Wrong pixelAspectRatio population in DASH

[avelad]

The current implementation of pixelAspectRatio in DASH is wrong. @par in DASH is "display aspect ratio" so the correct is populate this field with @sar

[TheModMaker]

In DASH, the @sar attribute specifies the sample aspect ratio, which is actually a synonym for "pixel aspect ratio". Confusingly, we incorrectly parse the @par attribute to mean that, when in fact it is the "picture aspect ratio", which is a synonym for "display aspect ratio".

So to summarize:

• @sar is the "sample aspect ratio" or "pixel aspect ratio", which is the aspect ratio of the pixels themselves.
• @par is the "picture aspect ratio" or "display aspect ratio" (DAR), which is the aspect ratio of the displayed image.
• The "storage aspect ratio" is the aspect ratio of the image in pixels. It is width:height of the pixels of the image.

[avelad]

Hum, I can change the name if you want.

Do you want me to change PAR name too?

[TheModMaker]

Since the @par is just the width:height of the image, apps could easily get this from those attributes. How about just changing the DASH parser to use @sar to fill the pixelAspectRatio property? I don't see a way to get the storage aspect ratio from DASH without just calculating it from the pixel aspect ratio and the displayed image.

[avelad]

I have several live streams with the following:

<AdaptationSet id="4" group="2" frameRate="25" mimeType="video/mp4" startWithSAP="1" contentType="video" par="16:9" minBandwidth="700000" maxBandwidth="1500000" minWidth="480" maxWidth="1024" minHeight="270" maxHeight="576" segmentAlignment="true">
      <Role schemeIdUri="urn:mpeg:dash:role:2011" value="main" />
      <SegmentTemplate timescale="25" initialization="boing-$RepresentationID$-scale=25-init.mp4" media="boing-$RepresentationID$-scale=25-$Time$.mp4">
        <SegmentTimeline>
          <S t="4278345222" d="50" r="149" />
        </SegmentTimeline>
      </SegmentTemplate>
      <Representation id="avc1_700000=1" bandwidth="700000" width="480" height="270" sar="1:1" codecs="avc1.640015" />
      <Representation id="avc1_1000000=4" bandwidth="1000000" width="640" height="360" sar="1:1" codecs="avc1.64001e" />
      <Representation id="avc1_1500000=5" bandwidth="1500000" width="720" height="576" sar="64:45" codecs="avc1.64001e" />
    </AdaptationSet>

720x576 is taken from a dvb-t source, the rest are converted qualities.
Actually applying the correct values ​​the video has 1024x576

If I understand you well ... really the problem is that the parsing in DASH is wrong?

[TheModMaker]

Actually, it looks like the @width and @height attributes specify the size in pixels, not the display size. So you could get the "storage aspect ratio" by taking the width:height. The @par is just (width:height)*@sar.

So, with a @width=720, @height=576, @sar=64:45, then @par=(720*64):(576*45) == 46080:25920 == 1024:576 == 16:9. This being the "display aspect ratio".

So in this case, the "storage aspect ratio" would be 720:576 or 5:4.

I propose we expose the 64:45 value as pixelAspectRatio in the API since width and height are already exposed. Currently, we would expose the value as 16:9, which would be incorrect.

[avelad]

Ok, I’ll update the PR to change the dash parser. pixelAspectRatio should be populate with @sar

[avelad]

I updated the PR, @TheModMaker can you review?

[shaka-bot]

All tests passed!