How to scan barcodes from the mobile screen itself?
Edit on Mar 10, 2020: I am also looking to do the same for QR codes.
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As already described in above comments, this would require taking a screenshot and saving it as an image file. Havind done that, there are several barcode scanners available in the playstore supporting "loading" of barcodes from image files, as e.g. QR BARCODE SCANNER or mentioned Google Goggles. I don't know of any barcode scanner capable of "scanning your screen directly".
EDIT: QR Droid seems to be capable of what you're looking for:
Scan QR codes from your camera, browser, SD card, or saved image
I'm frequently using Manatee Works 'Barcode Scanners' app towards such a purpose. Here's the Google Play Store demonstration application link, recently updated:
The app supports wide variety of 1D/2D codes, scans very fast and its free. The only downside is that the option to manipulate with the results are few, namely either copy or email, because its a mere demonstration.