Blog: TTTThis

Create a photo montage (imageMagick)

montage -geometry +0+0 -tile 10x *.jpg result.jpg

creates a montage of all the jpgs in the folder, 10 images wide (you can't specify by how many height.


Lighten, darken, increase contrast on (text) images, for readability (ImageMagick)

Lighten those pages

  • convert output.pdf -function polynomial 1,0,0,0 darkened.pdf
  • mogrify output.pdf -contrast-stretch 2%x20% music1C.pdf
  • convert -density 600 output.pdf output-%02d.jpg

(using rm to add security permissions then remove them after

sudo mv /etc/ImageMagick-6/policy.xml /etc/ImageMagick-6/

-----When done, you can restore the original with

sudo mv /etc/ImageMagick-6/ /etc/ImageMagick-6/policy.xml

3 step process:

  • convert your_pdf_filename.pdf output-%02d.jpg
  • convert output*.jpg -level 25% final-%02d.jpg
  • convert final*.jpg very_readable.pdf

(change the level value)

With the arg -threshold you get a "black and white" (only) image. But I want to keep the gray scale, which is possible with the arg -level: you keep the gray, letting the image with a darker or lighter gray scale. (referring to something like <<< convert output*.jpg -normalize -threshold 80% final-%02d.jpg >>>


Extract some pages from a pdf (qpdf)

install qpdf (FOSS)

qpdf originalDoc.pdf --pages . 1-10 -- outputDoc.pdf

For multiple sets of pages

qpdf --pages . 1-8 . 53-70 -- input.pdf output.pdf


Convert images to pdf (img2pdf)

(There is a way described for Imagemagick but it didn't work for me )

sudo apt-get install img2pdf

Open a Terminal in the folder with the images and do

sudo img2pdf *.png -o output_imgs.pdf

(assuming the images are pngs.)


Convert image to text (tesseract-ocr)

sudo apt install tesseract-ocr

or (although I don't think this is necessary)

sudo apt install tesseract-ocr libtesseract-dev tesseract-ocr-eng

Do an example. Name your file existingimage.png and open a Terminal in that folder and do

tesseract -l eng existingimage.png output_from_ocr cat documenttocreate.txt

(where -l specifies a language. To see all the languages, do man tesseract)

OCR means Optical Character Recognition

Convert image to pdf (not to txt)

tesseract -l eng input_for_ocr.png output_from_ocr pdf

Errors because 'Tesseract couldn't load any languages!':

Spanish: download from here

put it here /usr/share/tesseract-ocr/4.00/tessdata/eng.traineddata (You will have to open that folder as root)

Or maybe you can just use: sudo apt-get install tesseract-ocr-spa (although it might not save it to the location you want)

NOTE: After you install a language (or even if you don't) you might over-save the same file, and see an error message, but it's working anyway.