fastshot

A screenshot utility for KDE Plasma 6 on Wayland. Captures screenshots via KWin's D-Bus interface and saves them as PNG files.

Features

• Single-shot mode: take one screenshot and exit
• Loop mode: continuous screenshots at configurable intervals
• Image comparison: skips saving duplicate/similar screenshots using SIMD-optimized comparison
• Async PNG writing: screenshots are saved in background threads to avoid blocking capture

Requirements

• KDE Plasma 6 with KWin Wayland compositor
• systemd (for D-Bus session bus access)
• libpng

Usage

Single shot

fastshot [output.png]

Takes a screenshot of the active screen and saves it. If no filename is provided, uses timestamp format YYYY.MM.DD-HH.MM.SS.png.

Loop mode

fastshot --loop [options]

Runs continuously, taking screenshots at regular intervals. Only saves when the screen content has changed beyond the similarity threshold.

Options:

Option	Description	Default
--loop	Enable continuous screenshot mode	-
-d, --directory DIR	Output directory	~/desktop-record
-i, --interval SECS	Seconds between captures	45
-t, --threshold FLOAT	Similarity threshold (0-1). Only saves if similarity is below this value	0.99
-v, --verbose	Enable verbose logging	off
-h, --help	Show help	-

Example:

fastshot --loop --directory ~/screenshots --interval 10 --threshold 0.95

This captures every 10 seconds and saves only when the screen differs by more than 5% from the last saved image.

NixOS Module

A NixOS module is provided for running fastshot as a user service.

Configuration

{
  imports = [ ./path/to/fastshot/module.nix ];

  services.fastshot = {
    enable = true;
    user = "alice";            # required: user to run the service for
    directory = "screenshots";  # relative to $HOME
    interval = 30;
    threshold = 0.99;
    verbose = false;
  };
}

The service runs as a systemd user unit, starting after graphical-session.target. The user option restricts the service to only run for the specified user.

Building

With Nix

nix build

Or enter a development shell:

nix develop

Manual

cd src
gcc -O3 -mavx2 fastshot.c image-compare.c \
  $(pkg-config --cflags --libs libsystemd libpng) \
  -o fastshot

Requires: gcc, pkg-config, libsystemd-dev, libpng-dev

How it works

• Connects to the session D-Bus
• Calls org.kde.KWin.ScreenShot2.CaptureActiveScreen to capture the screen
• Reads BGRA pixel data from the returned pipe
• In loop mode, compares against the last saved screenshot using MSE-based similarity
• Converts BGRA to RGBA and writes PNG (async in loop mode)

The image comparison uses AVX2 SIMD instructions when available for fast pixel differencing.

Files

• src/fastshot.c - main program
• src/image-compare.c - SIMD-optimized image comparison
• module.nix - NixOS service module
• default.nix - Nix package definition
• tests/vm.nix - NixOS VM integration test

License

MIT