A "spotlight" focus dimmer for Windows. Whatever app is in front stays bright; everything else is dimmed. ~120 KB single .exe, pure C, no install.

Think of it as a vignette around your active app — close all the cosmetic windows of 2025 and just see the one you're using.

bitdim.exe

• A tray icon appears near the clock.
• Left- or right-click the tray icon → opens a small floating dialog near the cursor:
  • Slider sets dim amount (5 %–95 %)
  • ☑ Enabled toggles dim on/off
  • Exit quits the app
• Ctrl + Alt + D = global hotkey for quick on/off without the dialog.
• Click anywhere outside the dialog (or press Esc) to dismiss it; the dim state is kept.
• Multi-monitor: each display gets its own overlay; the active region is cut out wherever it falls. WM_DISPLAYCHANGE rebuilds overlays when monitors are plugged or unplugged.

Put any mingw-w64 toolchain on PATH — easiest option is w64devkit (32-bit or 64-bit). Then from a cmd prompt in the repo root:

make            :: release -> bitdim.exe
make debug      :: bitdim_debug.exe (-g -O0)
make clean

If gcc.exe, windres.exe, and make.exe are not yet on PATH, prepend the toolchain's bin\ first, for example:

set PATH=C:\path\to\w64devkit\bin;%PATH%

The Makefile is POSIX-style and forces SHELL = cmd.exe, so it never invokes sh. The project has no third-party dependencies — only the Win32 SDK headers shipped by w64devkit.

For each monitor a click-through layered top-most window is created (WS_EX_LAYERED | WS_EX_TRANSPARENT | WS_EX_NOACTIVATE | WS_EX_TOOLWINDOW) and painted solid black with SetLayeredWindowAttributes(LWA_ALPHA). To leave the active app visible we call SetWindowRgn with (monitor_rect MINUS active_region), literally punching a hole through the dim layer.

active_region is built as follows:

• The foreground window's owning process is identified.
• Regular apps: region = union of every visible top-level window that belongs to that process. Three Chrome windows therefore all stay bright as one app, and modal dialogs stay bright together with their owners.
• Shell processes (explorer.exe, ApplicationFrameHost.exe, ShellExperienceHost.exe, StartMenuExperienceHost.exe, SearchHost.exe, SearchApp.exe, SystemSettings.exe): region falls back to the foreground window's own rect. Otherwise opening the tray overflow would light up unrelated File Explorer windows.
• Maximized windows: the rect is taken from the host monitor's rcWork, because DwmGetWindowAttribute(DWMWA_EXTENDED_FRAME_BOUNDS) can return the pre-maximize bounds on some Windows / app combinations.
• Normal / snapped: DWM extended frame bounds (excludes the invisible Win10/11 resize border).
• Hidden / minimized / DWM-cloaked / shell-surface (Shell_TrayWnd, Progman, WorkerW, Shell_SecondaryTrayWnd) windows are filtered out and never contribute to the region.

A SetWinEventHook(WINEVENT_OUTOFCONTEXT | WINEVENT_SKIPOWNPROCESS) listening for EVENT_SYSTEM_FOREGROUND, EVENT_SYSTEM_MINIMIZE{START,END} and EVENT_OBJECT_LOCATIONCHANGE schedules a region rebuild whenever the user switches / moves / resizes windows. Updates are coalesced to ~60 fps with a single one-shot WM_TIMER so a 1000 Hz polling mouse can't flood the GDI path while a window is being dragged.

• 32-bit exe (i686 mingw-w64) → Windows 7 / 8 / 8.1 / 10 / 11, both 32-bit and 64-bit (the latter via WoW64).
• Manifest declares PerMonitorV2 DPI awareness and Common-Controls 6.0 visual styles, so the dialog renders in the modern Win10/11 theme.
• Win11-only nice-to-have: rounded dialog corners via DWMWA_WINDOW_CORNER_PREFERENCE (silent no-op on older Windows).

Ideas welcome — open an issue or send a PR.

• Per-app rules: never dim Discord / always dim Slack.
• Fullscreen detection: auto-disable for games / video.
• Smooth alpha animation on focus change.
• Custom dim colors (warm sepia, blueish night).
• Dim only inactive monitors (not just inactive windows).
• Persist alpha + enabled state across restarts.
• Configurable hotkey (Ctrl+Alt+D collides with some IDEs).
• Localization (currently English-only strings).

• Ctrl + Alt + D may collide with an app that has bound it (some IDEs use this combo for "duplicate line"). Quick workaround: turn dim off with the tray dialog before launching that app.
• Fullscreen apps still get an overlay on top — toggle dim off before launching games / fullscreen video.
• Some custom-chrome apps (WSL graphical apps, certain Electron variants) report exotic frame bounds and may produce a hole that's a couple of pixels off the visible window edge.

PRs and issues welcome — see CONTRIBUTING.md for the short version (build, code style, what kinds of changes are likely to be accepted).

MIT — see LICENSE. You can do almost anything with this code as long as the copyright notice ships with it.