1280×720 HiDPI mode does not work using SwitchResX, RDM or similar utility with an external monitor on a Retina MacBook Pro.
Use a custom scaled resolution with SwitchResX as a work around.
SwitchResX allows you to add custom “Retina” resolutions by adding exactly double the horizontal and vertical values of the desired HiDPI resolution.
Here’s a table that will achieve common 16:9 resolutions:
|Desired 16:9 Resolution||2x SwitchResX Custom Resolution|
*Due to a feature of OS X 10.10 Yosemite, use the given value for a 1279×720 approximate resolution. You could also use 2562×1440 for 1281×1440, but 1280×720 doesn’t work.
With a 2560×1440 display, 1280×720 yields the truest HiDPI result because it’s a 4:1 pixel mapping. It looks great on a 25″ WQHD display. The in-between resolutions might yield acceptable results, but for me there were some slight yet noticeable artifacts that disappeared at 1280×720.