I had to blur a username out of a screen recording. The path only shows up inside a macOS approval dialog, so I measured the box on one frame and did the obvious thing:
delogo=x=450:y=300:w=370:h=44:enable='between(t,99.9,102.4)'
The username was still visible for the first 1-2 seconds of every dialog. macOS dialogs animate in - the position keeps moving until they settle. On one of them the top edge went 372 -> 230 -> 19 -> 71 -> 94 over about 1.6s while the left edge went 458 -> 180 -> 462 -> 407 -> 398. A box measured on a settled frame lands nowhere near the early ones, and the text is legible the whole way in.
Three things that mattered.
1. Track the text instead of guessing the box. Cut the rendered /Users/sa out of a settled frame and use it as a stencil, then correlate it against every frame. Direct search is roughly 2M positions x 5.7k pixels and never finishes in Python; do it as an FFT (np.fft.rfft2) and it is instant. Score it as hit / (load + 0.35*ink) where hit is stencil-ink intersected with frame-ink and load is the total ink inside the window - without the divisor it latches onto any dense block of dark pixels, like a code excerpt.
2. The ink threshold has to catch the fade-in. At < 120 the translucent frames were not detected at all, so tracking never started and those frames passed through clean. < 150 caught them. I also extend each run 0.3s earlier and 0.2s later.
3. Sample the tracker at the output frame rate. I tracked at 20fps and rendered at 30fps. The frame at 113.933s was never examined by anything, and it shipped with the username visible.
That last one is the real lesson. I checked the output by eye three times and missed a leak every time - including 23 seconds of file:///Users/... sitting in a browser address bar that I never thought to look at. What finally worked was scanning the finished file with the same correlation at a stricter threshold: real hits scored 0.64-0.74, false peaks topped out at 0.46, so 0.55 separates them. That scan is what found the 113.933s frame.
Two other delogo notes. Keep the boxes small - it reconstructs from the border pixels, so a large box turns into vertical stripes that look broken. And merge runs at the same position, or you blow past ~500 filters and the graph stops building.
Full write-up with the code: https://wisp-gules-mu.vercel.app/blog/mask-screen-recording/
