Make text in PDFs easier to read when printed by deepening the dark colors used for text. You can draw an analogy between this script and browser extensions that automatically convert web sites to dark mode such as darkreader.
-
Install pre-requisite packages, for the command-line programs pdftoppm, img2pdf, and convert:
sudo apt install poppler-utils img2pdf imagemagick -
Copy the file 'ezread' in this repo to somewhere on your computer.
ezread input.pdf output.pdf
TLDR: contrast-stretching on rasterized page images.
-
Rasterization: Convert each page of the input PDF into a high-quality image.
-
Contrast-stretching of each page's image: Simple 'convert -normalize'. This is a simple technique to boost contrast in images. No AI or anything fancy. Yet.
-
Combining page images into a PDF: img2pdf to losslessly make a PDF.
I've started coming across more and more PDFs that have poor readability when printed on paper (both color inkjet and monochrome laser). I prefer to print PDFs vs reading on the screen.
This script fixes PDFs that have low contrast text: this will be most apparent as hard-to-read text on paper, when such PDFs are printed. May be apparent on low/medium quality displays, displays with bad gamma settings, as well as low-color, low-density displays such as ebook readers.
TLDR: displays display differently than printers print.
An LCD/CRT can set the intensity of every single sub-pixel (RGB) of every single pixel by adjusting the brightness of that subpixel's LED, in addition to whether subpixel is on or not.
But printers (inkjet and laser) cannot adjust the intensity of the color at every subpixel as well a display can. I think some models vary the quantity of ink at a subpixel, which helps somewhat, but printers primarily overlay color inks, and use dithering.
Therefore, ultimately, best results are achieved during printing, by matching the colors to be printed with the inks the printer has. Since black used to be a common color, all printers come with separate black ink.
Grays can be especially challenging for printers: Most printer algorithms will opt to use dithering (i.e., reducing effective resolution) to produce gray text. Or they may use color toner/ink in addition to black to get the shade of gray. The second technique won't work if you print in monochrome mode to save color toner/ink.
Printers try to do their best by mixing colors to achieve the closest match. More formally, the color gamut of printers is much smaller than the color gamut of most monitors (circa 2022). The effect will continue to get WORSE as displays improve -- OLED, HDR etc. Printers are fundamentally limited because they use fixed-color ink/toner. That is why photo printers use many more fixed-colored inks (home printers use three colors + black, printers that seven colored inks exist. See the the Canon product page).
Need to recheck: IIUC inkjet printers can vary the amount of a ink laid down at pixel. I think laser printers cannot even vary the amount of toner at a pixel per color -- its a binary decision) laid down at a pixel.
-
Output PDFs can be large: Approx 1 MB per page.
The input PDF's pages are rasterized at high dpi (300 dpi) and high quality, which makes them quite large (approx 1MB per page).
The output files are intended to be printed, not archived. The output PDFs are intended to be high-resolution, meaning "jagginess"/"blockiness" aka aliasing virtually indistinguishable from the input PDF.
Since the output PDF is essentially images of each page in the likely vector-format input PDF, to compensate, each output page is rasterized at relatively high pixel density (300 dpi) to match the practical densities of most home printers.
450 dpi is when it actually gets to be indistinguishable to my under a magnifying glass on my printers, so I left it at 300 dpi for now, because it seems to be a more "round" number wrt printer specs.
-
High temporary disk usage: About 16 MB per page.
I opted to use uncompressed ppm files, except in the last stage which uses JPEG, to reduce CPU usage in encoding/decoding of intermediate files. Fix: replace 'pdftoppm' with 'pdftocairo -png'
On my computer, this script appears CPU-bound, and I have a relatively large disk, so I opted to reduce CPU usage rather than I/O. The impact of I/O on the script's total runtime is not noticeable on a spinning hard disk, and the effect is drastically smaller on an SSD.
TODO: Add this section. For now: see https://imagemagick.org/Usage/color_mods/#normalize
Experiments were performed on a Canon ink tank printer, and a Brother laser printer. The OS whether Mac, Windows, or Linux didn't make a huge difference. The printer firmware can make some difference because each manufacturer uses different heuristics to balance speed, color accuracy, and ink usage, but the PDF file itself is the main factor.
eof