ttml_descriptor: text description skipped, if font usage present

When the ttml_descriptor is parsed and the font usage substructure is present (essential_font_usage_flag set), the variable i_data is decremented by 1 + p_data[0]. However, p_data[0] has already been incremented in the line before, though its old value is needed here (as p_data[0] refers to the font_count field).

https://git.videolan.org/?p=vlc.git;a=blob;f=modules/demux/mpeg/ts_psi.c;h=c93d0666290f89d504c05faf88849b67fcac8fb9;hb=HEAD#l743

        p_data += 1 + p_data[0];
        i_data -= 1 + p_data[0];

As a consequence, the len check just below fails and the text description is skipped by mistake.

Fix

Added attachment:

• 0001-demux-ts-fix-TTML-font_count-retrieval-24871.patch

yes, indeed, we need to invert the lines

c14cab8b

Thanks; works as expected!

we still lack samples though

I'm about to finish a new TTML test stream with font downloading, which uses this field. I will provide it to you as soon as it is available.

closed

Fixed as comment #24871 (comment 244966)