RE: WOFF-ACTION-176: Test hmtx transformation over google fonts corpus (how many lsb == x-min for all glyfs, what savings)

+1 on hmtx pre-processing.

The bigger question though is whether we want to modify the ‘flags’ field now, or postpone it until we actually want to introduce an additional technology into WOFF2. Reserved bits will always be there, the WOFF2 minor version can be introduced and/or upped at any time – it really is a matter of our preference whether we want the extensibility as a feature now or later (when an extension is planned).
Comments?

Thanks,
Vlad


From: David Kuettel [mailto:kuettel@google.com]
Sent: Thursday, May 21, 2015 4:04 PM
To: Cosimo Lupo
Cc: Levantovsky, Vladimir; Jonathan Kew; Esfahbod Behdad; public-webfonts-wg@w3.org
Subject: Re: WOFF-ACTION-176: Test hmtx transformation over google fonts corpus (how many lsb == x-min for all glyfs, what savings)

Wow, that is fantastic.  Thank you so much for collecting the data Cosimo.  I too would agree that an average savings of 548 bytes would not be sufficient savings to justify the increased complexity and likely increased decode times.

On Thu, May 21, 2015 at 12:53 PM, Cosimo Lupo <cosimo.lupo@daltonmaag.com<mailto:cosimo.lupo@daltonmaag.com>> wrote:
I just run a test on all the Dalton Maag Library fonts, as well as on a bunch of popular Google Fonts. The average saving per font is just... 548 bytes.
Interestingly, this average value is almost exactly the same for both DaMa fonts and Google Fonts (read below for the full stats).

Note that the average number of glyph per font ranged about 700 and something. Given the lsb is stored as un unsigned short, one would expect an average saving of 1400 bytes, but Brotli entropy coding seems to save a little less when lsb are not there.

Do you think the overall trade-off between compression gain and reconstruction cost is worth it?
I’m not very impressed, I must confess. I was hoping for a bit more...

Cheers,

C.


###

Dalton Maag Library

number of fonts: 164
average number of glyphs per font: 732
total size with hmtx: 8857900
average size with hmtx: 54011
total size without hmtx: 8767928
average size without hmtx: 53462
total difference: 89972
average difference: 548


"Top 50” Google Fonts from Skyfonts

number of fonts: 187 *
average number of glyphs per font: 708
total size with hmtx: 10758700
average size with hmtx: 57533
total size without hmtx: 10656396
average size without hmtx: 56986
total difference: 102304
average difference: 547


* (the number of fonts was 221 fonts, however 34 of them had different xMin and lsb, so I excluded those from the average)