How to change onscreen resolution when screen sharing with Intel Mac from M1 Mac

[Boidem]

Ok so. This is the actual reason I joined this forum but then got distracted with other threads and completely forgot. Anyway. Having bought a lovely new M1 iMac last year, I wanted to be able to keep my old MacMini (late 2012) and connect the two together, using the new iMac's screen as a single display for both. Screensharing, right? But. Although it all connects fine (via ethernet, no silly wireless nonsense), the screen image of the Mac Mini appears in a very low resolution; 1280x1024. I appreciate the max display resolution from the MM is up to 2560 by 1600 (Thunderbolt) or 1920 by 1200 (HDMI), but obvs what I'm actually getting is only half of that, so I get a small screen image in the middle of my iMac's screen, with no way of changing the size. I'm not looking for full screen resolution, just a bit bigger would be helpful. There is no option to change anything in the MM's settings:

See what I mean? 'Connected Projector' has never appeared before I connected the MM to the iMac. Previously, I used a 1920x1200 24" monitor. Clicking on 'Scaled' just gives two options; 1280 or 1024x768. Using Zoom on the Screen Sharing just makes the screen image larger but does not increase resolution, so it looks pixelated.

Is there any way I can get a larger screen image? Thunderbolt? Or does that not work? I'd like to keep the old MM running, as it can serve as a 'legacy' machine, and be useful in sorting out problems on other people's older iMacs, as I can roll it right back to like Tiger or something.

It is a 2.3GHz i7; 16Gb RAM, graphics show as 'Intel Graphics HD 4000', VRAM showing as 1536MB.

Please help me. Please.

 

[Juicy Box]

This is a common problem for running headless Macs.

There might be some third-party software that can fix the situation, but I think most people use an HDMI dummy plug. You could buy them on Amazon. They're not that expensive.

 

[Boidem]

Aha! I may, by complete accident, have solved it! So I clicked on Airplay, as that can connect the MM to myAppleTV connected to the telly, and that worked. And lo and behold, it also brought back the 1920x1080 option in Displays Preferences on the MM(not 1200 like my old monitor though), so whilst it's not great, it is at least a much larger image on screen. Which is fantastic.

 

[Boidem]

This is a common problem for running headless Macs.

There might be some third-party software that can fix the situation, but I think most people use an HDMI dummy plug. You could buy them on Amazon. They're not that expensive.

What is this thing?

 

[Boidem]

No no no! Even more brilliantly, if I change my iMac's scaling to 1920x1080, then I get a complete full screen of the Macmini's desktop!!! Makes everything on the iMac look a bit big and clunky mind, but I can always switch back once I've done with the MM.

I now hope this may be of use to others.

 

[Boidem]

Sussed it. The MM will display according to the last display device it was attached to. So if I connect to the Apple TV, then it defaults to 1080p. I found a Luna Display dongle thingy I bought a few years ago to connect the MacMini to my old iPad Air (obvs the new iPad Air and iMac don't need such a thing, as they can screen share/mirror from the OS now). This can display (on the iMac) as 2240x1260, but there's still pixelation and it's harder edged, so doesn't look very good. What I'm wondering now, is if I connect the MM to a 2560x1600 display, will the MM then save those settings when reconnected to the iMac in Screen Sharing? Cos that would be marvellous. 1080p is workable, but not great. Hey ho. At least it's not a stupid tiny little window anymore.

Perhaps one of those dummy HDMI or MDP plugs will work. Worth a punt, they're only a few quid. Thanks to @Juicy Box for that tip, and thanks so much to everyone else who's contributed so helpfully to this thread. 🤣

 

[Gudi]

The MM will display according to the last display device it was attached to. ... What I'm wondering now, is if I connect the MM to a 2560x1600 display, will the MM then save those settings when reconnected to the iMac in Screen Sharing?

2560×1600 is 16:10, you need 2560×1440 which is 16:9. Can you select this resolution manually in the display settings of both the iMac and the Mac mini?

 

[Lihp8270]

Sussed it. The MM will display according to the last display device it was attached to. So if I connect to the Apple TV, then it defaults to 1080p. I found a Luna Display dongle thingy I bought a few years ago to connect the MacMini to my old iPad Air (obvs the new iPad Air and iMac don't need such a thing, as they can screen share/mirror from the OS now). This can display (on the iMac) as 2240x1260, but there's still pixelation and it's harder edged, so doesn't look very good. What I'm wondering now, is if I connect the MM to a 2560x1600 display, will the MM then save those settings when reconnected to the iMac in Screen Sharing? Cos that would be marvellous. 1080p is workable, but not great. Hey ho. At least it's not a stupid tiny little window anymore.

Perhaps one of those dummy HDMI or MDP plugs will work. Worth a punt, they're only a few quid. Thanks to @Juicy Box for that tip, and thanks so much to everyone else who's contributed so helpfully to this thread. 🤣

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[Boidem]

2560×1600 is 16:10, you need 2560×1440 which is 16:9. Can you select this resolution manually in the display settings of both the iMac and the Mac mini?
View attachment 2128163

I don’t even have the option to change resolution on my iMac! How do you get that?? All I have is something about changing ‘scaling’.

 

[Gudi]

You right-click on the scaling options and chose show resolutions as a list.

