Ive <"ive### [at] lilysoftorg"> wrote:
> Bill Pragnell wrote:
> > (Actually you can use a ball photograph without processing, as long as you can
> > work around the 'pinched' area - to say nothing of the photographer!).
> >
>  > :-)
>
> err, I really hate it to sound so picky but actually you can't use a
> mirror ball photograph without processing - at least if the image is
> used as a reflection map - because it will look distorted.
>
> The generation of a photographic 'probe' works usually like this: take a
> two photos of a chrome ball from opposite directions.
> Remap both images from the 'mirror ball' projection (sorry this time I
> do not know the exact therm - but maybe there is none) to
> latitude/longitude projection and compose them. Then map from
> latitude/longitude to angular projection to get the so called probe.
>
> It is just a nice side effect that this way the reflection of the
> 'photographer' can be removed, the importand thing is: The 'mirror ball'
> *projection* is different to the 'angular' *projection*. But I admit
> they can very easily be confused.
>
> -Ive

Well you *could* use a mirror-ball without processing, but neither POV nor
megaPOV have a map-type that corresponds to it (as has been noted, map type 7
is not equivalent to mirror-ball and using it as such will present quite a bit
of distortion, I see this mistake made quite often on DeviantArt for example).
You could write a custom function that would unwrap and properly map the
mirror-ball, but this would involve lots of math and trig to figure out
correctly.

A HDRI map can be produced from a single mirror ball, however there are some
limitations:
-because of perspective/parallax, you don't quite get the full 360deg
-you tend to get a pinching and low resolution issues from the edges of the ball
(which reflects the rear of the scene)
-the photographer will be visible
There are situations where this may be acceptable (eg, the reflections are such
that the pinching effect or photographer wouldn't be obvious)
The *preferred* method for creating is as is described above, with two
perpendicular shots of the mirror ball (this allows both the distorion/low
resolution and photographer issues to be removed).  However, for the creator,
this requires meticulous processing in matching and masking/blending the two
together.

I find that the best way to handle HDRI maps is to convert them all to lat/long
format (I use HDRShop) whether from angular or mirrorball (just make sure you
use the correct corresponding map types for conversion or you will also get
major distortion, at least with lat/long, any severe distortion from incorrect
map types should be quite obvious also, this map-type is supported by both POV
3.7 and megaPOV as it is just your basic spherical panorama mapping.

-tgq

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