On 01/03/21 12:11, (Nuno Silva) wrote: > On 2021-03-01, Wols Lists wrote: > >> I've got a bunch of scans, let's assume they're text documents. And >> they're rather big ... I want to email them. >> >> How on earth do I convert them to TRUE b&w documents? At the moment they >> are jpegs that weigh in at 3MB, and I guess they're using about 5 bytes >> to store all the colour, luminance, whatever, per pixel. But actually, >> there's only ONE BIT of information there - whether that pixel is black >> or white. >> >> I'm using imagemagick, but so far all my attempts to strip out the >> surplus information have resulted in INcreasing the file size ??? >> >> So basically, how do I save an image as "one bit per pixel" like you'd >> think you'd send to a B&W printer? >> >> Even at 300dpi, I make that 300*300/8 ~= 10KB/in^2 or 800KB of >> uncompressed info for a page of A4, not 3MB. >> >> Cheers, >> Wol > > Somebody else might have a better suggestion, or perhaps a better > understanding of the JPEG format and of what needs to be tuned, but, for > example: > > convert origin.jpg -threshold 70% -monochrome result.jpg > > (And adjust the "-threshold percent" if needed. It might be that you > don't need thresholding at all, but if you do, it apparently must go > before "-monochrome".) > > (Depending on the receiving end, you could also explore other > formats. Here, if the scanned document can be stored in monochrome, I > usually use djvu.) > Thanks but no, I've already tried that. It makes matters worse!
I've messed about with the scanner, so it is now creating 800KB images, but I don't want to rescan everything I've done. The problem is that it is clearly saving the images as greyscale, not as black&white. And when I search for help, what I want is swamped by all the false positives for greyscale. Oh - and for Nuno - sorry tesseract is no use, they are NOT text. That's why I used the word "assume" - to make it clear that I want a 1-bit/pixel palette, not a 5-byte/pixel greyscale. Cheers, Wol
