Projectors and external screens

Covers the failure that shows up five minutes before a lecture: the projector works, but everything looks wrong.

Washed-out colours over HDMI

Colours look faded, blacks look grey, and the laptop's own screen is fine. It often looks correct at a lower resolution, which sends people hunting for a resolution problem.

It is not a resolution problem. The graphics driver is sending a limited RGB range (16–235, the television convention) while the projector expects full range (0–255), so the whole picture loses contrast.

xrandr --output HDMI1 --set "Broadcast RGB" "Full"

Get the real output name from xrandr on its own; it varies (HDMI1, HDMI-1, HDMI-A-0). If nothing changes, unplug and replug the cable: the setting applies at the next link training.

On Wayland this does not work. xrandr cannot reach the setting. On Intel graphics the equivalent is the kernel parameter video=HDMI-A-1:D combined with driver-specific options; on other drivers there may be no user-facing control at all. Switching to an X session for the lecture is the reliable answer, unsatisfying as it is.

xrandr --output HDMI-1 --set "Broadcast RGB" "Full"

Get the real output name from xrandr on its own; it varies (HDMI1, HDMI-1, HDMI-A-0). If nothing changes, unplug and replug the cable.

On Wayland sessions the setting is not reachable this way; an X session is the reliable workaround.

macOS negotiates the range itself and usually gets it right. When it does not, the control is not exposed in System Settings. A display override profile is the usual workaround, and it is fiddly enough that borrowing a different cable or adapter first is worth trying.

Background: the askubuntu answer describing the same fix.

Verify

devices-verify.sh Download

Says whether this is a Wayland session, what is connected, and whether the range setting exists at all

curl -fsSLO https://hutzelmann.github.io/thi-linux-macos-setup/scripts/devices-verify.sh
less devices-verify.sh   # read it first
sh devices-verify.sh --dry-run

Runs on its own. The documented values and helpers are built in, no clone needed. --dry-run prints what it would do and changes nothing.

That answers whether the fix above is available here. Whether the picture is right is the part below, and no script can see a wall.

Show something with a large black area and a large white area: a terminal on one side, a white document on the other. Correct full-range output gives you an actual black, not dark grey.

Known quirks

USB-C docks add their own failure. Some docks negotiate a lower link rate and silently drop to a lower refresh or resolution. If the picture is fine over plain HDMI and wrong through the dock, the dock is the variable.

Mirroring vs extending changes the negotiated mode. A resolution that works extended may not be offered mirrored, and lecture halls generally want mirrored.

Room-specific behaviour is not documented here. AV equipment differs between buildings and this page has one fix in it. If you work out what a particular room needs, that is worth adding.