Neil,
How well do we expect the MSLK to work for the sphere and corridor images?
Thanks

Ali,

In my implementation I used 5-6 levels (at least for the sphere and corridor images).

-----------------------------

Ramsin,

When the threshold test fails, you should just keep your previous estimate of the optical flow at that pixel. One of the advantages of multi-scale optical flow is that it can work in regions with little texture (without a huge window) because the optical flow is propagated from the low-res estimation.

Also I think you're confused about the thresholds..there should only be two thresholds. You should check that lambda2 > th1 and that lambda1/lambda2 < th2. I found that values of th1 = .01 and th2 = 100 worked fairly well.

----------------------------

Nikhil,

Below are two results from my implementation. For the corridor image I am using lambda2 > .1 & lambda1/lambda2 < 100 with a window size of 9x9 and 7 levels. For the sphere image, I am using lambda2 > .01 & lambda1/lambda2 < 100, windowsize = 15x15, 6 levels.

<img src="http://www.neilalldrin.com/shared/files/of_sphere.jpg" />

<img src="http://www.neilalldrin.com/shared/files/of_corridor.jpg" />

OK, I guess image tags don't work in this discussion board...here's links to the images:

sphere result

corridor result

Oh, one more thing I should mention: I'm using Gaussian smoothing at each level of the pyramid..

-Neil

Is explicit gaussian smoothing necessary? If we use imresize(), isn't it supposed to do that automatically?

From the matlab doc:

When the specified output size is smaller than the size of
the input image, and METHOD is 'bilinear' or 'bicubic',
IMRESIZE applies a lowpass filter before interpolation to
reduce aliasing. The default filter size is 11-by-11.

Hi Neil,

For the sphere image, you used lambda2 > .01 & lambda1/lambda2 < 100, windowsize = 15x15, 6 levels. Well, the sphere image is 200*200, then the pyramid should be 200, 100, 50, 25, 12, 6. Then how could we use the 15*15 window on a 6*6 image on the 6th level? I think the window has fixed size at each level, right?

Thank you!

Brian,

Yeah, you probably don't need to explicitly apply a Gaussian filter, but I was just making sure...

----

Yuzhe,

In my implmentation, if a window goes outside the image, I truncate it so it only contains valid portions of the image (an equivalent technique would be to zero pad the image). I found this helped a lot because otherwise the edges of the image don't get an optical flow estimate at low-res. I recommend doing this if you haven't already, but it's not really stated anywhere that you should do this so consider it optional.

OK, seems every time I make a post I end up confusing things for people. The optical flow results I posted for the corridor are from images 1 and 2 instead of 0 and 1. So if you're wondering why the optical flow looks funny that's why.

-Neil

When we refer to the two windows, (the second of which is shifted by the previous result in MSLK)

The gradient of the first window is used in producing A,
The difference of the two windows is used in producing B...

Is this correct?

could someone post any notes they took at today's Q & A session?

He said that the final will be heavily focused on Just in Time Compilation in Matlab and on the Sprite Operating System.

When will we get our grades?

Thanks!

What happened to our grades?!

The grades should be posted as of yesterday (april 4th). Sorry for the delay.

-Neil

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