Screen region filter

A small portable utility for applying colour filters to a specific region of your screen, based on the Windows Magnification Sample program.

Define a rectangle with two mouse clicks, then toggle inversion, grayscale, and brightness using keyboard controls.

Save up to 10 regions with their settings and recall them instantly with a numbered shortcut. Useful if you work in dark mode but regularly encounter bright content you can't control: shared screens in video calls, legacy apps, PDFs, e-learning platforms. This is a portable app - just run the executable. Shortcuts are configurable via a plain text file.

Usage

Defining a region

When the app launches it goes full-screen in selection mode (crosshair cursor). Click two points — top-left then bottom-right — to define the filter region. No click-and-drag required.

Filter controls

These keys are active once a region has been selected. The window must have focus (i.e. not be pinned — see below).

Key	Action
I	Toggle inversion on/off
C	Toggle colour/grayscale
W	Cycle brightness level: 100% → 80% → 60% → 40% → 100%

These can be combined - for example, inversion + 60% brightness, or grayscale + inversion.

Pinning (click-through mode)

Key	Action
Ctrl+Shift+P	Toggle pin/unpin (global hotkey, works even when the window doesn't have focus)

When pinned, the filter overlay becomes transparent to mouse and keyboard input — you can interact normally with whatever is underneath. The title bar remains draggable so you can reposition the window while pinned. When unpinned, focus returns to the filter window so you can adjust settings.

Saving and loading presets

Presets store the region position, size, and all filter settings (inversion, grayscale, brightness level).

Key	Action
Ctrl+1 – Ctrl+9	Save current region and settings to slot 1–9
1 – 9	Load preset from slot 1–9 (after launching the EXE)
0	Cycle through all saved presets in order

Any preset rectangle settings persist between settings in saved_rects.txt which is written to the working directory at the time the app is launched - this may differ from the directory the executable lives in, depending how you invoke it (shortcut, AHK macro, etc). Multiple running instances share the same preset file.

Multiple instances

Key	Action
Ctrl+N	Launch a new instance to define an additional filter region

Each instance operates independently with its own filter settings. Run as many as you need for different regions simultaneously.

Customising shortcuts

All shortcuts except the number keys and Ctrl+N are configurable via shortcuts.txt in the same directory as the executable. The file is created with defaults on first run. Changes take effect after restarting the app.

ToggleInvertKey=I
ToggleGrayscaleKey=C
CycleWhiteLevelKey=W
GlobalHotkeyKey=P
GlobalHotkeyModifiers=CTRL+SHIFT   # combine with +, e.g. CTRL+ALT