TTML Parser incorrectly scales root container region by cellResolution

[davemevans]

Environment

• The MPD passes the DASH-IF Conformance Tool on http://dashif.org/conformance.html
• The stream has correct Access-Control-Allow-Origin headers (CORS)
• There are no network errors such as 404s in the browser console when trying to play the stream
• The issue observed is not mentioned on https://github.com/Dash-Industry-Forum/dash.js/wiki/FAQ
• The issue occurrs in the latest reference client on http://dashif.org/reference/players/javascript/ and not just on my page
• Link to playable MPD file: http://rdmedia.bbc.co.uk/dash/ondemand/elephants_dream/1/client_manifest-snake.mpd
• Dash.js version: dev + Fix ttml span timing + moved image subtitles to div level #1427
• Browser name/version: Chrome 50.0.2661.102
• OS name/version: Win7

Steps to reproduce

Play MPD linked above. Occurs 100% of the time.

Observed behaviour

Root Container Region is being vertically scaled by some ratio of the cellResolution. This can be observed by inspecting the elements in the developer tools. I don't think this is correct. EBU 3380 states 'If the related media object is a video media object then the rendering plane shall be coincident with the rendering plane of the video media object' which I assume to mean it is the same size. The cell resolution is used for calculating line padding, font size etc, but the container should not be scaled by it.

I tried simply removing the "aspect ratio" passed to getVideoVisibleVideoSize from checkVideoSize but this appears to cause other scaling issues.

Console output

N/A

[davemevans]

To make this work with the MPD supplied, you need to pull in #1427.

However, this effect is noticeable without that patch. See http://rdmedia.bbc.co.uk/dash/ondemand/elephants_dream/1/client_manifest-all.mpd, which gives the same result without requiring #1427.

#1427 has now been pulled into dev.

[nigelmegitt]

Just in case it's relevant, the ttp:cellResolution parameter in a TTML document carries no information about the aspect ratio: it's merely used to divide the root container region into a cell grid that can be used as a coordinate system. The individual cells are not constrained to be square or any other aspect ratio: their aspect ratio depends on the root container region aspect ratio and the number of cells in each dimension.

[TobbeEdgeware]

@bbcrddave This is probably incorrect, but has been like this for a very long time. I've never worked with the actual HTML rendering and scaling, so if you know how to fix it, please do so.