Convert pdf greyscale to B&W

[flywire]

ImageMagick version

7.1.0-Q16

Operating system, version and so on

Win 10

Description

Trying to convert test.pdf from greyscale to B&W using magick "e:test.pdf" -threshold 75% e:new.pdf. Output is outlining objects rather than converting them to black.

new.pdf

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Edit: Added test.pdf

[fmw42]

This is what I get using IM 7.1.0.62 on Mac OSX Monterey:

magick 220787711-20aa00b2-f64e-46f7-9d86-257c20aeca1d.png -colorspace gray -auto-threshold otsu 220787711-20aa00b2-f64e-46f7-9d86-257c20aeca1d.pdf

220787711-20aa00b2-f64e-46f7-9d86-257c20aeca1d.pdf

Here is a png version of the output:

[flywire]

My bad, I hadn't added test.pdf.

I'm still getting the same result with magick "e:test.pdf" -colorspace gray -auto-threshold otsu e:new.pdf. Do you think it is a bug with Win10? I need to retore the dpi to 600 or 1200 too.

[fmw42]

Your PDF has an alpha channel. Remove it. So try

magick test.pdf -alpha off -colorspace gray -auto-threshold otsu test_new.pdf

test_new.pdf

[flywire]

ImageMagick® is a free and open-source software suite for displaying, converting, and editing raster image and vector image files.

Is ImageMagick converting the vector pdf to a 75dpi raster image and embedding it in the pdf? In this case that is fine.

[fmw42]

Yes. Imagemagick is a raster processor. If you need vectors, I believe you can load potrace on your system and IM will use it to output vectors, though they may not be in the form your want.

The default density is 72 dpi. If you want to increase density, then add -density XX where XX=300 or whatever you want before your output pdf.

[flywire]

magick -density 300 test.pdf -alpha off -colorspace gray -auto-threshold otsu new.pdf

new.pdf

[fmw42]

I must have expressed it wrongly. That increased the density by 4 by reducing the page size by 4 in each direction. I want to increase the sampling of the original vector image by 4 and retain the A4 page size.

[fmw42]

To increase density and save as A4, try

magick -density 288 test.pdf -alpha off -colorspace gray -auto-threshold otsu -resize "%[papersize:A4]" -gravity center -extent "%[papersize:A4]" -density 72 test_new.pdf

test_new.pdf

[flywire]

The command above with -density 288 didn't give me the resolution I was after:

but it was enough for me to work out how to get the following last year:

I don't understand how I generated the higher resolution last year. Increasing the density seems to generate the same low resolution file. Any thoughts?