I see now that my problem is with scanimage(1), not imagemagick. Thank you for your help.I can only try to help you if you have a problem with an image that you want to solve with ImageMagick.
I can't really help you if you have problems in the handling of your scanning and printing software.
This is another interesting point. I've googled and googled, but everybody who addresses this issue is dealing with TIFF. Now, I've been using this set up for years, scanimage(1), lpr(1), and PNM (it used to work for me), and I think TIFF is a microsoft format, so I thought that I should be able to continue using PNM, but maybe PNM has problems storing the density? Is TIFF just as good as PNM? Are there disadvantages to it?Well, maybe you should try to save your scanned images (in 'scanimage') in a 'better' format right at the start. (A format that can store the correct density, e.g. TIFF, PNG, BMP etc. - not PNM.) That would minimise your problem.
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What I think I did was to convert a file of density 300 by a factor of 300/72, yielding an effective density of 1250 dpi. I suspect that lpr did the same thing when I passed it a file with an improperly given density 72.Anyway - regarding your problem with the correct density value in the PNG file: That should be solved.
cts wrote:What I really need is the command to force the file to have the real density
You did that.
But thank you for your suggestion of posting the file - as soon as I find a good place to do so, I'll try that.