Queries on: Fixed position and style formatting for EBU-TT-D subtitles #1335
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I'm working on adding vertical positioning, approximative alignment (calculated from text positions to be left, center, and right) as well as text color. Should hopefully have a PR before the end of the month (March 2024). |
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This PR adds parsing of teletext styling, and rendering of the styling in output TTML and WebVTT subtitle tracks. Beyond unit tests, I've used the sample https://drive.google.com/file/d/19ZYsoeUfH85gEilQkaAdLbPhC4CxhDEh/view?usp=sharing which has rather advanced subtitling with two separate rows at the same time, where one is left aligned and another is right aligned. This necessitates two parallel cues to be rendered. It also has some colored text. Solve #1335. ## parse teletext styling and formatting Extend the teletext parser to parse the teletext styling and formatting. This includes translating rows into regions, calculating alignment from start and stop position of the text, and extracting text and background colors. The colors are limited to full lines. Both lines and regions are propagated in the TextSample structures. This is because the number of lines may differ from different sources. For teletext, there are 24 rows, but they are essentially always used with double height, so the number of output lines is 12 from 0 to 11. There are also corresponding regions are denoted "ttx_R", where R is an integer row number. A renderer can use either the line number or the region ID to render the text. ## ttml generation for teletext to EBU-TT-D Add support to render teletext input in EBU-TT-D (IMSC-1) format. This includes appropriate regions ttx_0 to ttx_11 signalled in the TextSamples, alignment and text and background colors. The general TTML output has been changed to always include metadata, layout, and styling nodes, even if they are empty. EBU-TT-D is detected by the presence of "ttx_?" regions in the samples. If detected, extra TTML elements will be added and the EBU-TT-D linePadding used as well. Appropriate styles for background and text colors are generated depending on the color and backgroundColor attributes in the text fragments. ## adapt WebVTT output to teletext TextSample. Teletext input generates both a region with prefix ttx_ and a floating point line number (e.g. 9.5) in the range 0 to 11.5 (due to input 0-23 as double lines). The output is adopted to drop such regions and convert the line number to an integer since the standard only used floats for percent values but not for plain line numbers.
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Closed by #1384 |
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System info
Operating System: Ubuntu 22.04
Shaka Packager Version: main, git tip
Issue and steps to reproduce the problem:
This is a query on the current state of Fixed position and style formatting for EBU-TT-D subtitles in shaka-packager.
As highlighted on this PR, EBU Teletext input following Level 1.5 of the core specification ETSI EN 300 706 V1.2.1 (2003-04) and webvtt in MP4 segments output are implemented. This works well, as tested.
However, fixed position and style formatting for the same aren't implemented (yet), which raises two questions:
Can this be extended to match what Amazon's Medialive implementation achieves?
Is such a feature within the scope of a downstream packager such as Shaka?
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