JPEG 2000 is the wavelet successor to JPEG that medicine and cinema kept. JPEG is lossy compression tuned for human vision, and no alpha channel at all. ImageMagick sits between the two, and this page says exactly what happens to a JP2 file on the way to becoming a JPG one.
What runs when a JP2 file becomes JPG
ImageMagick 6.9.12, on a machine we rent and watch. Your JP2 file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept. JP2 to JPG is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the JP2 into the JPG
The pixel dimensions of your JP2 file. The JPG comes out the size the JP2 went in, because nothing on this path resizes anything.
What JP2 to JPG costs you
Transparency. JPG has no alpha channel, so the engine composites your JP2 image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent JP2 logo comes back as a black rectangle.
Exactness. JPG is a lossy encoding, so the file you get back approximates the JP2 you sent. The artefacts appear once and never come out again, which is why converting the JPG back to JP2 afterwards restores nothing.
Where JP2 and JPG files come from
JP2. The 2000 successor to JPEG, using wavelets instead of cosines. The web never took it, and it stayed exactly where quality mattered more than adoption: medical imaging, digital cinema and archives.
If a model is going to read the JPG
A JPG file is pixels, and pixels are expensive for a model to read. Handed this JPG, an assistant spends its budget looking at the picture rather than reading it, and it will paraphrase what it thinks it saw. If your JP2 is mostly words, the useful move is to pull the text out of it, not to change which picture format holds it.
JP2 to JPG, measured rather than promised
Running JP2 to JPG against the real engine with a real file, ImageMagick wrote 1,568 bytes of JPG in under a tenth of a second, machine otherwise idle. That is one JP2 file on one day and not an average, which is why the number is given together with the file that produced it.
The JP2 to JPG verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the JPG file has anything inside it.
The same JP2 file, sent somewhere else
Other outputs the probe measured out of a JP2 file, so the cost of choosing JPG can be read against something. Sizes do not compare across engines, because each family was probed with its own JP2 witness file.
JP2 to PDF: 2,908 bytes in under a tenth of a second, verified by a format signature.
JP2 to PNG: 689 bytes in under a tenth of a second, verified by a format signature.
JP2 to WEBP: 370 bytes in under a tenth of a second, verified by a format signature.
JP2 to GIF: 3,512 bytes in 0.1 seconds, verified by a format signature.
JP2 to TIFF: 144,264 bytes in under a tenth of a second, verified by a format signature.
Other ways into JPG, and what they measured
Among the published routes into JPG, JP2 is the number 13 largest output of the 13 measured. The witness files differ, so this ranks the probe run and not your document.
HEIC to JPG: 1,666 bytes in under a tenth of a second.
PNG to JPG: 1,568 bytes in under a tenth of a second.
WEBP to JPG: 2,340 bytes in under a tenth of a second.
PDF to JPG: 5,782 bytes in 0.1 seconds.
GIF to JPG: 20,559 bytes in under a tenth of a second.
What we will not pretend about JP2 to JPG
A JP2 file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A JP2 to JPG run that passes 30 seconds is killed, and the ImageMagick process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
No text is read out of the JP2 anywhere on this path. Converting a picture to JPG never turns it into words, whatever the picture happens to contain.
ImageMagick was not saturated at sixteen simultaneous conversions: it held 47.5 a second while latency moved from 0.22 s to 0.34 s, and the service memory did not move off 51 MB. That is the measured headroom behind JP2 to JPG, not a marketing number.
One JP2 file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
JP2 to JPG: what people ask
What actually converts my JP2 file to JPG?
ImageMagick does it, 6.9.12, on a machine we rent and watch. Not a browser trick and not somebody else service: the JP2 file is uploaded once, ImageMagick runs once, the JPG comes back, and neither file is kept afterwards.
How long does JP2 to JPG take?
On the file the probe used, ImageMagick took under a tenth of a second and wrote 1,568 bytes of JPG. That is one real measurement on one real JP2 file, not an average and not a promise about yours: a larger JP2 takes longer, and past 30 seconds the run is stopped.
What do I lose going from JP2 to JPG?
The one to know about first: Transparency. JPG has no alpha channel, so the engine composites your JP2 image onto white before encoding. Without that step ImageMagick lays black underneath, and a transparent JP2 logo comes back as a black rectangle.
Why did my transparent JP2 background turn white in the JPG?
Because JPG has no alpha channel and something has to sit behind the picture. The engine puts white there on purpose. Left to itself ImageMagick puts black, and a transparent JP2 logo comes back as a black rectangle. If you need the transparency, send the JP2 to PNG or WebP instead.
Is JP2 to JPG free?
There is a free allowance every month, and one JP2 to JPG conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse JPG.