iMac 5k as a monitor: the Holy Grail

Keith Barkley wrote:

One thing we don't know: Was there a *hardware* change that would prevent the video *output* from becoming a video *input*.

Target Display Mode went away when the first 27' 5K Retina iMac came out.

That seems unlikely to be a coincidence. Especially since that iMac had Thunderbolt 2 ports, and it might have taken two cables running into that Mac to deliver a 5K video signal.

The 2017 iMacs have Thunderbolt 3 ports, and a Thunderbolt 3 cable can carry a 5K video signal - encoded as two DisplayPort sessions, each carrying data for half of the display. But that doesn't mean there is a hardware-accelerated "fast path" all the way from the Thunderbolt ports, through the GPU, to the screen. If there is any requirement for the CPU to do high-speed, real-time, video data transfer, you run into the issue of whether the "slow path" is up to that job.

And again, even if it turns out that the hardware is theoretically up to the job, someone would still need to write system software to support it.

You're not going to get that magically, for free, just by installing Windows or Linux.

Lars Hansen wrote:

To turn my 2017 5K intel iMac into a 5k monitor, I can rip the guts out from the iMac and install an aliexpress kit to convert it or I can use the Luna Display, but is there a physical reason that prevents someone from installing a different OS that essentially turns the computer into a monitor?

I wouldn't count on there being enough hardware acceleration, of the right kind, to let the iMac accept a real-time stream of 5K video data, and feed it to the LCD panel without lags and glitches.

Even if there was,

• Apple does not provide detailed low-level programming information for the iMac hardware. You would need to somehow figure that out.
• Writing and debugging a custom OS would be an enormous amount of work, even with full documentation.
• I haven't heard of anyone doing it. Are you the one volunteering to spend thousands and thousands of dollars worth of your own time, as a software developer, just on the off-chance that you can reuse that old iMac?

Stop wasting your time and effort, if you want an external 5K display buy one, a 2017 (or any 5K iMac ) will not work. Luna Display will however the performance is not reputed to be very good. There are several 5K displays available and even more 4K displays at very reasonable prices.

We see posts similar to yours daily and the answer is always the same.

FYI, this disclaimer is on the Luna Support home page:

The red emphasis is theirs, not mine. That basically says thar Luna (and, by the way, other similar software/dongle workarounds) is only for providing a place to park a static window or tool palettes.

One thing we don't know: Was there a *hardware* change that would prevent the video *output* from becoming a video *input*.

target display mode was always something the operating system had to support so you would have to find some app for windows or linux which knew the protocol to activate it, if such a app existed, also it was with the introduction of 5k imacs that ended target displays life so there could be a technical limitation in the mix too.

as you mention there are third party software which can do it

Use iMac as monitor: 5 Solutions - 9to5Mac

but they are often wireless and their performance would likely not be fit for games, movies and quick responses in general.

also a factor an imac acting as display in any form would still be using the AC watt power of a full computer where an external display would use but a fraction.

Can I improve screen sharing by using a usbc- to-usbc or thunderbolt cable? - Apple Community

The answer is still no.