27 December 2013

Windows 8.1 on high DPI

I’ve been working with Windows 8.1 on a Dell XPS 15 for about eight weeks now and I thought I’d share some of my experiences of working with display scaling as the Dell has a 3200x1800 display.

Being what Apple would term “Retina”, the display has a pixel density of almost 250 PPI, which is matched only by a handful of other Windows laptops at the moment. Until recently the limiting factor in this area has been that desktop operating systems have expected a display’s resolution to scale more or less linearly with its size meaning the pixels per inch didn’t change a great deal.

Using fonts for an example, 10 point text should be about 10/72 of an inch or 3.5mm high  (1 point = 1/72 inch). Windows, by default, renders 10 point text about 13 pixels high which, if you do the math, is assuming a PPI of 96. Some background on where this 96 comes from can be found in this MSDN article. In the case of printers – when you print 10 point text you will get text that is 3.5mm high regardless of the printer’s DPI. The higher the DPI the crisper the text will appear but the characters will be the same size. The same is not true for displays, however. This hasn’t been so much of a problem up until now because average pixel density has been between about 90 and 120 but now we’re nearer to 250 pixels per inch that same 10 point text is only about 1mm high which is essentially impossible to read.

Obviously with high DPI displays some solution to this and the reasonable scaling of other elements on screen is required so that we can have nice crisp visuals that aren’t comically tiny. The operating systems are finally catching up and in Windows 8.1 are some usable scaling options for a high DPI display but it’s fair to say that, yet again, Apple have led the charge in this department with their Retina options in OSX.

In Windows 8.1 the scalings are 100%, 125%, 150% and 200%. Set at 200% on the XPS 15, for example, this renders things like text at a size you would see on a 1600x900 display. The scaling happens differently depending on the application. For most classic desktop applications such as Chrome it simply does a crude resize – essentially rendering at 1600x900 and then blowing the image up so you get a lot of pixilation and rough edges. For “Metro” apps and some desktop apps the scaling factor is passed to the app which scales the sizes of the UI elements as appropriate but renders them using the full resolution of the display.

It’s reasonable but it’s far from perfect unfortunately as there are still a lot of visual elements which don’t scale quite right and every so often you encounter some custom rendered dialog that isn’t scaled at all and you have to break out the magnifier tool.

Another oddity, which may be exclusive to the drivers for the XPS 15, is that coming out of standby mode loses the scaling option. It switches back to 100% scaling and you have to switch to external display mode and back to force it to pick up the scaling again.

Hopefully things will improve with updates to applications and subsequent revisions of Windows.


Ryan Clarke said...

Thank you for the tip about switching to external display. I have the same issue with scaling set to 100% after coming from standby.

Nick said...

I've not had any scaling issues, I have the same laptop. I think I have mine set at about 125%. It's small in some places, but definitely still readable to me.

Nick said...

I've not had any scaling issues, I have the same laptop. I think I have mine set at about 125%. It's small in some places, but definitely still readable to me.