LinkedIn HDR Brightener
Re-encodes an image so its highlights render brighter than the white of the page around them — a clone of a tool that sells this as magic, built to show it is under six hundred lines of colour maths and a hand-rolled PNG encoder.
Runs 100% in your browser — check the network tabDrop an image here, or
JPEG · PNG · WebP · HEIC — never leaves your device
Under the Hood
This does not brighten your image. It relabels it. sRGB is a relativeencoding — code value 255 means “whatever your screen calls white.” PQ, the curve used by HDR video, is absolute: every code value names a real luminance in cd/m², up to 10,000. Re-encode a photo with PQ, declare its whites at 1,600 nits instead of the usual 203, and a colour-managed HDR display will genuinely drive those pixels brighter than the interface around them. That is the entire trick. In a feed of ordinary SDR images, yours appears to be lit from behind.
The gain is applied only to highlights, not to the whole image. Multiply every pixel by the same factor and you do get a brighter picture, but the shadows rise with the whites and it flattens into grey. Masking by luminance spends the display’s headroom where it actually reads and leaves the contrast intact — that is what the gain curve on the right is showing you.
The honest part: this is a copy. AthenaHQ, Superwhite and superGLOW all shipped versions of it, and I rebuilt this one after reading their approach. The technique is a few hundred lines of colour maths and a hand-written PNG encoder — no model, no backend, nothing proprietary. That is rather the point.
Go deeper
Two colour signals go into the file and they do different jobs. The cICP chunk (9 / 16 / 0 / 1 — BT.2020 primaries, ST 2084 transfer, full range) is what a browser reads when you open the PNG directly. The embedded ICC v4.4 profile carries the same information in its cicp tag, and that one exists because uploads get re-encoded: platforms strip metadata they do not recognise but preserve ICC profiles, since colour management is load-bearing for ordinary photos. The HDR signal travels inside the profile, disguised as routine colour data.
Sixteen bits per channel is not optional. PQ spreads its code values across a 10,000 nit range, so at 8 bits a boosted sky bands visibly. But 16-bit RGB is six bytes per pixel — a 2400px image is about 35 MB before compression, and photographs only deflate around two-to-one. Most images therefore have to be downscaled to fit LinkedIn’s 5 MB limit, and the stats panel shows exactly how many passes that took. The tools selling this trick do the same thing; they just don’t mention it.
It is also fragile, and worth knowing why before you use it. It needs an HDR display, a colour-managed browser, and battery saver switched off. It survives only as long as the platform keeps preserving ICC profiles. And it is fundamentally an attention arms race in a shared feed — Apple and Instagram have both clamped HDR headroom in exactly this situation. Somewhere past +4 stops it stops looking bright and starts looking broken.