In article <40a5cf0b@news.povray.org>,
 Stefan Viljoen <rylan@<deletehis> wrote:

> 1. What is a common trace time for complex images (lots of reflection /
> refraction, many objects)

Minutes to hours, for me.


> I realise this is subjective, but I have reasonably simple scenes (10 000
> objects, and about 30 reflective/refractive objects, ten light sources)
> that take literally weeks to render while various IRTC entries I have seen,
> with scenes literally thousands of times more complex (as regards APPARENT
> objects and reflections / refractions) done with PovRay, render in times of
> 2 or 4 hours. So IS two weeks tracetime (or even a month) for a 1152x864
> image acceptable, or am I doing something exceedingly stupid? Or is this so
> subjective its not worth an answer? I am referring to times on a
> "standard"-ish system - P4 2.0 GHz with 512MB on Rh9 Linux, the raytrace
> being the only user process running.

You're probably doing something stupid. ;-)
There are things that will take that long, but usually just odd 
experiments. Typical scenes don't take weeks to render.


> 2. How exactly to use adc_bailout?
> 
> To decrease render time in scenes with lots of reflections / refraction, it
> seems to me that I should use larger adc_bailout values (0.05+ ?) and lower
> max_trace_level values (5 or less?) The default values are taking simply
> too long - and apparently there is no difference for me yet, as a test
> scene is taking forty hours to get to a certain point, with or without
> adc_bailout set to 0.05 and max_trace_level set to 5, while having the
> reflective and refractive objects in that particular part of the image.

I generally set max_trace_level to at least 12. Higher values of 
adc_bailout will make POV stop tracing sooner. My guess is that you're 
doing something else that's causing the high render times.


> 4. Pointers / hints?
> 
> Any URL's or info on PovRay optimisation? I have often looked for
> optimisation guides for programming, but are there any for the PovRay scene
> language?

None that I know of which are specifically for optimizing. The manual 
has some general pointers for specific features like radiosity and 
photons.


> 5. Reasons for scanline renderers being faster - is scanline better?

It is for realtime display of the type used by today's games, and for 
other applications where speed is of great importance.

Raytracers simulate the actual paths light takes through the scene. It 
follows the light that in the real world would enter the camera back 
into the scene, and then figures the illumination from light sources and 
the environment which contributes to that light. It handles refraction, 
reflection, and diffuse illumination with trivial additions, and can 
easily be extended to include things like photon maps.

Scanline renderers, on the other hand, use a process more like painting 
or drawing. They consider the scene as a bunch of polygons, and project 
each polygon onto the screen, filling each polygon scanline by scanline 
and compositing it onto the existing image. They really can't do 
reflection or refraction, they have to fake it with things like 
environment maps...a view of the surrounding scene is rendered from a 
point inside the refracting/reflecting object, and that view is mapped 
on the surface of the object, using the normals and camera position to 
make it look something like reflection or refraction. You could do the 
environment map for every vertex or every pixel instead, but this would 
basically be doing raytracing with a scanline engine, and would probably 
be even slower than an actual raytracer. Scanline renderers usually use 
raytracing for accurate reflections, shadows, and transparent objects.


> I have seen images created with 3DStudio that would (I think) take YEARS to
> render with PovRay.

Unlikely. Raytracing actually performs better than scanlining with 
extreme amounts of detail.


> Which technique is better, i. e. which program is
> better - a scanline renderer (what does that mean? What is the difference
> between that and raytracing?) or a raytracer?

Raytracers are more accurate. Scanline renderers are generally faster 
for typical scenes being done today.


> I have heard about IPAS
> modules for 3DStudio that do raytracing - so 3DS can do both? Why is
> scanline so very much faster than raytracing, but looks just as good?

3DS can mix raytracing into its scanline renderings. I don't know if it 
does full raytracing...doing so would not be difficult, though. And 
scanline renders typically *don't* look just as good

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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