I redacted my username from a screen recording three times and missed it every time

Originally published on my own site. I make a desktop AI agent. I recorded a demo of it writing files, and the approval dialog showed /Users/ /Downloads/… in plain text. Blurring it out should have been a five-minute job. I checked the result by eye three times and missed a leak every time. The fix that finally worked was scanning the finished video with a machine. The conclusion first: you cannot verify a redaction by looking at it. The scan needs to be as much a part of the pipeline as the blur. A fixed rectangle does not cover a moving dialog The naive version: open one frame where the path is visible, measure the box, and paint over it. delogo=x=450:y=300:w=370:h=44:enable='between(t,99.9,102.4)' The result had the username visible for the first one or two seconds. macOS dialogs animate in. They keep moving until they settle. | time (s) | dialog top | dialog left | |---|---|---| | 98.4 | 372 | 458 | | 98.8 | 230 | 180 | | 99.2 | 19 | 462 | | 99.6 | 71 | 407 | | 100.0 | 94 | 398 | | 102.4 | 94 | 398 | A box measured at 100.0 s lands nowhere near the frame at 98.4 s - and the text is legible the whole way in. You can sidestep this by trimming each cut to start after the dialog settles. That was not available here: the whole point of the video was that it is uncut. So the box has to follow the text. Track it with an FFT correlation The dialog moves, but the font size does not. So take the rendered /Users/sa from a settled frame as a stencil, and slide it over every frame to find the best match. Done directly, that is 2 million positions × 5,700 pixels per frame. Python will not finish. Correlation is a convolution, and convolution is one multiplication in the frequency domain. def correlate(field, kernel): """Convolution via FFT. The direct loop does not finish.""" fh, fw = field.shape F = np.fft.rfft2(field) K = np.fft.rfft2(kernel[::-1, ::-1], s=(fh, fw)) out = np.fft.irfft2(F * K, s=(fh, fw)) kh, kw = kernel.shape return out[kh - 1:, kw - 1:] # align the origin to top-left Score on two things: misses and over-eager matches. def find(img, tmpl): d = dark(img) hit = correlate(d, tmpl) # stencil ink ∩ frame ink load = correlate(d, np.ones_like(tmpl)) # total ink inside the window ink = tmpl.sum() score = hit / (load + 0.35 * ink + 1e-6) score[hit /… for 23 seconds. I had been staring at dialogs. It never occurred to me to look there. You do not find what you did not think to look for. So scan the finished file At that point I stopped trusting the redaction and started scanning the output. TEMPLATES = [ (100.0, (452, 302, 150, 38)), # write_file … larger type (148.7, (850, 512, 90, 28)), # open_path … smaller type (152.0, (940, 12, 100, 30)), # address bar … different again ] One stencil is not enough. Different dialogs are different widths and set their text at different sizes. With a single stencil the smaller one never matched at all and instead latched onto false peaks around 0.30. Set the verification threshold higher than the tracking one. Measured: real hits scored 0.64-0.74, false peaks topped out at 0.46, so 0.55 separates them cleanly. At 0.45 the scan flagged 179 frames of browser iconography and buried the real result. The scan found one more leaked frame. Sample the tracker at the output frame rate The leak was at 113.933… s. I was tracking at 20 fps, so I looked at 113.90 and 113.95. The output is 30 fps, so the frame that actually ships is 113.933…. Nobody looked at it. # (start, end, frames per second to sample) WINDOWS = [ (113.4, 118.2, 30), # animating - sample at the output rate (150.8, 174.8, 5), # static - coarse is fine ] Sample the moving stretches at the output rate and leave the static ones coarse. Running everything at 30 means 4,000 FFTs and no result. Final pass: 2,225 frames scanned, nothing found. Takeaways - A fixed rectangle cannot hide something that moves. Dialogs animate in - Track with an FFT correlation; the direct search does not finish - Score against both misses and over-eager matches - Eyeballing it missed three times. One was a place I never thought to check - Scan the finished file. Sample the tracker at the output frame rate None of this is needed if the app never puts a username on screen in the first place, which is the real fix and the one I shipped. I kept the scan anyway. You do not find what you did not think to look for. The app in the recording is Wisp - a desktop agent with a 3D body that runs commands and writes files, and shows you exactly what it is about to do before it does it. Top comments (0)