178 lines
8.5 KiB
Plaintext
178 lines
8.5 KiB
Plaintext
Video Usability Information (VUI) Guide
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by Christian Heine ( sennindemokrit at gmx dot net )
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1. Sample Aspect Ratio
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-----------------------
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* What is it?
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The Sample Aspect Ratio (SAR) (sometimes called Pixel Aspect Ratio or just
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Pel Aspect Ratio) is defined as the ratio of the width of the sample to the
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height of the sample. While pixels on a computer monitor generally are
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"square" meaning that their SAR is 1:1, digitized video usually has rather
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odd SARs. Playback of material with a particular SAR on a system with
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a different SAR will result in a stretched/squashed image. A correction is
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necessary that relies on the knowledge of both SARs.
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* How do I use it?
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You can derive the SAR of an image from the width, height and the
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display aspect ratio (DAR) of the image as follows:
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SAR_x DAR_x * height
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----- = --------------
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SAR_y DAR_y * width
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for example:
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width x height = 704x576, DAR = 4:3 ==> SAR = 2304:2112 or 12:11
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Please note that if your material is a digitized analog signal, you should
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not use this equation to calculate the SAR. Refer to the manual of your
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digitizing equipment or this link instead.
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A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions
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http://www.iki.fi/znark/video/conversion/
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* Should I use this option?
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In one word: yes. Most decoders/ media players nowadays support automatic
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correction of aspect ratios, and there are just few exceptions. You should
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even use it, if the SAR of your material is 1:1, as the default of x264 is
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"SAR not defined".
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2. Overscan
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------------
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* What is it?
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The term overscan generally refers to all regions of an image that do
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not contain information but are added to achieve a certain resolution or
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aspect ratio. A "letterboxed" image therefore has overscan at the top and
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the bottom. This is not the overscan this option refers to. Neither refers
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it to the overscan that is added as part of the process of digitizing an
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analog signal. Instead it refers to the "overscan" process on a display
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that shows only a part of the image. What that part is depends on the
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display.
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* How do I use this option?
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As I'm not sure about what part of the image is shown when the display uses
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an overscan process, I can't provide you with rules or examples. The safe
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assumption would be "overscan=show" as this always shows the whole image.
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Use "overscan=crop" only if you are sure about the consequences. You may
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also use the default value ("undefined").
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* Should I use this option?
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Only if you know exactly what you are doing. Don't use it on video streams
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that have general overscan. Instead try to to crop the borders before
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encoding and benefit from the higher bitrate/ image quality.
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Furthermore the H264 specification says that the setting "overscan=show"
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must be respected, but "overscan=crop" may be ignored. In fact most
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playback equipment ignores this setting and shows the whole image.
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3. Video Format
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----------------
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* What is it?
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A purely informative setting, that explains what the type of your analog
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video was, before you digitized it.
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* How do I use this option?
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Just set it to the desired value. ( e.g. NTSC, PAL )
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If you transcode from MPEG2, you may find the value for this option in the
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m2v bitstream. (see ITU-T Rec. H262 / ISO/IEC 13818-2 for details)
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* Should I use this option?
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That is entirely up to you. I have no idea how this information would ever
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be relevant. I consider it to be informative only.
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4. Full Range
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--------------
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* What is it?
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Another relic from digitizing analog video. When digitizing analog video
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the digital representation of the luma and chroma levels is limited to lie
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within 16..235 and 16..240 respectively. Playback equipment usually assumes
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all digitized samples to be within this range. However most DVDs use the
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full range of 0..255 for luma and chroma samples, possibly resulting in an
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oversaturation when played back on that equipment. To avoid this a range
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correction is needed.
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* How do I use this option?
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If your source material is a digitized analog video/TV broadcast it is
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quite possible that it is range limited. If you can make sure that it is
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range limited you can safely set full range to off. If you are not sure
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or want to make sure that your material is played back without
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oversaturation, set if to on. Please note that the default for this option
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in x264 is off, which is not a safe assumption.
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* Should I use this option?
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Yes, but there are few decoders/ media players that distinguish
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between the two options.
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5. Color Primaries, Transfer Characteristics, Matrix Coefficients
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-------------------------------------------------------------------
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* What is it?
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A videophile setting. The average users won't ever need it.
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Not all monitor models show all colors the same way. When comparing the
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same image on two different monitor models you might find that one of them
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"looks more blue", while the other "looks more green". Bottom line is, each
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monitor model has a different color profile, which can be used to correct
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colors in a way, that images look almost the same on all monitors. The same
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goes for printers and film/ video digitizing equipment. If the color
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profile of the digitizing equipment is known, it is possible to correct the
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colors and gamma of the decoded h264 stream in a way that the video stream
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looks the same, regardless of the digitizing equipment used.
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* How do I use these options?
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If you are able to find out which characteristics your digitizing equipment
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uses, (see the equipment documentation or make reference measurements)
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then find the most suitable characteristics in the list of available
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characteristics (see H264 Annex E) and pass it to x264. Otherwise leave it
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to the default (unspecified).
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If you transcode from MPEG2, you may find the values for these options in
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the m2v bitstream. (see ITU-T Rec. H262 / ISO/IEC 13818-2 for details)
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* Should I use these options?
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Only if you know exactly what you are doing. The default setting is better
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than a wrong one. Use of this option is not a bad idea though.
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Unfortunately I don't know any decoder/ media player that ever even
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attempted color/gamma/color matrix correction.
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6. Chroma Sample Location
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--------------------------
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* What is it?
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A videophile setting. The average user won't ever notice a difference.
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Due to a weakness of the eye, it is often economic to reduce the number of
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chroma samples in a process called subsampling. In particular x264 uses
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only one chroma sample of each chroma channel every block of 2x2 luma
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samples. There are a number of possibilities on how this subsampling is
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done, each resulting in another relative location of the chroma sample
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towards the luma samples. The Chroma Sample Location matters when the
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subsampling process is reversed, e.g. the number of chroma samples is
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increased. This is most likely to happen at color space conversions. If it
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is not done correctly the chroma values may appear shifted compared to the
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luma samples by at most 1 pixel, or strangely blurred.
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* How do I use this option?
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Because x264 does no subsampling, since it only accepts already subsampled
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input frames, you have to determine the method yourself.
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If you transcode from MPEG1 with proper subsampled 4:2:0, and don't do any
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color space conversion, you should set this option to 1.
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If you transcode from MPEG2 with proper subsampled 4:2:0, and don't do any
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color space conversion, you should set this option to 0.
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If you transcode from MPEG4 with proper subsampled 4:2:0, and don't do any
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color space conversion, you should set this option to 0.
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If you do the color space conversion yourself this isn't that easy. If the
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filter kernel of the subsampling is ( 0.5, 0.5 ) in one direction then the
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chroma sample location in that direction is between the two luma samples.
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If your filter kernel is ( 0.25, 0.5, 0.25 ) in one direction then the
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chroma sample location in that direction is equal to one of the luma
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samples. H264 Annex E contains images that tell you how to "transform" your
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Chroma Sample Location into a value of 0 to 5 that you can pass to x264.
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* Should I use this option?
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Unless you are a perfectionist, don't bother. Media players ignore this
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setting, and favor their own (fixed) assumed Chroma Sample Location.
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