Read QR Code from an image.

Install ZBAR utility for scanning and decoding bar codes from various sources.

$ sudo apt install zbar-tools

Inspect available options.

$ zbarimg --help                                                               

usage: zbarimg [options] <image>...

scan and decode bar codes from one or more image files

options:
    -h, --help      display this help text
    --version       display version information and exit
    -q, --quiet     minimal output, only print decoded symbol data
    -v, --verbose   increase debug output level
    --verbose=N     set specific debug output level
    -d, --display   enable display of following images to the screen
    -D, --nodisplay disable display of following images (default)
    --xml, --noxml  enable/disable XML output format
    --raw           output decoded symbol data without converting charsets
    -1, --oneshot   exit after scanning one bar code
    -S<CONFIG>[=<VALUE>], --set <CONFIG>[=<VALUE>]
                    set decoder/scanner <CONFIG> to <VALUE> (or 1)

    --nodbus        disable dbus message

Decode QR Code.

$ zbarimg QRCode-wifi-HomeNetwork.png                 

QR-Code:WIFI:T:WPA;S:HomeNetwork;P:HomePassword;;
scanned 1 barcode symbols from 1 images in 0,02 seconds

Decode QR Code from an image with inverted colors.

$ zbarimg QRCode-wifi-HomeNetwork-dark.png

scanned 0 barcode symbols from 1 images in 0,01 seconds


WARNING: barcode data was not detected in some image(s)
Things to check:
  - is the barcode type supported? Currently supported symbologies are:
        . EAN/UPC (EAN-13, EAN-8, EAN-2, EAN-5, UPC-A, UPC-E, ISBN-10, ISBN-13)
        . DataBar, DataBar Expanded
        . Code 128
        . Code 93
        . Code 39
        . Codabar
        . Interleaved 2 of 5
        . QR code
        . SQ code
  - is the barcode large enough in the image?
  - is the barcode mostly in focus?
  - is there sufficient contrast/illumination?
  - If the symbol is split in several barcodes, are they combined in one image?
  - Did you enable the barcode type?
    some EAN/UPC codes are disabled by default. To enable all, use:
    $ zbarimg -S*.enable <files>
    Please also notice that some variants take precedence over others.
    Due to that, if you want, for example, ISBN-10, you should do:
    $ zbarimg -Sisbn10.enable <files>

It does not work, but you can use convert utility from the ImageMagick package to overcome this issue.

$ zbarimg <(convert QRCode-wifi-HomeNetwork-dark.png -negate -)

QR-Code:WIFI:T:WPA;S:HomeNetwork;P:HomePassword;;
scanned 1 barcode symbols from 1 images in 0,01 seconds

Nice.