vasvir / adaptive-thresholding

Create a PDF suitable for economic printing from photos

adaptive-thresholding

Create a PDF suitable for economic B&W printing from photos

Create a PDF suitable for economic B&W printing from photos

A little script that performs adaptive threshold in photos and converts them to B&W for higher contrast and ink/toner economy.

With the advent of Coronavirus and the remote learning spike a lot of children and professors started exchange learning material via electronic means.

Normally this is done via phones where applications like CamScanner perform adaptive thresholding. Adaptive thresholding is a must if you need to save some ink in order to print another day. Adaptive thresholding is not only environmentally friendly but helps your strained eyes by increasing the contrast of the generated image and lowers your bandwidth cap by producing images with much smaller size.

However many people are not aware of the adaptive threshold and the specialized scanner application and so they send images captured by the phone's camera (photos). These have the following problems in case you didn't read the previous paragraph.

1. They have large grays areas and degradation
2. They have small contrast (variation of gray) and shadows
3. They have much bigger size typically 1 to 7 ratio
4. Since they are bigger it takes more time to upload, download and occupy more space on disk

The problem though is that on Linux there is no easy to perform the adaptive thresholding.

• Gimp doesn't do it.
• There is no batch conversion (multiple images) utility

Solution

The following python script employs opencv and its python bindings to perform adaptive thresholding in all images in a directory (not recursively) and launches the default PDF viewer with the assembled PDF.

A desktop file is also provided if you want to have right click integration in your file manager for this kind of operations.

Dependencies

• python3-opencv
• python3
• xdg-utils
• imagemagick
• A default PDF viewer that is associated with xdg-open

In a normal desktop

sudo apt install python3-opencv

is all you need since the others must have been already installed.

Installation

Put or link th create-adaptive-threshold-pdf.py file in /usr/local/bin

sudo cp create-adaptive-threshold-pdf.py /usr/local/bin/

For KDE

Run this to locate where are the ServiceMenu folder (user and system)

kf5-config --path services

Put the create-adaptive-threshold-pdf.desktop in one of them

sudo cp create-adaptive-threshold-pdf.desktop /usr/share/kservices5/ServiceMenus/

1. Close all dolphin instances
2. Run kbuildsycoca5

   kbuildsycoca5
   

3. Run dolphin again
4. Locate a directory that has the scanned images
5. Righ click / Actions / Create PDF
6. Okular should start with all the images in the directory assembed in one tidy PDF suitable for economic printing that doesn't make your eyes and your pocket bleed.

Pitfalls

You may see

convert-im6.q16: attempt to perform an operation not allowed by the security policy `PDF' @ error/constitute.c/IsCoderAuthorized/421.

Here is the answer: https://stackoverflow.com/questions/52998331/imagemagick-security-policy-pdf-blocking-conversion

Troubleshooting

If for some reason the script doesn't launch a PDF viewer with the desired output it is time to investigate further. Open a terminal and run

create-adaptive-threshold-pdf.py /path/to/directory/with-images

The script is pretty verbose about each stage and it cleans everything up on exit so you don't have to take out the trash.

Hope that helps.