Of course, this introduces recompression artifacts, and is unnecessary
if no options are given that modify the final image (no -d, -c, -i).
But rather than worry about that, it would be better to ultimately find
a way to combine the original PDF page with the output PDF text in the
case where we want no changes to the original. This is good enough for
now.
The better option can apparently be achieved using pdftk background, or
probably better, PyPDF2's merge. If Tesseract PDF generation is used
then we need a way to remove the image. Tesseract PDF generation at 3.03
does layout better (I think) and also properly encodes the hidden layer,
which is less likely to give display issues (I think).
Ghostscript has the clunkiest imaginable syntax, obtuse documentation,
quirky behavior, and poor diagnostics... but it *actually works* unlike
pdftoppm/poppler which gets things wrong.
In this case I observed poppler incorrectly decompresses certain CCITT
encoded monochrome PDFs. So set up Ghostscript to do the job instead.
For the moment this performs monochrome -> RGB conversion via reportlab.
It appears to be possible to have a PDF with an embedded font that is
either unused or used only for whitespace. So check for some amount of
actual text instead.
Appears to be necessary to disable each state of the pipeline that is
inactive, not just initial and terminal stages of an inactive segment.
If nothing else this makes what is going on more explicit.
pdftoppm in recent versions (0.26.4,5) seems to be incapable of
producing valid TIFFs, so have it dump a .pnm file and let ImageMagick
figure out how to convert it to TIFF. This is not ideal, but at least
it works.
convert .pnm -deskew <...> .pnm seems to have a bug that produces an
invalid .pnm file which later causes tesseract (specifically,
leptonica) to choke (using 3.02/1.71 as versions, respectively). Will
change pipeline to use tiffs internally since they are less stupid.
