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strange results in rescaling video

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MarcoTKY
strange results in rescaling video
on Jul 16, 2007 at 6:21:08 am

Hi, this is not really a problem but something I don't understand well.
I get clips from my fs4prohd in mov or avi in standard DV resolution 720x576. If I scale and encode the video (flash encoder or quicktime conversion) down to base 400xsameratio i get clips full of "waves" on movement, not on still subjects. This happends in flv, mp4 and wmv. If i do exactly the same but on base 380 or 420 this wave effect is insignificant and very very little.
There are multiples to respect we downsizing the video?
I always respect the PIXEL RATIO but it seems to be just a part of the job.
Any suggestions? Thaks!


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Danny2007
Re: strange results in rescaling video
on Jul 16, 2007 at 12:29:07 pm

See the post below called "400x300 FLV Not right?" - it maybe that you are sauffering from the frame size not being divisable by 8.

What product are you using for encoding?


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MarcoTKY
Re: strange results in rescaling video
on Jul 16, 2007 at 4:59:25 pm

I'm using flash encoder on mac os x. If I understand well I always have to choose numbersi divisible by 8. Correct?


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cow
Danny2007
Re: strange results in rescaling video
on Jul 16, 2007 at 6:10:40 pm

The Flash encoder can kick out good quality output, however I'm not sure about its scaling or deinterlacing abilities.

As for being divisable by 8, that should be on both axis e.g. 384x288, 320x240, 416x312.


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MarcoTKY
Re: strange results in rescaling video
on Jul 17, 2007 at 4:34:10 am

Hummm... I did some tests but the best result is 480x384. There is a matematical rule behind this? Do you know the technical reason why must be divisible by 8 per each side?
Thanks for your precious help!


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Danny2007
Re: strange results in rescaling video
on Jul 17, 2007 at 5:51:25 am

480x384 is not 4:3
480x360 is 4:3

So I'm confused as to why you got the best results by squashing your frame

8x8 is not a rule, it's not essential to adhere to the divisible by 8 idea but:

http://www.google.com/search?q=8x8+macroblocks+&hl=en&start=0&sa=N



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MarcoTKY
Re: strange results in rescaling video
on Jul 17, 2007 at 6:01:59 am

Now you uderstand why I'm confused too. If you want I can post a llink of some examples. The 480x384 ratio did not come froma dream or the bingo but from the the 5:4 export dialogue of FCP. I got good results by chance and I kept them.
At this point, beeing so many variables (i did some googloling with your keywords) I believe the best thing to do is to write down a table og "good sizes" and work with it...
Thanks for your help.


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Danny2007
Re: strange results in rescaling video
on Jul 17, 2007 at 4:32:33 pm

If you shot 4:3 you should export and remain as 4:3 whatever the settings are within your FCP sequence - remember that 720x576, 768x576, 640x480 are all 4:3 frame aspect - however 720x576 uses non-square pixels whereas the others (and all computer screens) use square pixels.


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