Hi,
I must admit I'm confused. Doesn't help that math is not my friend...
so please forgive my questions.
Sensor size for display purposes only?
Focal length in meter? So in your example a 50mm focal length is f =
0.050? And this should be stored as pixels?
Could be in any unit as long as you know (and store/display it). Most important
point here is that focal length, sensor size, and point coordinates are in the
same unit.
The horizontal dimension of image sensor would in your example with
22.3mm be dx = 0.0223?
Number of pixels is then the x resolution, so lets say nx = 1920.
sx = 0.0223 / 1920
Sorry, got that wrong in the last mail.
sx is actually the pixel density. Unit of sx should be e.g. pixel/mm, hence sx
= nx/dx
For your example:
sx=1920/22.3 ~ 86.1 pix/mm
f = 50 mm
Fx = f * sx = f * nx / dx ~ 4304 pix
Units are: mm * pix / mm - pix
Fx = 0.050 * 0.11615
I then choose an object/point that I know is say 5 meters from the
left border of an image? X = 5
And that object/point I know is 6 meters away from the camera. Z = 6
With this I get a horizontal position in pixels that is u =
0.00484? What is this value and how is it used?
In this case, it would only matter that X and Z are of the same unit as f and
sx. In the example above: X=5000, Z=6000
u = 3586.6 pixel
Actually, you would have to add the coordinate of the principal point, which is
usually in the centre of the image. Hence your point would be
at
u' = 3586+1920/2= 4546 pixel
As you can see I don't understand anything. I can't see how this will
Sorry and thanks for pointing out my mistake.
make it easy to get the correct field of view from a known focal
length and image sensor size. How will you translate a vertical fov to
a focal length with your method? I do not doubt that having the focal
Converting to fov (alpha_x is horiz. fov) is:
alpha_x = 2 * atan ( dx/(2*f))
alpha_y = 2 * atan ( dy/(2*f))
alpha_x = 0.4388230571228 rad = 25.143275522848388 degree
Your example point would have an angle of
atan (5000/6000) = 0.69473827619670325 rad = 39.8 deg
to the right. That would mean it's far outside the fov (which extends 25.1/2 =
12.5 deg to the right). The coordinate u' is far bigger than 1920, so my
calculation seems correct this time ;-)
Any 3d computer vision book/lecture usually starts with this, e.g.
http://www-inst.eecs.berkeley.edu/~ee225b/sp07/lectures/lec12.pdf
Google for pinhole camera model and computer vision for further reference.
length as pixels is useful for your work, but can't see the relation
to what this sensor patch is trying to accomplish.
Just for clarification, I'm not involved in the patch. I'm just providing a
different point of view that might be interesting for the authors of the patch
or the community. I found it quite easy to work with the focal length in pixel
(which is usually called camera constant) as it denotes everything the
projection needs to know about the scaling and easy to convert from/to fov,
focal length, etc.
Best regards,
Ejner
On Mon, May 30, 2011 at 10:10 PM, Lars Krueger lars_e_krue...@gmx.de
wrote:
How about storing the focal length using pixels as the unit of
measurement? This way a lot of code gets a lot simpler. The sensor size is
then used
for display purposes only.
Assuming
f - focal length (in e.g. meter)
dx - horizontal dimension of image sensor (same unit as f)
nx - number of pixels
sx - pixel size in same unit as f ( = dx / nx)
Fx - horizontal camera constant ( = f * sx)
X - horizontal position of a point in meter
Z - distance of point in meter
u - horizontal position in pixel
you go from
u = f*(dx/nx)*X/Z
to
u = Fx*X/Z
Computing the Fx can be done in the UI, while all code needs only one
variable during the projection.
That's a quite common convention in computer vision. It comes from the
fact that focal length and pixel size have to be multiplied to get the final
scaling from meters to pixels during projects, but only pixel coordinates
can be measured during calibration. Hence, pixel size remains unknown as
only the total scale from meter (calibration object size) to pixels (image
coordinates).
We use this convention in all our 3D computer vision products and
applications for about five years and never had problems. Just enter the
correct
scale for the calibration object and you're done.
Original-Nachricht
Datum: Mon, 30 May 2011 17:48:05 +0200
Von: Ejner Fergo ejner...@gmail.com
An: bf-blender developers bf-committers@blender.org
Betreff: Re: [Bf-committers] Sensor Size
Hola,
I have updated the patch to revision 37009:
http://projects.blender.org/tracker/index.php?func=detailaid=24427group_id=9atid=127
I have also written a short documentation:
http://wiki.blender.org/index.php/User:San/Real_Focal_Length
About code review, I think I read that only committers can upload to
the code
