All JPG saves uses compression, you can reduce the amount of compression but you cannot 'turn off' all compression. And note that the quality of the compression is not universal, it can be dependent upon the app you are using. 'Free' software often uses lesser quality JPG compression while more costly graphics apps invest time and money in their JPG compression algorithms. Don
Edit: In many graphics apps there is a 'slider' or other control which lets you adjust the amount of compression. It may fool you into thinking you can turn it off but the lowest setting still applies some compression. Compression is the whole point of JPG, if it was totally uncompressed then you might as well just use TIFF.
My previous image editor offered "lossless JPEG" as a save option. It exists, it's googleable. You are right tho, unless it says "lossless", even at 0 compression (100% output size), some algorithms are in play to reduce size (from google). True "Lossless" preserves all colors and resolution but apparently isn't too common.
I've since switched to IrfanView which has some lossless JPG options but apparently not on save. Too bad. DxO PhotoLab doesn't have lossless options, just a 0-100% slider. And it apparently saves to TIFF, which surprised me.
I've blown up my uncompressed high-resolution JPGs beyond what I needed with exceptional results. Usually these are camera images with obscene resolution, but you would be hard-pressed to get better expandability in TIFF ... if it were even possible (from my camera). I suppose RAW mode -> save as TIFF, but then I'd just stay in RAW mode.
There is a lossless in the JPG spec but I was not aware that it has bene implemented anywhere; what would the point be? The entire reason for JPG was to reduce image size, if it does not reduce any image data then the file sizes will be the same as uncompressed TIFF. You can test to see if your app is truly lossless but scanning an image, saving in TIFF and also saving in 'lossless' JPG, then check the files sizes. If the 'lossless' file size is smaller, than the image is compressed. Any compression = loss of image data and quality.
My original point is that for saving reference images the best quality image is desirable. As you pointed out earlier, you can always make a image files smaller but once the image data is lost you cannot bring it back again. Don
Funny, I kept digging because I wanted to see what RAW mode actually was (suggested as the best lossless way to save camera images). And that's exactly what it is ... a raw and complete dump of sensor data with no processing whatsoever.
As it turns out, they ARE making scanners now (so says google) that save in RAW format. Likely too large to be practical when saving thousands of pages, but something to research.
I totally agree that the best quality saved is the better plan. I'm not convinced that a high res uncompress JPG loses any detectable detail to TIFF; simply, I'm happy with my huge JPGs. They do start to take up a ton of room, tho. And I usually copy and reduce these to 35% or lower to use on web pages.
I don't want to hijack the thread (and likely won't) but my wife's scanner has all but died and I might as well look for another. While I look to see if RAW-saving scanners are only used in medical applications or what ... what is the consensus on the BEST scanners for working with stamps? I have my camera, and if I set it up for large batches it can be fast and efficient, but not like a scanner that's always ready for singles or a whole page. My phone works ok for the occassional SOTN scan (although at least one argued differently; worked for me) Scanner recommendations? I suppose I should search here too.