HDRI_test1 uses a map generated with MegaPOV 1.2.1. From top to bottom, this 
was rendered with MegaPOV map_type 1; MegaPOV map_type 7; and finally with 
version 3.7 beta 32.

HDRI_test2 uses a map from a hdri collection, and was rendered identically 
with megapov and 3.7.

The scenes rendered with map_type 7 are clearly different. So, in what 
situations is this particular parameter used? The documentation is unclear 
about this.

Thanks for any info.

Thomas

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"Thomas de Groot" <tDOTdegroot@interDOTnlANOTHERDOTnet> wrote:
> The scenes rendered with map_type 7 are clearly different. So, in what
> situations is this particular parameter used? The documentation is unclear
> about this.

It depends on the shape of the light probe. If you have a lat-long projection,
like this:

http://www.trinity3d.com/media/dosch_design/dosch_hdri/City_At_Night/example2.jpg

....you use map_type 1, in which the top and bottom rows of the image correspond
to the north and south poles of the sphere.

If, however, you use spherical projections (i.e., as photographed from mirror
balls), such as this:

http://gl.ict.usc.edu/HDRShop/tutorial/images/uffizi_probe.jpg

....you need the map_type 7, which the standard pov 3.6 doesn't support.

I think (but have never tried) you can set up the povray camera to produce
either projection if you're generating your own probe.

Bill

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> HDRI_test1 uses a map generated with MegaPOV 1.2.1. From top to bottom, this 
> was rendered with MegaPOV map_type 1; MegaPOV map_type 7; and finally with 
> version 3.7 beta 32.
> 
> HDRI_test2 uses a map from a hdri collection, and was rendered identically 
> with megapov and 3.7.
> 
> The scenes rendered with map_type 7 are clearly different. So, in what 
> situations is this particular parameter used? The documentation is unclear 
> about this.
> 

   It all depends on the map you're using... in both of your tests, the HDRI 
used seems to be equirectangular, so this is why map_type 1 works, and 
map_type 7 doesn't because it was designed for another type of projection. I 
don't recall how it's named, but there was a recent post by clipka 
explaining just this and other interesting facts about the different 
projection types used for HDRI.

--
Jaime

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Bill Pragnell wrote:
> "Thomas de Groot" <tDOTdegroot@interDOTnlANOTHERDOTnet> wrote:
>> The scenes rendered with map_type 7 are clearly different. So, in what
>> situations is this particular parameter used? The documentation is unclear
>> about this.
> 
> It depends on the shape of the light probe. If you have a lat-long projection,
> like this:
> 
> http://www.trinity3d.com/media/dosch_design/dosch_hdri/City_At_Night/example2.jpg
> 
> ....you use map_type 1, in which the top and bottom rows of the image correspond
> to the north and south poles of the sphere.
> 

thats's right

> If, however, you use spherical projections (i.e., as photographed from mirror
> balls), such as this:
> 
> http://gl.ict.usc.edu/HDRShop/tutorial/images/uffizi_probe.jpg
> 
> ....you need the map_type 7, which the standard pov 3.6 doesn't support.
> 

I think you are confusing things here a little. What you call spherical 
projection is named angular projection (as used for so called light 
probes) and those 'probes' are different to mirror ball photographs 
because they are already post processed and corrected for the outer 
distortion of mirror ball photos. So the 'Uffizi' - and the well known
'Kitchen' - probe are directly usable with MegaPov's map_type 7.
Mirror ball photos have to be processed with some external tool.

-Ive

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Ive <"ive### [at] lilysoftorg"> wrote:
> I think you are confusing things here a little. What you call spherical
> projection is named angular projection (as used for so called light
> probes) and those 'probes' are different to mirror ball photographs
> because they are already post processed and corrected for the outer
> distortion of mirror ball photos. So the 'Uffizi' - and the well known
> 'Kitchen' - probe are directly usable with MegaPov's map_type 7.
> Mirror ball photos have to be processed with some external tool.

I didn't know the precise terminology for it, so my 'spherical' was just a
description! I realise there is more to it than just a photograph of a ball,
but I was trying to keep it simple to distinguish between the two projections.

(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!).

:-)

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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

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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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Ive <"ive### [at] lilysoftorg"> wrote:
>  the importand thing is: The 'mirror ball'
> *projection* is different to the 'angular' *projection*. But I admit
> they can very easily be confused.

I am well aware of the difference, and that using a 'mirror ball' projection
with map_type 7 will produce a distorted reflection. All I meant was that in my
experience (I have used both types of projection in hdri-lit renders), it can be
good enough (scene geometry and view permitting). Comparing the techniques will
make the distortion obvious, and an expert may spot it immediately, but it's
not the end of the world! :-)

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"Thomas de Groot" <tDOTdegroot@interDOTnlANOTHERDOTnet> wrote:
> HDRI_test1 uses a map generated with MegaPOV 1.2.1. From top to bottom, this
> was rendered with MegaPOV map_type 1; MegaPOV map_type 7; and finally with
> version 3.7 beta 32.
>
> HDRI_test2 uses a map from a hdri collection, and was rendered identically
> with megapov and 3.7.
>
> The scenes rendered with map_type 7 are clearly different. So, in what
> situations is this particular parameter used? The documentation is unclear
> about this.
>
> Thanks for any info.
>
> Thomas

To further clarify the differences for you:

lat/long (spherical panorama): Each x coordinate represents the angle around the
sphere from 0 to 360deg, each y coordinate represents the vertical angle from
-90deg to +90deg.  This results in the poles being quite stretched in the
unmapped image, but this is not an issue.

mirror ball: A direct image (or could be generated using software) of a mirrored
ball.  Each point of the visible side of the ball reflects the scene based on
the angle of incidence.  The centre reflects straight back, progressing to a
reflection from behind the ball at the edges (or as close as the camera
parallax/perspective will allow).  The progression from the front to rear
reflection is non-linear as it is related to the angle of incidence with the
surface of the sphere.  This results in a pinching effect from the edges of the
ball due to poor resolution at this area.  These are not usually used for direct
mapping, but are usually converted to lat/long or angular map types.

Angular (also called probe sometimes):  Appears similar to mirror-ball as it is
circular and represents the scene from front to back as it progresses from the
center to the edges.  However, the progression here is linear (i.e.,
centre=0deg, 1/2way out=90deg, edges=180deg).  The linear progression helps to
preserve an adequate resolution for the entire map area and is also very simple
to unwrap and map.

-tgq

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Bill Pragnell wrote:
> I am well aware of the difference, and that using a 'mirror ball' projection
> with map_type 7 will produce a distorted reflection. All I meant was that in my
> experience (I have used both types of projection in hdri-lit renders), it can be
> good enough (scene geometry and view permitting). Comparing the techniques will
> make the distortion obvious, and an expert may spot it immediately, but it's
> not the end of the world! :-)
> 

;) definitely not!

I just wanted to make it clear for everybody that there is a difference 
and (as Trevor also mentioned) I've seen quite some images where this 
resulting distortion *was* visible and it was obviously not the 
intention of the artists but lack of knowledge about those projection types.

-Ive

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