From Pov-Ray help:

6.11.11.2.5  error_bound

The error_bound float value is one of the two main speed/quality tuning
values (the other is of course the number of rays shot). In an ideal world,
this would be the only value needed. It is intended to mean the fraction of
error tolerated. For example, if it were set to 1 the algorithm would not
calculate a new value until the error on the last one was estimated at as
high as 100%. Ignoring the error introduced by rotation for the moment, on
flat surfaces this is equal to the fraction of the reuse distance, which in
turn is the distance to the closest item hit. If you have an old sample on
the floor 10 inches from a wall, an error bound of 0.5 will get you a new
sample at a distance of about 5 inches from the wall.

If I understand this right, we can calculate pretrace block size to match
with error_bound using formula:

  pretrace (_end) = error_bound * minimum_reuse

We have to assume that low_error_factor is one. This pretrace value is
smallest useful pretrace_end value and lower values are waste of time. When
always_sample is set to off this value should be used to get equal detail
when using radiosity always_sample on. Am I correct?

Radiosity samples are distributed evenly if there is one pretrace step. Is
there any use to set pretrace_start to different value from pretrace_end
value?

Matti

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Matti Karnaattu wrote:
> 
> [...]
> 
> If I understand this right, we can calculate pretrace block size to match
> with error_bound using formula:
> 
>   pretrace (_end) = error_bound * minimum_reuse

No, pretrace_end is the pretrace block size as fraction of the render
size.  error_bound on the other hand refers to scene geometry.  Therefore
you can't give a precise best pretrace_end for every render.  It strongly
depends on the view of the scene and the camera.  

> We have to assume that low_error_factor is one. This pretrace value is
> smallest useful pretrace_end value and lower values are waste of time. When
> always_sample is set to off this value should be used to get equal detail
> when using radiosity always_sample on. Am I correct?

As already said you are making wrong assumptions from the beginning but
concerning planning the pretrace another radiosity parameter is important
too: nearest_count.  It strongly affects whether new samples are taken or
not, both during pretrace and during final render.  And always_sample does
not really turn taking of samples completely off during final trace, IIRC
it just sets nearest_count to 1.

> Radiosity samples are distributed evenly if there is one pretrace step. Is
> there any use to set pretrace_start to different value from pretrace_end
> value?

Yes, there is - quality of results tends to get better with larger
pretrace_start because the choice of positions where radiosity samples are
taken is better balanced.  The difference in quality of course might be
low compared to the slowdown.

Christoph

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