# Re: [sharing] Kinect depth-sensitive blob tracker, 3D bg subtraction, etc.

Zachary Seldess

Hi ygreq,

Sorry for the delay. Two things about my earlier culling plane coordinates that may have been a problem:
1. 2 meters on the x axis (relative to the camera) may not be visible in the space you're working, as it you'll need a lot of depth to see that far to the side of the camera.

2. As I wrote, sometimes the culling is inverted from what you want, due to how the software determines which side of the plane is front and back (i.e. which side needs to be culled), based on the orientation of the points. So I needed to invert the y coords of the first two points to get the desired effect. If you want to see the math behind the culling, take a look at the shader file: zs.kinectcullplane.jxs

Again, three points is one simple way to define a plane. Four points is not. You can easily define four points that don't sit on the same plane (try it, you'll see). So you just need to do some visualization and careful thinking about this and it should become a bit clearer. Skim the definition section for a plane on wikipedia: http://en.wikipedia.org/wiki/Plane_(geometry)

I could definitely come up with a way to may this culling visual and intuitive, just don't have the time right now.

I've captured a few screenshots for clarification. First, I reduced the x values from -2. to -0.5, which will the allow the plane to be visible at much closer distances. I've also captured the inverted plane, so you can see what happens, and the original left cull plane together with a symmetrical right culling plane, so you can see how to mirror the planes. Move the points around, see what happens, and based on coordinate system information in the info.rtf, see if you make sense of it all. Hope that helps.

best,
Zachary

[attachment=220807,5406] [attachment=220807,5407] [attachment=220807,5408]

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Apr 11, 2013 at 4:49pm #219703