Hiding the words so you can hear them (blurring out burned-in subtitles for language learning)

Short-form video comes with words stamped on it. Creators burn subtitles right into the picture so a clip still makes sense on mute, in a loud room, thumb already moving. For scrolling, that's smart. For learning a language, it quietly gets in your way: your eyes read the line before your ears have a chance to catch it, and you walk away thinking you understood the audio when really you just read it.

So across a great many of the videos in Reelang we find those baked-in subtitles and blur them out, and we pair that with a subtitle track of our own that you actually learn from. Here's the thinking, and then, in plain terms and then in detail, exactly what it takes to build.

The part that gets worse the better you get

Early on, a little text on screen is a friendly crutch. It keeps you from drowning. But as you climb, the whole job of comprehension has to move from the page to the ear, and the hardest part of real speech, the way words blur together, drop, and run into each other, is exactly the part a subtitle lets you skip.

What learners and the research say

I want to be honest that this isn't settled science, and it isn't a settled argument among learners either. Spend any time in the subtitles debate on r/languagelearning and you'll find every position held with conviction. One learner describes subtitles as a crutch they only noticed after quitting: "my reading improved more than my listening. Once I turned off subtitles, I realized just how poor my listening skills actually were." Another, in a thread literally titled "Are subtitles holding back my listening skills", swears by a two-pass routine: "Read, then listen. Watch with subtitles, then rewatch without. Stop and re-listen to blurred phrases until you can hear each word." And plenty push back just as hard: "I have found no downsides to using subtitles, and many downsides to fretting about whether I'm hurting myself by using subtitles."

The research is split in much the same way, and mostly lands on it depends on your level. Subtitles reliably help you follow a clip in the moment. And for intermediate-to-advanced learners, a study on same-language subtitles found they actually improved listening more than watching with none at all, so long as the subtitles were in the language being learned rather than the viewer's own. There's no universal right answer, only a right answer for you, today, at your level, in the mood you're in.

Which is the whole point. Since nobody can pick that for you, the useful thing is to hand you the switch. Almost everywhere else, the words are baked in and the choice has already been made.

Two different things

People sometimes lump these together, so it's worth pulling them apart.

Our own subtitle layer. There's always a Smart Subtitle track available on Reelang, but it's ours, not the one the creator burned in. Every word is tappable: tap it and you get what it means and how it's being used right here, in this sentence, not a dictionary page dumped on top of the video. And we pay attention to what you tap. If you keep reaching for the same kind of word, we quietly line up more videos over the coming days that circle back to it, no lesson, no flashcards, just natural repetition of the things you were curious about. If a particular tense keeps tripping you up, we can ease you toward it rather than dropping you straight in. Subtitles, here, are something you learn from.

Blurring the burned-in subtitles. This is a separate thing entirely. Nobody needs the creator's on-screen text while they're trying to listen, so we find it and blur it out softly, so it reads as intentional rather than a black bar slapped over the frame. Then we leave you a toggle: off when you want to test your ears, back on in a tap when you want to check yourself against the words. It's already on across a large and growing slice of the catalog, and if you land on a clip we haven't reached yet, you can have it scanned on demand.

Finding words in a picture is harder than turning them off

When subtitles are a separate track, hiding them is a checkbox. When they've been painted into the video, there's no checkbox, because there's no track. Those words are just pixels, the same as a face or a street sign. So before we can hide them, we have to find them: work out that a shape on screen is writing and not scenery, follow it as it appears and disappears through the clip, and pin down exactly where it sits and for how long.

How we actually do it

The plain version first: every video we bring in gets watched by a machine that can read text in a picture. It marks every place words appear, where they sit, and for how long. That short list of boxes and timings is saved next to the video, and when you press play the app blurs those exact spots live on your screen. The video file itself is never touched.

Now the longer version, for anyone who wants it.

Finding the words. We run an open-source OCR model, PaddleOCR, on our own machines rather than paying a cloud vision API per frame. We use only its detector, the half that draws boxes around text, not the half that reads the letters: to blur a subtitle we need to know where it is, not what it says. Watching every frame of every clip would be slow and costly, so it goes in two passes. A cheap probe samples a handful of frames from the middle of the video and asks one question, is there burned-in text here at all, and most clips that don't have any stop right there. The ones that do get the full pass, about a frame a second, and each detected box is scored on how subtitle-like it is. Burned-in subtitles tend to sit low in the frame, run wide, and center horizontally, so a box doing all three is probably a subtitle and a logo in the corner probably isn't. We group the surviving boxes into cues by when and where they appear, pad each cue by about a second on either side so the blur doesn't flicker between words, and if nothing looks like a subtitle we keep an empty result and move on.

Keeping it small. What we store is tiny: a list of cues on the video's record, each one a start time, an end time, and a box in the video's own pixels. That's the whole artifact. A clip we've scanned and found nothing on is marked differently from one we've never scanned, so improving the detector doesn't mean re-scanning the entire library.

Blurring it live. The player is built in React Native and also runs on the web. It takes those pixel boxes and maps them onto wherever the video actually sits on your screen, which shifts with your device and with whether the clip is letterboxed. It grows each box a little and feathers the edge, so the effect reads as a soft focus rather than a hard rectangle, then blurs only those regions: a real blur (expo-blur on phones, a CSS backdrop-filter on the web), not a gray bar painted over the frame. Because it all happens at playback from that little list, the toggle is instant, and so is press-and-hold to peek, which lifts the blur off a line for as long as your finger is down.

Where it runs. The detector has covered a large part of the catalog already, and it keeps going. Wherever it has found burned-in subtitles, the blur and its toggle are right there in the player, and anything it hasn't reached yet can be scanned on demand: point it at a clip and it gets scanned for you.

The tools, bent to fit. Trigger.dev runs the whole thing as background jobs; the OCR job asks for a larger machine and retries on a larger one still if a long video runs it out of memory. Our own tappable subtitle track is powered by Deepgram, a different job entirely from the burned-in text we're covering up, and one we cared enough about to give its own post.

It doesn't always get it right

This is AI, so it makes mistakes. Once in a while it blurs something you actually wanted to read, a name on screen, a price, a sign, a bit of on-screen writing that was part of the joke. When that happens you can tell us, and those reports are exactly how the thing gets sharper. It's a feature we keep improving, video by video and report by report, rather than one we shipped and walked away from.

The through-line

Reading has its place and so does listening; the only thing worth insisting on is that you get to decide which one you're doing right now. Somewhere else, the words are baked in and the choice is made for you. Here, you get the switch.