How can I change DPI on OS X? Simple question, can't find a single simple answer on the entire internet. Although from what I have read, this might be a driver or hardware limitation.

My problem
I have a 1440 x 900 native pixel resolution. I want to run at a 1280 x 800 scaled resolution. 

I know you can change this in the display settings, but then you see a horrible blur because of the hardware upscaling. OS X has built it DPI upscaling, and has been resolution independant for a while now. Why can't I change the perceived resolution at the software level?

My attempts
I've force enabled HiDPI modes via Quartz Debug and it has given me one HiDPI mode. 720 x 450 PX @ 2 DPPX. I've installed SwitchResX and although I can add custom resolutions, I can't force it to use the graceful scaling I want, and I can't see a way to manage the DPI or DPPX.

To summarise
I want to run my native MacBook Air 13-inch display at 1440x900 PX and have OS X upscale to this from 1280x800 PX @ 1.125 DPPX, still 128 DPI, using the same DPI scaling technology that makes "retina" displays look so clear.

I would love this option but I appreciate it may not be possible. I love my MacBook Air and don't want a rMBP, but I also have terrible eyesight and would prefer a 1280 width, but not with the horrible upscaling that is currently my only option making me squint even more.
Not sure if this will help some people, but I figured this out for myself

I have a 27" LG WQHD (2560x1440) display connected via HDMI to my 2014 13" Macbook Pro Retina.


Enable Mirror Displays





On the external monitor, select Optimize for: Built-in Retina Display




The final result isn't perfect, but it makes my 27" external display look like a beautiful retina display.


  Two downfalls:



the macbook has to be open in order for the external to continue running in HiDPI mode.
the external monitor has an aspect ratio of 16:9 but the laptop is 16:10. There are thin vertical black bars on the left/right of my external monitor.