RE: Incremental Transfer Demo now has the option to retain glyph ID's

Thank you Garret (and Rod behind the scenes, I assume)!
This is really exciting development, looking forward for more (in form of support for Brotli Patch Mode) coming soon. Considering that the incremental transfer is the only mode that retains working layout features across segments, and offers the most efficient data transfer in term of size – combining it with Brotli Patch Mode sounds like a winning combination.

I also couldn’t help but notice that Cyrillic1 and Cyrillic2 subsets use [correspondingly] Russian and Ukrainian text as content examples – nice touch!

Thanks,
Vlad


From: Garret Rieger <grieger@google.com>
Sent: Monday, April 8, 2019 4:50 PM
To: w3c-webfonts-wg (public-webfonts-wg@w3.org) <public-webfonts-wg@w3.org>
Subject: Incremental Transfer Demo now has the option to retain glyph ID's

I've updated our public incremental transfer demo with a few new capabilities (https://fonts.gstatic.com/experimental/incxfer_demo):

  *   Support for selecting which subsetter to use. Currently you can select harfbuzz which does not support GSUB/GPOS, or fontTools which does support GSUB/GPOS subsetting. Using fontTools should give the most accurate view of what patch sizes would look like in a real implementation.
  *   Added the ability to optionally retain glyph id's in the subsets. If enabled then glyph id's will be stable between patches.
Some things I've noticed in playing around with the new options:

  *   Retain gid's looks like it slightly reduces the overall transfer size when GSUB/GPOS are included. My guess is that this is because the patches are more efficient if glyph id's are not shifting in each patch. For example if you add all of Latin, Cyrillic and Vietnamese patches:

     *   With retain gids total transfer size is 45k vs 46k for no retain gids.
     *   Retain gids costs 29%  more then the optimal size vs 34% more then the optimal size for no retain gids.

  *   When GSUB/GPOS are not included, then we see the opposite behaviour. Here the retain gids ends up transferring more. This makes sense to me since retain gids will add some extra data to the font file for the empty glyph ids and without GSUB/GPOS there's not as many opportunities to reap compression benefits.
  *   That said, the above two points represent only two data points so I'd take it all with a grain of salt.




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