[gentoo-user] Re: [OT] Saving an image as black and white

[<nunojsilva@ist.utl.pt> (Nuno Silva)]

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.)

-- 
Nuno Silva