Troubleshooting • Spoofing HDMI using [cmdline.txt]

I have a Pi4B with a new Trixie and camera.
Using Python I can Preview on the Desktop and via rVNC can access the Desktop and all is fine.

However without the monitor attached to the Pi at start up the VNC output is terrible. This is a known.
I can start with monitor and then unplug and VNC is fine. The Pi has to have a monitor to set up display output.
It's not a screen resolution that is bad, that I can fiddle but the fps is just 1ps and not at all suitable for a remote monitor

So I am trying to spoof the existence of an HDMI so the VNC output will be fine.

There are posts about the issue(s) and the recommend is to edit the [ cmdline.txt ].

Quote from Picamera2 Library

3.4. Remote preview windows
. . . . .
When using VNC:
• The QtGL (hardware accelerated) window works adequately if you also have a display connected directly to your Raspberry Pi
• If you do not have a display connected directly to the Pi, the QtGL preview will work very poorly, and the Qt preview window
should be used instead

For spoofing code
viewtopic.php?p=2168314&hilit=spoof+hdmi#p2168314

It exists in [ /boot/firmware ]
So far so good
but
a) The file is un-writable
b) I removed the microSD to try and edit in my laptop running Ubuntu but the directory is empty
c) used another version of Trixie on a separate SD in the Pi and tried to access the file but the directory is again empty

d) Swapped SD's to check if I had trashed the directory but no, it was full of files and a couple of directories and the [cmdline.txt] file showed.

So it looks like 'this' file is created at boot and hence does not exist to be edited.

So have I got this 'right' and either
a) how can I edit the 'live' file or
b) how to get info read into it at boot.

Thank you

Statistics: Posted by amoun — Wed Feb 11, 2026 7:24 pm — Replies 3 — Views 101

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