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Re: [xml-dev] Holographic XML

  Regardless of what Wikipedia says, I've actually made holograms 
before.   The old fashioned way with lasers and film.
The hologram film stores the interference pattern.   This cannot be 
stored (easily) on strictly 2D film.
The film has a thickness of photoactive material that is required to 
store the interference pattern.
It does not store one 'dot per pixel'.  It requires a thichkness of 
material all of which is photosensitive in order to record
the entire interference pattern at each x/y coordinate.   You cant store 
holograms on normal photographic film.
Interestingly, this is why its so difficult to create holograms 
digitally. You cant use a regular printer to produce them,
and the calculations are in 3D not 2D.  That, and its a difficult 
transformation to take a 3D image and construct the interference
pattern needed to reconstruct the image.   Its possible though.    But 
its computational expensive and I am aware of no
output technology currently that can do it.  (You cant print the darn 
things !) .
( Do note that holography is not the same thing as stereo images making 
the "3D" movies today").

Now there is a kind of 'fake holography' made if embossed diffraction 
gratings which is effectively 2D (ignoring the fact they are 'embossed',
sorta like a vinal record, the data is effectively 2d).  These were 
popular back in the 70s on cerial boxes and toys.
But they are not true holograms, the quality is extremely low and the 
range of angles of view is very low and discrete.

On 9/8/2010 5:16 AM, Norman Gray wrote:
> Greetings.
> [physics niggling]
> On 2010 Sep 7, at 17:12, David wrote:
>> First off, physical holography (on film() requires 3D not 2D film.   The holographic pattern is encoded in a "thickness" of film" and cant be done like normal photography on purely 2D film.
> I don't think that's the case.  A hologram is stored on 2-d film.  Perhaps you're conflating this with holographic storage?  The wikipedia article on that[1] is a bit garbled, but points to [2], which makes it clearer that '3-d' holographic storage is effectively a lot of 2-d stores stacked on top of each other, only one of which is read at a time.
> [[ In case anyone's interested: the way that photographs work is that they store the intensity of the light at each point in the plane of the film or sensor.  But that's not all the information that's in the light field at that point -- phase information is missing.  What a hologram does is to store the intensity _and_ phase information present in the field, in a particular plane (ie, 2-d structure) intersecting the field, in such a way that both can be resurrected at a later stage.  The fact that you now have the complete reconstructed field to play with is what allows you to focus at different points in the image, or look at it from different angles. ]]
> Best wishes,
> Norman
> [1] http://en.wikipedia.org/wiki/Holographic_storage
> [2] http://en.wikipedia.org/wiki/3D_optical_data_storage

David A. Lee

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