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  1. Member
    Join Date: Mar 2008
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    Weīre filming with DV 720 x 480 widescreen and convert this to MP4 (x264, AAC). The result is for streaming via LAN and Internet. Neighborhood and relatives, friends in different continents.
    Bitrate of the video stream is only 1500 Kbps, since many have only 2 mb connections.

    Previously we used 640 x 360 px for the video. But after a discussion about pixel size, we have tried 624 x 352. And also 640 x 352. Because of the divider 16.
    We put the different videos online, so people could compare. But the results of the comparison gave out no clear winner. Differences were not evident. Sometimes 640x360 was better - sometimes 624x352.

    So: What would be the best size, at least theoretically, for a 16:9 video in X264?
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  2. What would be the best size, at least theoretically, for a 16:9 video in X264?
    The input size with the same pixel aspect ratio as the source material,..
    (x264 + 1500kbit/s should be fine for SD streaming assuming you use decent x264 settings,...
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  3. Member
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    Originally Posted by Selur View Post
    What would be the best size, at least theoretically, for a 16:9 video in X264?
    The input size with the same pixel aspect ratio as the source material,..
    (x264 + 1500kbit/s should be fine for SD streaming assuming you use decent x264 settings,...
    Thanks, Selur.
    But we already get excellent picture quality in a 16:9 format in our streaming video

    The question was: Theoretically, what would be the best, the most correct pixel size for a 16:9 SD stream?
    As I said, we used 640x360 before - no problem, than we tried 640x352 and 624x352.
    Because of the multiples of 4, multiples of 16 ....
    Visually theyīre quite the same : all look pretty good. And noone figured out the difference....

    So itīs just my curiousity that lead me to the question.
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  4. Okay, what is to consider:
    0. the content type (natural, cartoon, noise, details rich/arm,...)
    1. Human Visual System (depending on the content and it might be better to i.e. encode anamorph, change the contrast,...)
    2. Dimensions and pixel distance of the playback device combined with the knowledge what resizer are used to fit the dimensions of the playback device. (i.e. depending on the resizer and the input it might be better to have a high detail preserving low resolution output or a low detail preserving but edge enhanced high resolution output)
    3. How the encoder handles the different resolution and details regarding detail preservation and creating new artifical details (i.e. due to ringing&co)
    4. How to calculate the objective quality to compare the combinations. (or you need to do a lot of blind tests)

    Since 0. changes, 1. is not really well known, 2. also varies normally and computational objective quality is a dream you normally end up with the following conclusions:

    1. assuming you input has the details to support it, more pixel are always better (that's why they came up with anamorph encoding for systems with a fixed pixel size like dvds) unless you encoder will produce a too high datarate or starts to produce to much encoding artefacts. (but like I said x264 should be fine for normal sd material and 1500kBit/s)
    2. if you downsize (not the case her) a lot smooth a bit before downsizing
    3. a bit of denoising before reencoding can really make a huge difference
    4. if you want to / need to use metrics like ssim to get a rudimentary grab on automated quality comparisons

    Cu Selur
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  5. aBigMeanie aedipuss's Avatar
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    without getting too technical... the highest quality will be produced when you eliminate re-sizing. the dvavi is 720x480 with non-square pixels. to convert (not re-size) to square pixel mpeg-4 only change the horizontal to 848 (mod16). try it at 848x480 and see how it compares.

    you can use the same bitrate.
    --
    "a lot of people are better dead" - prisoner KSC2-303
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  6. @aedipuss: Would be nice if you could explain your reply a bit further.

    to convert (not re-size) to square pixel mpeg-4 only change the horizontal to 848 (mod16).
    How do you do that, I mean getting from anamorph 720x480 to square pixel 848x480 without resizing?
    Why change to square pixel and not stay anamorph?
    Why mod16, resizing to mod16 instead of mod2, mod4 or mod8 should not really change much (I doubt it would be a noticeable change) when using x264?

    Thanks!
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  7. aBigMeanie aedipuss's Avatar
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    going from anamorph 720 to 8xx changes the number of pixels but not the screen size. yes it "resizes" the horizontal number of pixels but not the vertical.

    ntsc widescreen dvavi is weird in that 480x16/9 = 853.3 not a number you'd like to encode to. pal is much cleaner at 576x16/9= 1024 which also is mod16. for ntsc anything from 848 to 864 can be used there is no one best number they are all close enough. mod16 compliance i've use for 20 years just because it was needed for older encoders.
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    "a lot of people are better dead" - prisoner KSC2-303
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  8. ah, okay, I just thought I missed something fundamental in my years of video editing.
    (one question is still open: "Why change to square pixel and not stay anamorph?", unless the client doesn't support, it sounds like the best way to go for me,..)
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  9. aBigMeanie aedipuss's Avatar
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    that's just it, the anamorphic aspect ratio flag in mp4 isn't very well supported. square pixel will play on all.
    --
    "a lot of people are better dead" - prisoner KSC2-303
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  10. Always Watching guns1inger's Avatar
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    If you are already resizing to 640 x 360 (or variants thereof) the retaining the anamorphic features obviously isn't a priority anyway
    Read my blog here.
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  11. get rid of anamorph for sure, as you do ,
    stay with whatever you do, you are using x264
    you are limited by bit rate, nothing to do much here,
    do not notice a difference between divider 16, 8, ...etc, there used to be a time a might add borders or crop in Avisynth to get divider 16, because I was told to, but not anymore

    The only thing that comes on my mind is to think about format profile and I'd be curious about is how it is handled by latest Pads , Ipods, Iphones or phones generaly ? I 'd say stay with Baseline @3.0 ?
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  12. Member
    Join Date: Mar 2008
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    Thanks to all of you for replying. Pretty nice stuff to read.

    @aedipuss
    Yes we tried 848x480 just for fun, same bitrate (we canīt get higher). And, yes. At least it "seams" to look better at full screen (we use JW Player for streaming, people get a YouTube like feeling, easy to use- for kids and grandmas)
    Maybe we stay with that. At least for while

    And the reason we use square pixels: you will always get the 16:9 AR. Even when people download the video and watch it locally on WMP or any other player.

    But as I read in your answers, and what we noticed by our (nothing official) tests: forget about the multiple factor. You canīt see the difference. (at least not with a bitrate at 1500 kbps)
    640x360 looks just like 624x352, and maybe 848x480 looked only a bit different because it was new to our eyes.

    @ _Al_
    Exactly my thinking !
    And yes, we feed to Ipad, and even phones too. Whereever these people are, whatever they use. (Itīs private family -, neighborhood stuff)
    Right now we use Main @ 3.0 - no complaints.

    And X264 really is an excellent codec. At our 1500 bitrate, lot of people are saying, that this streaming video looks better than their normal TV stations... Big thanks to the X264 team !!!
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