At 10:41 AM 21/07/2019 -0600, you wrote: On Sun, Jul 21, 2019, 4:16 AM Joseph S. Barrera III via cctalk <cctalk at classiccmp.org> wrote: There are still plenty of bad scans being done today, for various reasons. The technology of producing a final digital copy continues to improve and has a way to go yet. *This* is why I strongly oppose destroying rare docs to scan them, now. Better to wait till non-destructive scanning methods become available. Points: 1. Both the DPI and bits/pixel affect the visual result. Having shaded pixels on curved edges makes the eye see a smooth curve, where the same resolution in two-tone (B&W) would look jagged. Achieving an optimal balance of resolution and shading levels for various types of content and fineness of detail, vs file size, is a bit of an art. But ultimately it's a simple test: look at the paper original, and your final result on screen (at 1:1 final scale.) Does the quality look the same? Is your copy how the original publisher would have wanted the doc to appear? People only auto-producing PDFs rarely catch on to this, because PDF ONLY encodes as one of: two-tone B&W (fax mode), or JPG (or JPEG2000 rarely) or the excreable JBIG2 (Never use this!) Experiment with PNG encoding, via a tool like Irfanview, which allows flexibly setting PNG bits/pixel, raw, indexed color or gray scale. PNG is a lossless encoding, and so the only resolution loss is by your choice while rescaling in post-processing. 2. The resolution you scan at, and the final presentation resolution, won't be the same. Especially when the pages include elements like screened color or B&W images. To deal with these properly you MUST scan at a resolution several times higher than the screen dot pitch. Otherwise there will be moire patterns (beats) between the scan sampling and the screening dots. Then you post-process to eliminate the screening, and end up with a truly tonal image at the resolution the eye would perceive when viewing the original screened image. This avoids any moire patterning, realizes the original publisher's visual intent, and enables minimizing the final file data size. B&W text should be encoded with at least 16 gray levels available to edge shading. ie 4 bits/pixel. B&W tonal images need at least 256 level gray scale, or the eye sees quantization of shades (aka posterization.) Colour images need either 24 bit/px, ie 8 bits each for RGB, or if there are a limited number of flat colours an indexed color scheme may work. 256 colors or less, ie an 8 bit index per pixel. Typical utilities will generate the color table automatically (which can sometimes ba a pain.) PDF does not allow any of these kind of user choices. 3. The final page images, don't have a 'dots per inch' dimension. They have only total number of pixels in H & V. When doing final page image down-scaling and choice of encoding, you have to make an aesthetic decision on final pixel dimensions. If your original page was A4 (8.5" wide) and you scanned at 600 DPI, that's 5100 pixels wide. But you'll likely find that the final copy can be scaled to around 1000 to 1200 pixels wide, with 4 bits/px (if B&W text), for an on-screen page image indistinguishable from the original. 4. All post processing should be done in 24 bit RGB, at the full scan resolution. Keep staged backups. NEVER use any indexed color scheme when scaling, rotating, etc. The result is unavoidably bad. The final two steps should be: rescale to desided X-Y pixel size, THEN down-code to final color system and file encoding. There's a discussion of this in http://everist.org/temp/On_scanning.htm In general, 'acceptable' resolution VERY MUCH depends on the content. Just wondering if you're aware of the freeware util Irfanview? https://www.irfanview.com/ It's very capable for batch processing large sets of images. Rescaling, changing coding, cropping, etc. Guy