颜色为黑白，从PDF到IMG再到PDF

Question

Ok, so I am taking full color PDFs and trying to convert them to black and white images to then make them black and white PDF's.

Currently I am attempting to do this with imagemagick and i am for the most part successful with breaking them PDF into separate images. However once creating the separate images I want to make full size black and white copies next to the color copies as well as thumbnails thereof of both. Which that part I feel I'll be able to figure out on my own.

What I am having trouble figuring out is how to get them to become black and white. Not necessarily grayscale, since grayscale will still use colored ink in most printers to come up with the varying shades of gray. From which I am trying to avoid.

Is there a means I can do this with imagemagick or is my thought process all wrong?

Also worth noting is I am trying to do this through a browser and server side process with PHP

Answer 1

I think you are looking for "Two Colour Quamtisation" as described in Anthony Thyssen's excellent work here.

Basically, you use quantisation to pick the best two colours to represent your image, then force the darker one to black and the lighter one to white. So your processing becomes:

convert image.png -colors 2 -colorspace gray -normalize result.png

Btw, I presume you found and are using pdfimages (part of the poppler package) to extract the images in original quality.