On 3/1/25 01:20, Martin via groups.io wrote:
Don't know where I read this, but it said: better use high resolution with JPEG compression than low resolution without. For the same file size, that is.
Don't believe everything you read. There's a vast amount of misunderstanding and bad information about JPEG floating around; it has been that way for a very long time. This is a result of people thinking it's just fine to use technology without first learning even the most basic things about it, while at the same time cheerfully ignoring the advice of people who do know something about it.
My experience: 300dpi is largely sufficient even for schematics, with compression. If the original is bad, has exceptionally small lettering or such, I take 600dpi as a precautionary measure.
At the museum we use 600DPI as a minimum for most things. Data storage is cheap, compression works great, and cheap paper is aging quickly.
Of course JPEG does the artefacts around sharp contrasts, but as long as its perfectly readable I prefer that to humongous files that take minutes to open...
That depends on the goal, but I personally believe that when any document is scanned, it should be treated as if it is the only known copy (which it may be), and that getting it scanned may be a matter of life and death for someone in the future (which it may be). Either may be true; we have no way to know.
The artifacts introduced in JPEG compression, in particular those which result in 8x8 blocking (JPEG's DCT and other steps operate on 8x8 pixel blocks) make subsequent OCR very difficult. This can never be undone, as JPEG is a lossy compression algorithm. JPEG *discards data* in changing the nature of an image file to exploit weaknesses in the human vision system. This is detrimental to most any sort of subsequent processing that may need to be performed.
So having researched this exhaustively many years ago, and having been "all up in there" with the JPEG algorithm, I stand by my assertion that JPEG should never, ever be used for something like this. There are plenty of other compression algorithms which will result in the same, if not better image size reductions.
-Dave
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Dave McGuire, AK4HZ
New Kensington, PA
Dave McGuire