Hi everyone,

I've got a question regarding bounding boxes. I've just started using
POV-Ray again (the last time I did was a long time ago, back when programs
like DKBTrace and Vivid 1.0 were still around), and I decided to render a
(very small) version of the "Last Guardian" scene by Johnny Yip (which won
POVComp 2004) as a test.

This was successful, too, but I noticed two things: one, the render took
very long (about 8:20 hours on a P4/2.66 GHz/512 MB RAM, for a 120x160
picture); and two, the intersection stats that POV-Ray dumps at the end of
the render show that the success rates for many of the shapes are extremely
low, and that there are far more intersection tests with the actual shapes
than bounding box tests - about 35 billion vs. only 500 million (see
below).

Is this to be expected, or does it hint at possible bugs in POV-Ray's
automatic bounding box generation? Thanks. :)

--- Intersection test stats ---

Blob                              2453              97      3.95
Blob Component                     910             420     46.15
Blob Bound                       12265             910      7.42
Box                          147470815        35655239     24.18
Cone/Cylinder               1063684672         2824318      0.27
CSG Intersection              98485645         2242437      2.28
CSG Union                   1332219932          968874      0.07
Height Field                  16208116           52465      0.32
Height Field Box              16208116          258793      1.60
Height Field Triangle           254001           52833     20.80
Height Field Block              514866          153635     29.84
Height Field Cell              2996414          140677      4.69
Isosurface                    13916160         1245999      8.95
Isosurface Container       32828694974        13926910      0.04
Isosurface Cache                235795           45256     19.19
Lathe                             1252             296     23.64
Lathe Bound                       1252             717     57.27
Plane                         22451753        15033221     66.96
Prism                           112560           37668     33.46
Prism Bound                     725170          432584     59.65
Sphere                        24797823         3543791     14.29
Sphere Sweep                 205080375          268461      0.13
Torus                         49374207          152711      0.31
Torus Bound                   49374207          193932      0.39
Bounding Box                 533615869       127430453     23.88
Light Buffer                2709788124      1107570558     40.87
Vista Buffer                  16121912        13135844     81.48

--- Intersection test stats ---

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> Is this to be expected, or does it hint at possible bugs in POV-Ray's
> automatic bounding box generation? Thanks. :)

As far as I know, that scene used a lot of CSG and blobs and such.
Especially CSG is hard to bound automatically. It's not really a bug, but a
known draw-back of using bounding boxes. This has become a much larger issue
lately, now that PC's can handle a lot more than in the early days, and
bounding boxes aren't always efficient in terms of actually bounding an
object (as mentioned above, especially when lots of CSG is involved). Have
you tried +ud in the commandline? If I'm not mistaken, that shows the
bounding boxes, and you could have a look at the troubles for yourself. :-)

-- 
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>

Post a reply to this message

"Tim Nikias" <JUSTTHELOWERCASE:timISNOTnikias(at)gmx.netWARE> wrote:
> Have
> you tried +ud in the commandline? If I'm not mistaken, that shows the
> bounding boxes, and you could have a look at the troubles for yourself. :-)

I'll give that a try, thanks! :)

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