APS5_VIDEOOUT_TELEMETRY_DIR=<directory> appends one JSON object per line to <directory>/videoout.jsonl: a contract line for the run (commit and pid), a pad line whenever the published pad state changes in its buttons, sticks, L2 or R2, a present line after the first present and every 30 after that with the count and rate the window title shows, and a drops line with the number of lines the writer discarded, reported every ten seconds while it grows and once at a normal exit. A bounded queue of 4096 lines and one writer thread batch the file, so neither the present nor the input path waits for it; a pad change that the full queue refused stays pending for a later publish, and the frame count moves from UpdateTitle's function-static to DisplayWindow so the input path can name the present it belongs to.
Without the variable no file is opened and no thread is started: a present or an input update costs one call that checks a guard variable and returns null.
Adrià Franch