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105 lines
4.8 KiB
105 lines
4.8 KiB
From Gerhard Monzel AT sap.com
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Date: Mon, 21 Jan 2002 09:36:34 +0100
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While producing mpeg videos, you have to give special attention to the
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desired image size of your output video (this is a big difference to
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avi-encoding). Because mpegs, especially VCD and SVCD can be played in
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consumer players, there are some restrictions in "resizing" !
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Usually image sizes for VCDs are 352x240 (NTSC) or 352x288 (PAL), for
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SVCD 480x40 (NTSC) or 480x576 (PAL) -> some consumer players can play VCD
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sized videos as SVCDs too and vice versa.
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As a further difference to AVIs, the aspect ratio of the video is stored
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in header of the mpeg-videos. Therefore most players (pc and consumer
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players) will display the right image proportions no matter what X/Y dimensions
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your video is encoded.
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In your case of a 720x576 vob in 16:9 aspect ratio, you should use
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480x576 as destination image format (PAL) and (force) apsect ratio to 16:9.
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The resulting video will look like expected on PCs with e.g. mplayer,
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which will honore the aspect ratio, but many consumer players have problems
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switching to 16:9 format in VCD/SVCD mode -> in this case you need a resizer
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capable of generating black bars on top/bottom of your video.
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This can't be done with the trancode resizer, but with the internal mpeg-
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module resizer (I mean -y mpeg not -y mpeg2enc). You can also mix the resizer
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of transcode with the resizer of the mpeg-module too.
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On base of this knoweledge you will concluse, that following options
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should produce a "original" looking picture on a 4:3 aspect ratio:
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"-B 5 -y mpeg -F s,3 ..."
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"-B 5" => 720x416 (formats image near to 16:9),
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"-F s,3" => 480x576 (formats image to SVCD size with black bars on top
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(80 pixels) and botton (80 pixels), so it will look well on 4:3 display.
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Because of using only high speed resizing options, the transcoding
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process will run in better speed than using zoom or other slow rescaling
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options. Unfortunately transcode itself can't handle black bars for output
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video (in AVI world there is no need of this feature and transcode begans as
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AVI transcoding tool). Of cause this described solution won't work
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with other export modules e.g. "mpeg2enc" (it lacks of internal black bar
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resizer).
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There is no suggestion to calculate filesize for mpegs with variable bitrate
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(usually used in SVCD mode), because filesize depends on kind of your
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source video (luminances, motion etc.) and the quality value (quant_value)
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you are using while encoding. Only the max. possible size is known by the
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max. bitrate restriction for SVCD and the count of frames.
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to avoid problems with oversized mpeg-files (won't fit on CD), the mpeg-
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encoder module can forced to split the encoded file in several parts.
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The size per part in MB is given by the "max_file_size"-Parameter used in
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conjunction with a user-profile
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(-> remember option -F s,3,<name of user-profile>, where this profile may
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look like:
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max_file_size = 700
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...
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The encoding speed in SVCD-mode varies in a wide range, because of the
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big influence of bitrate control method: the encoder calculates the actual
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bitrate for all frames of one GOP (group of pictures) and if the max.
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bitrate limit is exceeded, the whole GOP will be encoded again with a higher
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quantizer value until bitrate fit into limit. Bitrate (for SVCDS) depends
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mainly from "quant_value" (lower value / better quality -> higher
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bitrate). In conclusion higher quality leads often to lower speed, so it's
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your decision what is more worth for you :-)
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But there there are some tricks to reduce bitrate for mpeg-2 typed streams
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without loosing much quality:
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- use of parameter "qscale_type = 0" in your own user-profile (worth to try)
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- use image size 352x288 instead 480x576, but with smaller "quant_value"
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(3-5) in your user-profile
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(most consumer SVCD players have no problem to play such sized streams)
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maximum effect results in combination of both hints.
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To observe actual bitrate during encoding you should set transcode option
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"-q 2", so you can run short encoding turns to find out the best quant_value
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settings.
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To test the maximum encoding speed you should use option -F 1,3
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so you'll see how much the SVCD bitrate control will drop speed.
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Common problems:
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----------------
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Green artefacts may be caused from wrong resizing too, please try with
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options of the above description.
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Using "-V" option (needed to encode mpegs) will force import modules to
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use internal YUV-format instead of RGB.
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But there are several different YUV-Formats and for example some of it
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has swapped chroma (UV-) components.
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Weird color on encoded mpegs is not a bug, therefore you can use the "-k"
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option to "reswap" the croma components.
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_______________________________________________
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transcode-users mailing list
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transcode-users@theorie.physik.uni-goettingen.de
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http://www.theorie.physik.uni-goettingen.de/mailman/listinfo/transcode-users
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