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We are digitizing a huge collection of paper repair manuals (not violating copyright) and the print quality and evenness of tone of the text is quite excellent.

I am using using cell phone images for a test and saving as various resolution jpegs (from LR) to test file size in order to estimate storage needs.

Even at quality even as low as 10%, the resultant images viewed at screen size on tablets are still quite legible.

Does this make sense in terms of how jpeg compression works?

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    \$\begingroup\$ Are there gray-scale or color photos in the manual? If just black & white, CCITT compression will create compact file sizes which any decent scanner can produce a PDF using CCITT compression. \$\endgroup\$
    – qrk
    Commented Apr 30 at 5:54
  • \$\begingroup\$ Re, "Does this make sense in terms of how jpeg compression works?" What would you do differently if somebody said No? In the end, the only thing that really matters is whether or not it works for you. That being said, IMO, you should at least try a few other formats (including 1-bit-per-pixel, CCITT Group IV) so that you can figure out what works best for you. \$\endgroup\$
    – Ohm's Lawman
    Commented Apr 30 at 12:47
  • \$\begingroup\$ JPEG was designed for compressing continuous tone (natural world photograph-type) images. Separately, how critical are your storage requirements? In my experience, this isn't as much of a concern nowadays - but you may have special circumstances. \$\endgroup\$
    – osullic
    Commented Apr 30 at 13:14
  • \$\begingroup\$ "Quite legible".... ask your users (especially those with glasses). "legible" and "usable for work without undue strain even under marginal lighting" are two very different things. \$\endgroup\$
    – xenoid
    Commented May 1 at 10:09
  • \$\begingroup\$ Thank you all for your comments. We are at the very beginning of this project and, since human resources are at a premium, I want to understand as much as possible to avoid backtracking to redo work. In response to what has been said here, we/I have decided to maintain all of the original images (in HEIC format) but transform all of them into somewhat compressed jpeg images for ease of use and then into digital books for some certainty of compatibility no matter the platform. Thanks again. \$\endgroup\$
    – The _traveler
    Commented May 1 at 16:01

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Does well printed black text stay essentially unchanged on a highly compressed image of a page of text?

No. You can always make smash potatoes from a file.

You need to define what you mean by "stay essentially unchanged" You are using two specific words (stay unchanged) bonded by a very loose one (essentially).

Another word we need to define is "Quality at 10%". Quality is a process, that can not be represented as a percentage.

Let me link you to a really old test I did but defining 10% of the file size vs. an uncompressed file. Those are real numbers we can compare.

It is in Spanish, but I want you to see that some areas are affected more than others. A perfect black on the right image means no change in the information.

https://otake.com.mx/Apuntes/PruebasDeCompresion/3-CompresionJpg10Porciento.htm

But another point is that there is not just 1 JPG algorithm. There are at least two. One that uses smaller blocks. Some programs define it as 4:4:4 or 1x blocks.

But, in my opinion, as you are asking on a photography site, not in a document archival site:

  1. Save your original photos. The cost of 1 or 2 GB of disk space, vs the work you are investing in taking the photos, is INSIGNIFICANT.

  2. When you are ready to deliver or generate a PDF, then you can play with the compression and resolution. If for some reason you need a change, you go back to your originals.

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    \$\begingroup\$ And it the images are really grayscale (R=G=B for all pixels) the Jpeg will be much smaller because the chroma channels become uniform. And some psot procesing can make the background truly uniform (usually fully white) this can also have a large influence on the final size. But beware of exotic JPEG formats, some are not supported everywhere. \$\endgroup\$
    – xenoid
    Commented May 1 at 10:13
  • \$\begingroup\$ True! I will extend my answer addressing the two points. \$\endgroup\$
    – Rafael
    Commented May 1 at 13:31
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    \$\begingroup\$ Here here! Data storage is cheap. Lost information is not. Extra points for smashing potatoes. \$\endgroup\$
    – Jeffrey J Weimer
    Commented May 14 at 3:44
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Does this make sense in terms of how jpeg compression works?

You are talking about compressed resolution; yes, it makes sense.

If I use my 12.2MP cell phone camera to take an image, and view it so it fills the screen on my Macbook Pro Retina with default setting, it is being displayed at just under 2MP. That is 6x resolution compression... doesn't matter if the compression is done during display, or during the saving of the file. I.e. an even higher resolution image is just going to be compressed more when displayed at ~2MP on my monitor. And a reduced resolution jpeg is just being compressed less during display.

The main difference is that if you compress the resolution of the file itself you can't really go back. Instead you would probably be better off looking at compressing the data used to create the file; since it is a grayscale image the data could probably be compressed quite a lot with minimal quality loss... or some combination of both.

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