adobe's Photoshop. So, let's just observe that right now.
In order to illustrate one of the examples of active contours, we're going
to be using, again, the image processing online website where, as we have seen
before, there is many very interesting image processing algorithms implemented.
And particularly, now, we are in the Chan-Vese webpage,
which is an implementation of one of the approaches for active contours developed
by Tony Chan and Luminita Vese. Before we show you one of the examples
running actually in real time, let's just basically run the demo provided by the
author of this website. So here, we see contours evolving.
This is a very clear example of a contour evolving towards the segment.
Here, we have another one, just look at this one.
And here, we see once again, how contours are evolving and yet once again.
So, that's a basic idea of active contour. This was a very fast video, just
to illustrate one of the examples. But let's just run one, so we're going to
have more time, to look at it. Wwe're going to run the demo online as we have
done in the past for other algorithms. I'm going to pick this, example, it's
just one example, I'm not going to change the parameters, we don't know exactly
since we haven't discussed the technique, what this parameter means, so I'm going
to just, leave the default and I'm going to run it.
Remember, now is rhyming as we see here, but it's rhyming in the server wrap that
is provided by the imaging processing online.
The algorithm is actually much, much faster, than we can see, it actually can
be run on real time, but it's run on the server.
And here is the result. We see once again, the initial contours,
these intial contours at the very beginning are the initial contours and
then, we see the boundaries. The curves have evolved toward those
boundaries. The curves have evolved to segment out
the object of interest. There are some spurious objects and we
are going to understand why they happen, but mostly, it has done a great job
basically, in finding the boundary of the flower, which is probably the object of
interest for this image. And we see once again, these contours
evolving. And this is, as I say, something that
we're going to learn next week. Exactly the formulas, exactly the math
behind it. But this illustrates the idea.
Thank you for this part of the video. It's now time to illustrate using Adobe
Photoshop one implementation of active contours.
In particular, we're going to be using this tool in active contours that is a
selection tool and look what's going to happen.
Please observe my mouse. Look how I'm moving the mouse and the
contours are being automatically attracted to the boundary of the flower.
You know, how I just clicked and it's basically catching the flower.
I almost didn't move my mouse, I just click inside the object of interest, and
we capture the flower. Let me do that again here. I'm going to
just click and it's capturing this part of the flower.
So, you see how we can help, and we have already learned about user interaction by
doing a few clicks inside the flower. Basically, a technique related to active
contours, move my clicks towards the boundaries of the object of interest.
This is a very, very interesting technique,
this time, implemented inside Adobe Photoshop.
And this provides in our demo of active contour type of approaches.
Thank you. These examples that we just saw should
motivate you for the material that is coming next week.
Before we go into that, we have a couple of additional and very, very exciting
things to learn about video segmentation. So, I see you in the next video.
Thank you.
Guillermo SapiroProfessor
