[Robotics] Camera calibration parameters

[Simon Lacroix]

	Benoit,

Thanks for the information, it answers our question !
To make things clear:  the 'pixsize' as defined by your formula is the  
size of the scene element corresponding to one pixel in the image,  
whereas the pixel size Gilberto was originally referring to is "the  
size of one pixel of the image plane", i.e. of a CCD / Cmos imaging  
device.

The clue we were missing is the following :

On 11 mai 10, at 20:55, Benoit Bolsee wrote:
> [...]
> The lens attribute is a value that represents the
> distance in Blender unit at which the largest image dimension is 32.0
> Blender units.

Explanation: the intrinsic calibration matrix K of a camera is :

(a_u	0		u_c)
(0		a_v		v_c)
(0		0		1)

This matrix transforms a 3D point P(X,Y,Z) in the 3D camera frame to a  
2D point p(u,v) in the image plane in pixel coordinates: u = KP (see  
e.g. http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/OWENS/ 
LECT9 for frame and notation conventions, but this is not necessary  
here).

(u_0, v_0) represent the intersection of the optical axis with the  
image plane in pixels coordinate (for a Blender "ideal" camera, they  
are capsize[0]/2 and capsize[1]/2).
(a_u, a_v) represent the focal length expressed in pixels : a_u = f /  
pixSize_u, where pixSize_u is the length of the physical pixel of the  
imaging device according to the u image direction, and f is the lens  
focal length.

Now that we know the meaning of the attribute 'lens', we can compute  
a_[u|v]:
a_u = capsize[0] . lens / 32.0
(and we have to consider the aspect ratio if it makes non-square  
pixels).

Simon



>
>> -----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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