APS Excellence in Physics Education Award
November 2019

Science SPORE Prize
November 2011

The Open Source Physics Project is supported by NSF DUE-0442581.

## Mobile origin post and replies

Mobile origin - video from moving boat
Laët Vib
5 Posts

Hello,

I'm trying to modelling the movement of the dive of a Gannet from a boat video record.
I am confronted to the problem that I have no fixed point where I could set the origin, and since the boat -hence the video- is moving, the trajectory cannot be modelized just frame to frame.
Is there a way to set a mobil origin? Should I set it at each frame? In that case, is it possible to stack 2 images, so that I can point the same point frame after frame ?

I would be much grateful for any help

Laëtitia

### Replies to Mobile origin - video from moving boat

Re: Mobile origin - video from moving boat -

lookang
222 Posts

https://iwant2study.org/ospsg/index.php/interactive-resources/physics/02-newtonian-mechanics/01-kinematics/175-sailing-motion-with-moving-reference-frame

OSP@SG blog
OSP@SG Digital Library

Re: Mobile origin - video from moving boat -

lookang
222 Posts

I also have a YouTube. https://youtu.be/Lu86kmk2PMQ Enjoy!

OSP@SG blog
OSP@SG Digital Library

Re: Mobile origin - video from moving boat -
Paul Nord
26 Posts

Relative motion is tricky.  If you have some "fixed" object in the view in every frame, create a second point and track that object in every frame.  Then your moving object's position can be calculated as just the distance to that object.  If the fixed object position is (xf, yf) and the moving object is (xm, ym) then the real position is just (xm-xf, ym-yf).

Doing this for any length of time from a boat requires some careful consideration of the camera's perspective.  A camera is mapping the entire view into a flat space of pixels.  Rotation, pitch, and yaw of the camera will change the relative position of the objects in the view.  Extracting a position in 3-dimensional space from that is tricky at best.

Re: Mobile origin - video from moving boat -
Laët Vib
5 Posts

Hi, thanks for the responses !!
Yes, I was thinking of something like a fixed point as reference to compare with, but unfortunately, there isn't :(
I tried to set a system of axes for every image (I have enough images per sec (25) to be able to fit the origin approximately at the same point), and then pointing my bird. I'll see how it works. It is probably not the fastest way, so if there's any other trick, I'm in!