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Forums Compression Techniques strange results in rescaling video

  • strange results in rescaling video

  • MarcoTKY

    July 16, 2007 at 6:21 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 720×576. 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!

  • Daniel Low

    July 16, 2007 at 12:29 pm

    See the post below called “400×300 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?

  • MarcoTKY

    July 16, 2007 at 4:59 pm

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

  • Daniel Low

    July 16, 2007 at 6:10 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. 384×288, 320×240, 416×312.

  • MarcoTKY

    July 17, 2007 at 4:34 am

    Hummm… I did some tests but the best result is 480×384. 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!

  • Daniel Low

    July 17, 2007 at 5:51 am

    480×384 is not 4:3
    480×360 is 4:3

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

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

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

  • MarcoTKY

    July 17, 2007 at 6:01 am

    Now you uderstand why I’m confused too. If you want I can post a llink of some examples. The 480×384 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.

  • Daniel Low

    July 17, 2007 at 4:32 pm

    If you shot 4:3 you should export and remain as 4:3 whatever the settings are within your FCP sequence – remember that 720×576, 768×576, 640×480 are all 4:3 frame aspect – however 720×576 uses non-square pixels whereas the others (and all computer screens) use square pixels.

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