Hi Bruno On Jul 18, 2:05 pm, Bruno Postle <[email protected]> wrote:
> Yes focal length is independent of image orientation, but it is > meaningless to anyone who hasn't learnt the 'craft' of photography. > > The different sensor-size thing combined with 'equivalent' focal > length numbers is a real mess - At least any technically minded > person can grasp the idea of 'angle of view'. > > Plus there are also whole classes of images that you might use as > input for hugin where 'focal length' has no physical meaning: I'm talking about mathematics, not physics. If you claim to know the "field of view" of an image then you have defined a focal length. It is exactly the derivative of distance by angle, at the projection center. Angle is measured in dimensionless units (radians) so the focal length is always a distance, in whatever units you use to measure your image. As a practical matter you need a focal length in pixels to reproject an image, or do any other computation that depends on angles of view. So you have to specify that somehow, else it is no use loading that image into Hugin. For "real" images made with a lens and a digital sensor, it is best to state the designed focal length of the lens and the physical spacing of the sensor pixels. For "ideal" images it is just as good to specify the field of view and projection function. Another way to define the focal length in pixels is as the reciprocal of the angular pixel spacing at projection center. So another good synonym for "focal length" is "1 / angular resolution". If you feel the term focal length should only apply to lenses, you are free to call this parameter something else when the image was not made by a lens; but it is the same parameter anyhow. I often call it the "angular scale factor", and in the libpano code Dersch usually calls it the "distance parameter" -- nice neutral terms, though somewhat opaque. It is also sometimes loosely called the "magnification factor", but properly that term refers to the ratio of two focal lengths (a dimensionless quantity). > If the problem you are trying to solve is "hugin treats portrait and > landscape images differently", then maybe there is a simpler > solution. No, the problem I am trying to solve is "hugin uses an unreliable representation for the single most important image parameter". I'm not trying to force any user to think differently, or act differently; I just think that internally, hugin should treat focal length as the primary quantity and field of view as a derived one. The absurdity of having the angular scale go wrong when you turn or resize an image would then disappear, and several technical improvements would become easier to implement. Best, Tom --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "hugin and other free panoramic software" group. A list of frequently asked questions is available at: http://wiki.panotools.org/Hugin_FAQ To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/hugin-ptx -~----------~----~----~----~------~----~------~--~---
