[PNG] Feb 21, 2022 meeting topics

Hello everyone,
For our Feb 21, 2022 meeting let's discuss:

   - TPAC 2022 planning has begun. TPAC is W3C's yearly meetup. The last
   two years, TPAC has been remote. W3C is considering a hybrid approach this
   year (in-person with strong support for remote participants).

   TPAC 2022 will be in Vancouver, Sep 12th-16th.

   As part of TPAC 2022 planning, W3C wants Chairs to gather thoughts from
   their groups:
      - How likely are you to attend?
   - What safety concerns should be considered? Already noted are:
         - new COVID variants
         - quarantine restrictions
         - 7-day average COVID trends

         - tRNS ambiguity (GitHub issue #29)
   <https://github.com/w3c/PNG-spec/issues/29> [1] -- The existing spec
   says unused bits are 0. I guess this was intended to inform encoders to set
   them to zero. But for a decoder, any bits being set to 1 means this image
   does not conform to the spec.

   There is ambiguity in whether the bits MUST be 0 (and what to do if they
   aren't) or if they can be any value and a decoder should ignore the unused
   bits.

   Unfortunately, different decoders have taken different interpretations.
   libpng (and apps that use it, such as Chromium-based browsers) choose to
   ignore the unused bits. Firefox sees the chunk as being invalid and ignores
   the whole chunk. So if we choose to clarify the ambiguity, we'll break some
   wide-spread implementation. Nevertheless, I think it would be good for the
   PNG spec to be clear instead of ambiguous.

Thank you. See you at the meeting.
- Chris Blume

[1] https://github.com/w3c/PNG-spec/issues/29