How to simulate a retina display (HiDPI mode) in Mac OS X 10.8 Mountain Lion on a non-retina display?

Question

How can you simulate a retina display (HiDPI mode) in Mac OS X 10.8 Mountain Lion on a non-retina display?

Answer 1

Edit: (5/31/2016)

For users trying to do this on El Capitan, please read the FAQ on SwitchRes's website. Also, if something's still not working after you did all the steps in the FAQ, consider uninstalling and reinstalling SwitchResX. That solved the issue I was having on one of my laptops.

Original:

After reading through several forums, websites, blogs.

I am here to present a solution for users with 15" MacBook Pro with Retina display connected to a Thunderbolt Display.

First of all,

1. Terminal command of modifying plist
2. Quartz Debug
3. Holding option and select "Scaled" in System Preferences
4. ResolutionTab (Mac App Store)

These methods DO NOT work for MBPr with Thunderbolt Display, for whatever reasons. You will not see the HiDPI options to be selected.

The only tool I found that actually gives us the options is SwitchResX.

However another problem exists here.

Most users with this setup, I believe, are trying to use 1280x720 HiDPI because it's half the native resolution of the TBD.

According SwitchResX's FAQ, in some cases it is not possible to set to this resolution because of a bug within OS X itself.

Here's a screenshot for your reference:

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After contacting the developer, he presented a workaround - adding one more pixel - which worked for me.

1. Install SwitchResX and open it from System Preferences.
2. Go to Thunderbolt Display tab, and add a Custom Resolutions with Scaled Resolution at 2562 x 1440 Here's a screenshot
3. Save using command + s. (or simply close the window and use the prompt up)
4. Restart the laptop.
5. Go to SwitchResX and select the new custom resolution in the Current Resolution tab. (Sometimes it doesn't show up right away, play around with it and it should.)

Here you go.

I hope this answer gets to users with this setup because it is really frustrating to use 16:10 resolution on a 16:9 display.