QR Text being padded with 'ec 11' which doesn't allow for smaller versions.

[rlightner]

trafficstars

Encoded Text: WIFI:S:*MyWifi-B0B5;T:nopass;P:;;

Encoded Bytes: 42 15 74 94 64 93 a5 33 a2 a4 d7 95 76 96 66 92 d4 23 04 23 53 b5 43 a6 e6 f7 06 17 37 33 b5 03 a3 b3 b0 ec 11 ec 11 ec 11 ec 11 ec 11 ec 11 ec 11 ec 11 ec 11 ec 11

For some reason it's padding with ec 11, which is now allowing me to use a smaller version

[ricmoo]

I don't quite understand what the encoded bytes are. How did you get them? The encoded bytes for your string should be:

57 49 46 49 3a 53 3a 2a 4d 79 57 69 66 69 2d 42 30 42 35 3b 54 3a 6e 6f 70 61 73 73 3b 50 3a 3b 3b

which would need at least a version 3 QR code with L or M EC, or version 4 at any level.

[rlightner]

exactly. That's the exact string being passed in, with the latest code from this repo, so I'm a bit at a loss.

[ricmoo]

But how are you converting the string into those bytes? How are you printing them to the console? It may be an issue with either of those...

[rlightner]

char ssid[13]; uint64_t chipid = ESP.getEfuseMac(); snprintf(ssid, 13, "*MyWifi-%04X", (uint32_t)chipid);

Which calls a method with this signature: void Draw_QRCode(const char *string)

And that in turn calls: qrcode_initText(&qrcode, qrcodeData, 3, 0, string);

[nayuki]

WIFI:S:*MyWifi-B0B5;T:nopass;P:;; The encoded bytes for your string should be [...] which would need at least a version 3 QR code with L or M EC

To my surprise, that string can be encoded at v2 L, if you segment the characters such that the first 9 are in alphanumeric mode and the remaining 24 chars are in byte mode. The bit stream (before padding) is 267 bits for optimal segmentation, or 276 bits for encoding the whole text in byte mode.

You can confirm this on my demo page (only statistics, no rendering): https://www.nayuki.io/page/optimal-text-segmentation-for-qr-codes

[nk9]

For anyone wondering the answer to the original question: the amount of data which a QR code can store is a function of both the version (the size and subdivision pattern of the grid) and also the level of error correction. If you're seeing "ec 11" padding on the end of your decoded QR code, the library has had to level up to a larger version to store the provided data with the given error amount.

You may be able to avoid this by tweaking the error parameter to be less forgiving.