Sorry for the not so catchy subject, I don't know how to phrase the question more precisely.
My requirement is: I use an app scanning QR codes directly via camera for a specific purpose. Sometimes it would be good to scan a QR code from an image stored in the gallery instead, but unlike many other apps this one is lacking the capability to browse the gallery for that purpose. The only way to get this working would be to emulate the camera in a way making the app think it is scanning a live image, but actually the user can first select the target image from the gallery.
Is there an app which can do this? I am using A Samsung Galaxy S7 with Android 8 stock firmware, no jailbreak.
Attention: This question is not asking for QR code reader apps in general, please read thoroughly.
Update: The app GalleryCam serves a similar purpose but only works for apps opening the camera with a button where it can hook into in order to display a pop-up menu. My target app is different, it does not work in connection with GalleryCam.