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Posted by Toby on 11/16/06 00:15
"Jukka Aho" <jukka.aho@iki.fi> wrote in message
news:wxz6h.52469$7e.19341@reader1.news.jippii.net...
> Richard Crowley wrote:
>
>>>>> Interesting that say converting from NTSC to PAL is best? I've
>>>>> always found it better the other way. It's always better to down
>>>>> convert from a superior format than try to add data upwards.
>
>>>> Spatial interpolation is significantly easier than temporal.
>
>>> That's true as a general statement, but does it work as an answer to
>>> the question? I'd argue that for a perfect PAL <=> NTSC conversion
>>> you will have to do temporal interpolation regardless of the
>>> direction of the conversion.
>
>> There is no "perfect conversion".
>
> Some conversion methods are more perfect than the others. :)
>
>> However going from 30FPS to 25FPS at least gives you more to work with
>> than you need. Going in the other direction requires the
>> hardware/software to "use its imagination" to fill in the missing pieces.
>> Which one do you think is easier to do?
>
> It's easy to fill in the missing pieces by duplicating frames. Dealing
> with the excess by doing the converse (dropping frames) is equally easy.
> Both these simple methods lead to a similar kind of distortion: aliasing
> in the temporal dimension. Motion that was steady and smooth in the
> original clip no longer isn't: it has a beating discontinuity at those
> points where frames were duplicated or dropped.
>
> The only way around this problem, except for slowing down or speeding up,
> is isolating the moving objects/scenes, calculating motion vectors for
> them, and synthesizing new frames out of thin air which depict moving
> objects at locations where they must have been in-between the original
> frames, but which were never actually shot with a camera.
>
> --
> znark
Yes, this is true. I wonder why all the NTSC->PAL conversions I have seen
look better than the PAL->NTSC ones. Perhaps it is just a question of the
quality of the converters used.
Toby
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