VTT Writer performances

[nywhere]

Hi,

Source is an SCC with 1033 captions.

• Converting it to TTML is ok
• Converting it to VTT is very slow (10s on MacBook pro)
• Converting it to TTML then to VTT is very slow (10s on MacBook pro)

source.scc.zip

Top 3 Time-Consuming Operations:

_process_element (isd.py:413)

Consumed 35.644s cumulative time
Called 2,165,274 times
Recursive function (note the ncalls format: 2165274/48445)

_compute_styles (isd.py:400)

Consumed 11.755s cumulative time
Called 150,222 times

set_style (model.py:339)

Consumed 5.424s cumulative time
Called 10,642,028 times

Please let me know if you need more details.

[palemieux]

The current algorithm is not optimized when the input document both generates a large number of regions with indefinite temporal intervals and a large number of captions/subtitles: all regions must be visited for each captions/subtitle (NxM problem).

Couple of options come to mind:

• reduce the number of regions generated when reading an SCC document by coalescing regions with similar dimensions (probably a good idea in any event)
• optimize the ISD generation algorithm (probably following a pattern similar to that at sandflow/imscJS@b728b68)
• add multi-processing support (not sure it is entirely worth the effort)

[nywhere]

The first option definitely makes sense.
Option 2 would be nice too.

Maybe some caching on computed style could help too, if applicable of course.

[palemieux]

@nywhere Checkout #477. I would appreciate your feedback if it speeds up your workload.