I would like to get to the bottom of what I am doing wrong that leads to black and white documents having a bigger filesize than color.
My process for a color TIFF is like this:
① tiff2pdf
② ocrmypdf
③ pdf2djvu
Resulting color DjVu file is ~56k. When pdfimages -all runs on the intermediate PDF file, it shows CCITT (fax) is inside.
My process for a black and white TIFF is the same:
① tiff2pdf
② ocrmypdf
③ pdf2djvu
Resulting black and white DjVu file is ~145k (almost 3× the color size). When pdfimages -all runs on the intermediate PDF file, it shows a PNG file is inside. If I replace step ① with ImageMagick’s convert, the first PDF is 10mb, but in the end the resulting djvu file is still ~145k. And PNG is still inside the intermediate PDF.
I can get the bitonal (bilevel) image smaller by using cjb2 -clean, which goes straight from TIFF to DjVu, but then I can’t OCR it due to the lack of PDF intermediate version. And the size is still bigger than the color doc (~68k).
update
I think I found the problem, which would not be evident from what I posted. I was passing the --force-ocr option to ocrmypdf. I did that just to push through errors like “this doc is already OCRd”. But that option does much more than you would expect: it transcodes the doc. Looks like my fix is to pass --redo-ocr instead. It’s not yet obvious to me why --force-ocr impacted bilevel images more.
#askFedi