Newsletter 46: describe mixed-case QR codes

Author: harding

_includes/specials/bech32/09-qrcode.md

@@ -30,17 +30,42 @@ same dimensions).
 
 Unfortunately, the `?` and `&` needed for passing additional parameters
 in a BIP21 URI are not part of the QR code uppercase character set, so
-only binary mode can be used.  Additionally, BIP21 specifies that query
+only binary mode can be used for those characters.  Additionally, BIP21 specifies that query
 parameter names such as `amount` and `label` are case sensitive, so
 uppercase versions of them aren't expected to work anyway.
 
-Still, anywhere you want to display just an address in a QR code, you
-should consider using all-uppercase for bech32 addresses and [bech32-like
-addresses][News 44 bech32].
+{:#qrcode-edit}
+However, QR codes can support mixed character sets and doing so will
+always be at least slightly more efficient when used with a string that
+either begins or ends with an all-caps substring containing a bech32
+address.  This is because the minimum allowed size of a bech32 address
+(14 characters) combined with the efficiency gain from using uppercase
+mode (31.25%) exceeds the worst-case overhead of switching modes (20
+extra bits).  At least two QR code encoders we're aware of,
+[libqrencode][] (C) and [node-qrcode][] (JS), automatically mix
+character sets by default as necessary to produce the least-complex QR
+code possible:
+
+![BIP21/bech32 mixed character mode](/img/posts/2019-05-bip21-bech32-qr-mixed.png)
+{:.center}
+
+In summary, when using bech32 addresses in QR codes, consider
+uppercasing them and any other adjacent characters that can be
+uppercased in order to produce smaller and less complex QR codes.
+(However, for all other purposes, bech32 addresses should use all
+lowercase characters.)
+
+*Correction:* an earlier version of this section claimed that QR codes
+which included BIP21 query parameters needed to use binary mode.  Nadav Ivgi
+kindly [informed us][ivgi tweet] that it was possible to mix character
+modes, and we've updated the final two paragraphs of this section
+accordingly.
 
 [bech32 easy]: {{news38}}#bech32-sending-support
 [rfc3986]: https://tools.ietf.org/html/rfc3986#section-3.1
-[news 44 bech32]: {{news44}}#bech32-sending-support
+[ivgi tweet]: https://twitter.com/shesek/status/1131733590235131905
+[node-qrcode]: https://github.com/soldair/node-qrcode#mixed-modes
+[libqrencode]: https://fukuchi.org/works/qrencode/
 
 [^only-bc]:
     Bech32 addresses have three parts, a Human Readable Prefix (HRP)