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

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

On 2021-03-01, Wols Lists wrote:

> 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

Sorry, my bad - I was checking the file sizes, but I didn't notice the
larger one was the new, "monochrome" version. More coffee needed, it
seems.

-- 
Nuno Silva