After you cover a secret, the next decision is the file you keep and the file you send. PNG is a lossless container for the pixels you painted. JPEG is a lossy encoder that chat apps love. Confusing those two jobs is how people either fear the wrong bug (“WhatsApp will unredact my bar”) or ignore the real one (a weak smear that already left glyph shape before anyone recompressed it).
This page owns export format after a real cover. It does not re-teach when to prefer a bar over a smear. That fork lives on blur vs black bar. Edge waste versus middle paint lives on crop vs black bar.
Open larger image in a new tabWhat PNG promises (and what it does not)
PNG stores the samples you saved without adding a lossy pass of its own. If you replaced every sample under a solid bar, the PNG holds that uniform region. Open it later and those pixels are still the cover you painted.
Lossless is not automatic privacy. An uncovered PNG is still uncovered. A translucent highlighter that never replaced samples is still a tint. A PDF text layer is a different object entirely (PDF box vs PNG bar). PNG helps clarity of the cover. It does not invent deletion you never performed.
What JPEG does to a finished redaction
Once pixels are fully replaced in a flattened raster, later JPEG encoding cannot reconstruct the removed originals. A solid black bar that replaced the samples stays a bar after a messenger recompresses it. JPEG ringing at a hard edge is a compression fringe, not the secret coming back.
Weak blur, a translucent highlighter, a gap in the box, a layer that was never flattened, and leftover metadata are separate problems. JPEG can make a weak smear look worse by ringing around leftover glyph shape. That is leftover structure, not JPEG undoing a real fill.
Why start from PNG if the chat app will recompress anyway?
Because you control the source. A PNG of a solid bar is a uniform region. JPEG cannot reconstruct the samples you replaced. A JPEG you made yourself of a light blur is already ringing around leftover structure before WhatsApp, iMessage, or Slack take their cut. Two compressors stacked on a smear are how glyphs get an outline. One compressor on a real bar is usually an ugly edge, not a readable interior.
If the app has a document or file attachment path, use it. That path often preserves PNG. The camera-roll “photo” path often does not. The sticker path almost never does. Choose the path that treats the file as a file.
Open larger image in a new tabWhat ringing looks like on a redaction
Light and dark fringes along a sharp boundary. On a black rectangle the fringe sits at the edge. The inside stays empty if you replaced those pixels. JPEG did not restore the original text.
On a blur, the “boundary” can be the character itself. Compression artifacts can trace leftover glyph shape. That is the failure PETS-style leftover structure rhymes with: a person staring at the JPEG can sometimes name a digit because the smear was weak, not because JPEG undid a fill. Research context: PETS 2016.
Heavy bars still belong in PNG first for clarity, not because PNG is a privacy charm. Do not pick JPEG to “make the blur look smoother,” and do not treat a mushy export as a way to hide a gap you left in the box.
Do not screenshot the screenshot to flatten it
Capturing the editor window a second time with the OS snip can resample again, pick up window chrome, and pick up notifications. Use Download. The download is the flatten. A snip of the browser UI is a new screenshot with a new set of accidents.
The same rule applies to photographing the monitor “so the layers are gone.” You added Camera metadata and another optical pass. Flatten in software. GPS is a separate leak: EXIF and GPS.
How to finish a JPEG you already have
Most photos arrive as JPEG, and that is fine as a starting point. Paint opaque black rectangles over every digit run and name, then save a new file, preferably PNG, so the cover is baked into the pixels rather than sitting as a highlighter layer on the original.
- Open the .jpg in a raster editor (Paint, Preview Markup, or BlurThis).
- Choose a filled black rectangle, not a highlighter tint. Cover each run with padding past descenders and digit gaps.
- Zoom to 100%. If you can still guess a character, enlarge the cover.
- Save or download as PNG. Attach that copy. Leave the camera original off the thread.
Device Markup recipes differ by OS and belong on the device guides, not as a four-platform dump here. The format rule is the same everywhere: replace samples, then keep a lossless copy of that cover until you must hand the file to a lossy channel.
When the only send path is a lossy photo bubble
Send the barred PNG into that path and accept their compressor. Do not pre-export JPEG “to match.” Do not shrink, screenshot, and re-encode three times to hit a size limit. If you must shrink, shrink the PNG in the editor and download once. Keep the original off the thread. Rotate tokens that appeared on screen even under a bar you are not sure survived the bubble.
Email is often kinder to PNG than a chat bubble. A bug tracker that accepts attachments is kinder than a status update that recompresses inline images. Pick the channel that treats the file as a document when the secret was worth barring. If the only channel is the bubble, the bar still has to be real pixels before you drop it in.
Export habit that survives messengers
BlurThis downloads PNG on purpose so the bar you drew is the bar in the file. Do not switch the download to JPEG to match the chat app. Open the export, zoom, then attach that file: download the PNG from this tab.
