Hallöchen!
I suggest to remove the Thoby projection from Lensfun. Its
definition is
r = 1.47 * f * sin(0.713 * theta)
with r being the distance from the sensor centre and theta being the
angle of the incoming light ray. The problem is that 1.47 * 0.713
is not equal to 1, which means a shrinking of the image by 5%.
Frankly, such global scaling of an image is a no-go for a projection
formula because it yields a wrong value of the focal length f.
The Thoby projection does not occur in peer-reviewed literature.
The general projection
r = a * f * tan(theta/a) (or with sin instead of tan)
is sometimes discussed, but "a" must be fitted per lens. There is
no general value for "a".
The Thoby projection was created Michel Thoby for the Nikkor 10.5mm,
but this can also be -- and is in Lensfun -- described with
equisolid + distortion. By the way, the 5% shrinking is due to the
fact the the focal length of this Nikkor is slightly larger than
10.5mm.
To summarize:
* This projection is not referred to outside the PanoTools world.
* It was created only for the Nikkor 10.5mm, and no research has
been done whether it contains any generality.
* It has never been used in Lensfun's database.
* It does not add accuracy in distortion correction, not even for
the 10.5mm.
* It spoils all subsequent image processing that relies on an
accurate focal length.
Alternative: If we keep it, I propose:
- We change the factor 0.713 to 0.680272108844. This un-does the
shrinking, but does not distort the projection any further.
- We document that "our" thoby also enlarges the image by
approx. 5%.
What do you think? Does anybody know of any usage of the Thoby
projection? Did I overlook a discussion of it in literature? Did I
overlook a usage outside the PanoTools? Has anybody reported
benefit from it?
Tschö,
Torsten.
--
Torsten Bronger Jabber ID: [email protected]
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