Which android barcode scanner apps can scan 2D codes like QR, Data Matrix, aztec etc. from a mobile phone screen (the code is displayed on the phone screen)? I looked at Barcode Scanner and QR Droid and QuickMark but either they don't support many 2D codes or they don't do good scanning from phone screens.
ZXing (pronounced "zebra crossing") is an open-source, multi-format 1D/2D barcode image processing library implemented in Java, with ports to other languages.
Download ZXing from here.