AutoInput Screen Capture problems

I'm trying to use AutoInput's screen capture to periodically watch the screen and use OCR to see if specific text appears on the screen. I'm trying to watch a game and respond to text on screen for certain events.
The trouble is that each time I try to get the screen shot it appears as if some overlay appears on the screen for a moment while saving and interrupts the game. I need to be able to check every three seconds or so, so the repeated pauses is a problem.
Is there a way to reliably get a screenshot that acts silently without causing the game to think it's not in the foreground anymore?
I tried using "screencap -p /sdcard/capture.png", via Tasker Shell actions, but it keeps creating an empty file.
I've seen other standalone apps that can watch the screen for text in other games, like CalcIV for Pokémon Go, so I'm pretty sure it can be done easily enough.