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Re: [WebVTT] It's not easy being green

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Wed, 18 Mar 2015 09:57:53 +0000
To: Michael Dolan <mdolan@newtbt.com>, "public-tt@w3.org" <public-tt@w3.org>
Message-ID: <D12EF9AF.1CE57%nigel.megitt@bbc.co.uk>
That would be Michael Borthwick I guess? His presentation is available in video at http://michaelborthwick.com.au/closed_captioning_online_streaming_video_dfxp_webvtt.html if anyone wants to look at it for themselves (withoutl being negative, I just want to be clear that's a pointer not a recommendation).

This issue applies also to conversion from Teletext. "Green" in Teletext, according to ETS 300 706 is a "full intensity" colour, in 4 bit RGB it's {0, 15, 0}, so it maps to {0, 255, 0} in 8 bit RGB, also known in CSS colours as "lime". There's also Teletext "Half green" which is {0, 7, 0} which maps to CSS "green" {0, 80, 0}.

I believe this is a conversion constraint and there's no change needed to any of our specification documents.

Kind regards,


From: Michael Dolan <mdolan@newtbt.com<mailto:mdolan@newtbt.com>> Date: Tuesday, 17 March 2015 00:03
To: "public-tt@w3.org<mailto:public-tt@w3.org>" <public-tt@w3.org<mailto:public-tt@w3.org>>

I received an email from someone studying SMPTE-TT and WebVTT who wrote and presented a paper about 18 months ago titled, “'It's not easy being green - a closed captioning for web case study'”.  Among other things, he observed that we map CEA 608 “green” to SVG/CSS3 “green”.

FYI, the SVG/CSS3 RGB value of “green” is 008000, not 00FF00 you might expect (which is the color “lime”):

Note in contrast that “red” and “blue” are both full brightness.

Of the various 608 mapping documents….SDP-US uses only RGB values (no color names) and explicitly defines “green” to be 00FF00.  However both SMPTE-TT (RP2052-10) and WebVTT say to map 608 “green” to TTML “green” which is minimally inconsistent with SDP-US. The same would be true for 708 mappings, of course.

Of historical note, CSS1 says the color palette came from the “Windows VGA palette”, which seems to have come from the CGA text color palette:

(although the above  defines the brighter green as “light green”).

We could assume it is supposed to be maximum brightness like “red” and “blue” (i.e. 00FF00), but on the off chance that “green” text overlay is actually handled differently in the billion TV’s out there I’ll ask a couple of set manufacturers….



Michael A DOLAN
TBT, Inc. PO Box 190
Del Mar, CA 92014
(m) +1-858-882-7497

Received on Wednesday, 18 March 2015 09:58:26 UTC

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