I've just completed the rendering of a cyclical animation which took several
days to complete. As the output is to be for television I switched on
povray's field rendering and also antialiasing.

I was expecting to see each generated image having sets of duplicated
consective lines which happens with field rendering when antialiasing is
switched off (i.e. line 1 and 2 identical, 3 and 4 identical, etc...).

However with antialising switched on, I was surpised to see every line was
uniquely different. And zooming into an image I can see that each line is
antialiased, but when viewed in total the image looks pixelated as if there
was little of no antialiasing present. I suspect that the antialiasing with
field rendering is not working "correctly" - perhaps a bug?

Is there anyone who knows how povray is supposed to perform antialiasing
when field rendering is switched on?

Thanks

Colin

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I think that this may be a bug.

I've had a look at the source code and it reveals that POV-Ray behaves in a
strange way when anti-aliasing is used with field rendering.

If you have anti-aliasing switched off, it behaves the way most people
expect. It calculates and renders only the odd-numbered lines of a frame -
on the screen the even numbered lines appear as black lines, but in the
output file they are an exact copy of the calculated lines. In other words
line 1 is calculated and output as line 1 and 2, then line 3 is calculated
and output as line 3 and 4, and so on. For the next frame only the
even-numbered lines are calculated and duplicated in a similar way.

However, when you switch on anti-aliasing the behaviour changes. For a
particular frame, POV-Ray calculates and renders ALL of the lines (odd and
even) individually as if field rendering was not switched on! But then it
anti-aliases the odd-numbered lines, leaving the even lines with no
anti-aliasing. For the next frame all of the lines are calculated and
rendered, and then the even lines are anti-aliased with the odd lines left
with no anti-aliasing.

This can be seen in the function Start_Non_Adaptive_Tracing() in render.c

This is very strange. The whole point of using the field rendering feature
of POV-Ray is to save time by only calculating HALF of the lines in each
image, but when anti-aliasing is switched on ALL of the lines are being
calculated (although only half are anti-aliased). So it seems that POV-Ray
is wasting a lot of time unnecessarily calculating the "unused" lines!

If any of the POV-Ray development team read this, can you comment on whether
this  is a BUG?

Thanks

Colin




"PovRAY" <cbd### [at] my-dejacom> wrote in message
news:3ccc7162@news.povray.org...
> I've just completed the rendering of a cyclical animation which took
several
> days to complete. As the output is to be for television I switched on
> povray's field rendering and also antialiasing.
>
> I was expecting to see each generated image having sets of duplicated
> consective lines which happens with field rendering when antialiasing is
> switched off (i.e. line 1 and 2 identical, 3 and 4 identical, etc...).
>
> However with antialising switched on, I was surpised to see every line was
> uniquely different. And zooming into an image I can see that each line is
> antialiased, but when viewed in total the image looks pixelated as if
there
> was little of no antialiasing present. I suspect that the antialiasing
with
> field rendering is not working "correctly" - perhaps a bug?
>
> Is there anyone who knows how povray is supposed to perform antialiasing
> when field rendering is switched on?
>
> Thanks
>
> Colin
>
>
>
>
>

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Colin wrote:
> Is there anyone who knows how povray is supposed to perform antialiasing
> when field rendering is switched on?

I'm not quite sure, but I think this is the way that it has to work. 
Anti-aliasing requires knowing the colors of surrounding pixels, and 
that includes pixels to the top and the bottom. The only way to know 
them is to calculate them.

Note that from your description, POV *is* saving time when you field 
render with anti-aliasing switched on: it isn't anti-aliasing the 
"unused" lines.

Jerry
-- 
http://www.hoboes.com/jerry/
"Give a man a fish and you feed him for a day. Teach him to fish, and you've
depleted the lake."--It Isn't Murder If They're Yankees
(http://www.hoboes.com/jerry/Murder/)

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Thanks for the quick response Jerry. Yes I agree that it is saving some time
by not anti-aliasing the unused lines.

I had a closer look and although the anti-aliasing is done by shooting extra
rays through the pixels on each "used" line, it first needs to decide
whether a particular pixel actually needs to be anti-aliased in the first
place. It does this by comparing the calculated colour of the pixel with its
neighbour to the left and ABOVE! Thus it needs to know the colour of the
pixel on the "unused" line above, but it seems that it doesn't actually use
the neighbouring pixels to perform the anti-aliasing itself.

Colin

"Jerry" <jer### [at] acusdedu> wrote in message
news:jer### [at] netplexaussieorg...
> Colin wrote:
> > Is there anyone who knows how povray is supposed to perform antialiasing
> > when field rendering is switched on?
>
> I'm not quite sure, but I think this is the way that it has to work.
> Anti-aliasing requires knowing the colors of surrounding pixels, and
> that includes pixels to the top and the bottom. The only way to know
> them is to calculate them.
>
> Note that from your description, POV *is* saving time when you field
> render with anti-aliasing switched on: it isn't anti-aliasing the
> "unused" lines.
>
> Jerry
> --
> http://www.hoboes.com/jerry/
> "Give a man a fish and you feed him for a day. Teach him to fish, and
you've
> depleted the lake."--It Isn't Murder If They're Yankees
> (http://www.hoboes.com/jerry/Murder/)

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Hello,

I've have the same problem with v3.1. Is it the same in v3.5???

William.

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