Created attachment 350132 [details]
TTML file to demonstrate that tabs aren't suppressed

The TTML spec has an issue in which tab (U+0009) characters that are
first in a sequence of whitespace characters are not suppressed at the
start and end of line areas. This issue was reported in [1] and the
editor of the TTML specs confirmed that this was not the intention
behind the spec.

The editor has created an issue to fix this in both the TTML1 and TTML2
specs [2], giving a proposal of what the spec should say. Patch to follow
updates ttmlparse to implement the intended behaviour as proposed, in
which tabs in the input are converted to spaces before processing.

The attached file (tabs.xml) demonstrates that the code at present won't suppress tabs; run as follows:

gst-launch-1.0 ttmlrender name=r videotestsrc pattern="gradient" ! video/x-raw,width=1280,height=720 ! autovideoconvert ! r.video_sink filesrc blocksize=16777216 location=tabs.xml ! ttmlparse ! r.text_sink r. ! ximagesink

[1] https://github.com/w3c/imsc/issues/224
[2] https://github.com/w3c/ttml1/issues/235