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Perspective Filter - Output Positions in Pixels post and replies

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How to set correct horizontal and vertical scales of output plane?
Jo Mayer
8 Posts

I am using Tracker 5.1.5 and need to compensate the perspective error. Therefore I use the perspective filter and set the input square. How do I get the correct output parameters for the rectangular dimension? Because Tracker always distort the video to much, as you can see attached. Is there any possibility to set the horizontal and vertical scales of the output rectangular instead of the pixels? Or can I calculate the output pixels with any tool? Thanks in advance.

Attached File: Tracker-Output.png


Replies to How to set correct horizontal and vertical scales of output plane?

Re: How to set correct horizontal and vertical scales of output plane? -
Douglas Brown Avatar
Douglas Brown
339 Posts

When correcting for perspective distortion, the INPUT corners should be dragged to positions that you know are at the corners of a plane parallel to the motion to be tracked. Then in the OUTPUT you drag one or more corners to change the shape of the rectangle so that the vertical and horizontal scales are the same. There is no automatic way to do this, you simply have to make sure there are vertical and horizontal objects with known dimensions in your video (in the plane of the motion) so you can adjust the output until both dimensions are scaled correctly.

I hope this is clear!  Doug


> Re: How to set correct horizontal and vertical scales of output plane?
>
> I am using Tracker 5.1.5 and need to compensate the
> perspective error. Therefore I use the perspective
> filter and set the input square. How do I get the
> correct output parameters for the rectangular dimension?
> Because Tracker always distort the video to much,
> as you can see attached. Is there any possibility
> to set the horizontal and vertical scales of the output
> rectangular instead of the pixels? Or can I calculate
> the output pixels with any tool? Thanks in advance.



Re: How to set correct horizontal and vertical scales of output plane? -
Wolfgang
146 Posts

Tracker help has an excellent description of the perspective filter.

https://tracker.physlets.org/help/video_filters.html



Re: How to set correct horizontal and vertical scales of output plane? -
Jo Mayer
8 Posts

Thanks for the fast response. Let me just show a little example (see attached). I have this rectangular as refernce object and I know that it is 20 cm long and 5 cm high. So the Output for Point 0 is the same as the Input and the Output for the y Coordinate of Point 3 is ((699-484,6)/20)*5=451,1. Is that correct?

Attached File: Example.png



Re: How to set correct horizontal and vertical scales of output plane? -
Douglas Brown Avatar
Douglas Brown
339 Posts

I think you have the right idea. But it is not necessary that the output for point 0 be the same as the input. Since your calibration grid is quite small you will probably need to reduce the size of the output rectangle in order to see the whole image. The critical number is the ratio in the output of (x2 - x0)/(y2 - y0) which in your case should be equal to 20cm / 5 cm = 4. I've attached a screenshot of how I set the output for the image you attached and you can see that x2 - x0 = 200 pixels and y2 - y0 = 50 pixels.


> Re: How to set correct horizontal and vertical scales of output plane?
>
> Thanks for the fast response. Let me just show a little
> example (see attached). I have this rectangular as
> refernce object and I know that it is 20 cm long and
> 5 cm high. So the Output for Point 0 is the same as
> the Input and the Output for the y Coordinate of Point
> 3 is ((699-484,6)/20)*5=451,1. Is that correct?

Attached File: perspective.jpg



Re: How to set correct horizontal and vertical scales of output plane? -
Jo Mayer
8 Posts

Dear Douglas,

thanks for your help. Now I understand it.



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