Proposal for RandR version 1.6, Leases and EDID-based output grabs

Pekka Paalanen ppaalanen at
Wed May 3 07:08:09 UTC 2017

On Tue, 02 May 2017 07:45:25 -0700
Keith Packard <keithp at> wrote:

> Pekka Paalanen <ppaalanen at> writes:

> > I presume that if "desktop" is set to "true", it implies that the HMD
> > is capable of showing a simple 2D canvas in stereo without any special
> > rendering and with the default video mode. That is, creating a sort of
> > a virtual 2D monitor. That would be nice.  
> I was thinking that 'desktop' would be true for non-HMD displays that
> didn't need the VR compositor. If you've got a VR compositor and want to
> paint the desktop inside the VR environment, then I think you'd want to
> create a synthetic monitor and hand images from that to the VR
> compositor each frame.

Hi Keith,

do you mean to list all kinds of display devices in the database? I was
assuming it would list only HMDs, so not in database would imply it's a
normal display and good for extending the desktop to.

Or did you mean it for exceptions? As in, define a range of HMDs, but
the vendor put a few normal displays in the middle of the range, so one
needs to be able to exclude those?

The reason I mentioned "virtual 2D display" was that I recall hearing
that actually exists in some HMD hardware. If you don't do anything to
enable a 3D mode, the HMD will process the signal to produce a virtual
2D display in front the user. In such case, there is no need for a VR
compositor, the plain old 2D image signal will be shown correctly on a
plane in the virtual space by the HMD hardware itself.

I asked on #openhmd and was pointed to "Cinematic mode" on PSVR, and
found e.g. this:

Mind the note towards the bottom: you don't actually need a PS4 to use
it - so it must be something built into the HMD. However, reading more
details from
reveals that there is actually a separate processor box providing the
cinematic mode. Sounds like it's your VR compositor as a middle-man
hardware device rather than just a program. :-)

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