[Robotics] Camera calibration parameters

[Benoit Bolsee]

Hi Gilberto,

The focal distance doesn't make sense for an openGL camera as it works
by projection. You can however talk about focal distance in Stereo mode
where it is the distance from the camera at which the two images of an
object will be displayed at the same screen position.

Another value of interest is the near distance (available as camera.near
attribute): it is the distance in blender unit (i.e. meter) from the
camera below which an object is not displayed. This near value is
usually small: 0.1 for example.

The image size (size of pixel in distance unit) can be derived from the
camera.lens attribute. The lens attribute is a value that represents the
distance in Blender unit at which the largest image dimension is 32.0
Blender units. 
So the formula to get the pixel size as a function of the distance to
the camera is:

Pixsize = 32.0/lens * dist

Hope this helps,
Benoit


> -----Original Message-----
> From: robotics-bounces at blender.org 
> [mailto:robotics-bounces at blender.org] On Behalf Of Gilberto Echeverria
> Sent: mardi 11 mai 2010 16:03
> To: robotics at blender.org
> Subject: Re: [Robotics] Camera calibration parameters
> 
> 
> Thanks for the suggestions. It would indeed be strange to 
> calibrate the 
> camera in the simulation. We expected to avoid this step 
> inside Blender, 
> since the data required should be already defined and exist 
> somewhere in 
> Blender. The focal length seems to be a modifiable property of the 
> camera object in Blender, but the image size is still evading 
> us. For the moment we'll try to estimate this measurement 
> from the pattern 
> image.
> 
> Thanks.
> Gilberto
> 
> 
> On 05/11/2010 03:28 PM, Paul Fitzpatrick wrote:
> > Another (somewhat strange, but fun) method would be to run 
> a standard 
> > calibration program (e.g. from OpenCV).  You'd need to make a plane 
> > with a pattern like this on it:
> >     
> http://www.vision.caltech.edu/bouguetj/calib_doc/htmls/pattern.pdf
> > Then light it, animate it moving around, record several 
> views from the 
> > camera, and run the calibration program.
> >
> > Cheers,
> > Paul
> >
> > On 05/11/2010 06:41 AM, koen buys wrote:
> >    
> >> Hi Gilberto,
> >>
> >> I once figured this out in a previous version of blender. Together 
> >> with Herve Legrand. I will forward you our experiments 
> back then. You 
> >> need both the intrinsic and the external calibration matrix. The 
> >> external should be fairly easy to find. The internal one was more 
> >> difficult. I hope they didn't change the API of that part.
> >>
> >> Best regards,
> >>
> >> Koen Buys
> >>
> >>
> >> On 11 May 2010 11:14, Gilberto 
> Echeverria<gilberto.echeverria at laas.fr
> >> <mailto:gilberto.echeverria at laas.fr>>  wrote:
> >>
> >>      Hello everyone,
> >>
> >>      As part of the Open Robots Simulator (now called 
> MORSE), we are now
> >>      connecting Blender with other robotics software 
> modules that do the
> >>      processing of the data sent by the simulation. In the case of
> >>      image data
> >>      from a camera, these modules expect the image to have 
> an associated
> >>      calibration matrix.
> >>
> >>      The calibration matrix is composed with the focal 
> point, the focal
> >>      length and the dimensions of the acquired image as a 
> number of pixels
> >>      per unit of distance. For the simulated camera in 
> Blender, we can
> >>      consider the focal point to be (0,0). For the other 
> two parameters we
> >>      have been unable to find the data in Blender. We are 
> using the Blender
> >>      camera object and the VideoTexture module to generate 
> captured images.
> >>      Does anyone know where this information could be found in 
> >> Blender?
> >>
> >>      Best regards
> >>
> >>      Gilberto
> >>      _______________________________________________
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> >>
> >>
> >>
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