 

[Boidem]

OMactual G I DIDN'T EVEN KNOW YOU COULD DO THAT!!!! 🤣🤣🤣

But; and this'll make you laugh; I didn't even know that my display was only at 2560x1440; I naturally assumed it was set at the max res of 4480x2520! To my eyes, it looked wonderful. 🤭 And when I tried that trick above, and set it to the max res, all the text is sooo tiny! Is that normal? How can I make the text and icons etc bigger?

Am I missing something fundamental here? I suspect I am, and that everyone will now laugh at me.

I feel such an idiot.

 

[profcutter]

OMactual G I DIDN'T EVEN KNOW YOU COULD DO THAT!!!! 🤣🤣🤣

But; and this'll make you laugh; I didn't even know that my display was only at 2560x1440; I naturally assumed it was set at the max res of 4480x2520! To my eyes, it looked wonderful. 🤭 And when I tried that trick above, and set it to the max res, all the text is sooo tiny! Is that normal? How can I make the text and icons etc bigger?

Am I missing something fundamental here? I suspect I am, and that everyone will now laugh at me.

I feel such an idiot.

If you set to the max res of the display, Mac OS renders everything pixel for pixel, and you get tiny text and UI elements. Scaled means that the display is still running at full res, but OS X is internally upscaling everything to match a “looks like” resolution. So icons have more pixels, so they appear larger, for example. It’s the retina part of retina display: you can have the same real estate as a lower pixel density screen, but all nicely rendered clean and sharp.

This is different than just choosing a lower resolution. That forces the screen to upscale, so you don’t get the larger icons and the internal smoothing of OS X HiDPI mode, and you just get blocky images and text.

 

[Boidem]

Ah I SEE. I understand it now. So the 'lower' resolutions aren't actually lower, they're still 4480x2520 pixels, right? Just set to LOOK like lower resolutions. To confirm this, I did a test by doing screenshots of the whole screen in different 'resolutions', then comparing them in Get Info. 2240x1260 and above are at max res. 1920x1080 is 3840 × 2160, 1600x900 is 3200 × 1800, and 1280x720 is 2560x1440 in actual pixels. This is why, that although 1920 now fills the screen when screensharing with the MacMini, it still looks a little 'soft' cos it's obvs 'upscaling' or something. so; if I can get a Mini Display Port dummy plug that allows 2560x1440, then I can switch between computers on the screen, with no quality loss at all.

I understand I'd need a MDP plug, as the HDMI on the MM will only allow up to 1920 resolution, even though HDMI can carry a 4k signal. Correct? This is VERY confusing...

 

[profcutter]

As far as the first part of your reply, then yes exactly right. As long as it’s listed as a HiDPI resolution, it should do the high quality upscaling. As far as the dummy MDP goes, that sounds right, but I don’t know if I’ve ever seen one, rather I’ve seen the HDMI ones for sure. And yes, then you’d be limited to HDMI resolution.

 

[Gudi]

So the 'lower' resolutions aren't actually lower, they're still 4480x2520 pixels, right?

In HiDPI mode (Retina) all resolutions are drawn @2× with twice as many pixels horizontally and vertically. This reasonably sharp image is then scaled up or down to fit the native resolution of the screen. Only the Standard option (2240×1260) requires no additional scaling as it already fits perfectly.

4480 × 2520 native resolution
2560 × 1440 @2× = 5120 × 2880 "More Space" (downscaling to 4480 × 2520)
2240 × 1260 @2× = 4480 × 2520 Standard (no scaling)
1920 × 1080 @2× = 3840  × 2160
1600 × 900 @2× = 3200  ×  1800
1280 × 720 @2× = 2560 × 1440 "Larger Text" (upscaling to 4480 × 2520)

 

[Boidem]

Ah. So this is interesting. As I was taking screenshots, one of them actually shows as having a resolution of 5120x2880, yet this display is 4480x2520. How can this be? Image pixel size in Photoshop shows as having a resolution of 144ppi, yet the 4480 version has just 72ppi. That is just weird. Surely my screen cannot display more pixels than it physically has? What's going on???

 

[profcutter]

Ah. So this is interesting. As I was taking screenshots, one of them actually shows as having a resolution of 5120x2880, yet this display is 4480x2520. How can this be? Image pixel size in Photoshop shows as having a resolution of 144ppi, yet the 4480 version has just 72ppi. That is just weird. Surely my screen cannot display more pixels than it physically has? What's going on???

View attachment 2128632

View attachment 2128633

It’s just as Gudi described. OS X renders the image at the 5120 size, then downscales it to match your display. That’s why there’s all the talk about the persnickety nature of matching displays to macs. They perform best with 27 inch 5k displays, rather than 27 or 24 inch 4k displays.

 

[Gudi]

Image pixel size in Photoshop shows as having a resolution of 144ppi, yet the 4480 version has just 72ppi. That is just weird.

This stems from the original Apple Macintosh (1984). The built-in display was a one-bit per pixel, black-and-white, 9 in/23 cm CRT with a fixed resolution of 512 × 342 pixels, using the Apple standard of 72 ppi (pixels per inch). Initially, the only printer available was the Apple ImageWriter, a dot matrix printer which was designed to produce 144 dpi WYSIWYG output from the Mac's 72 dpi screen.

Depending on whether you produce images for digital or print, you would save them with either 72 or 144 pixels per inch. Digital images have no natural length dimension, all screens and printers have different ppi/dpi features. So in order to produce images of a predictable size the Photoshop file format (.psd) has to make assumptions on how tightly the pixels will be packed. By default that's 144.