windows 8 - How to design Metro UIs with fonts that look good on any resolution?

Question

When one looks at the Guidelines for fonts, we see that fonts are specified in points. A point is 1/72 of an inch so it is an absolute measure: a 10 points character should show at the exact same absolute size on any monitor at any resolution. That would make sense to me as I want to be able to read text -at the same size- whether on a 10 in tablet or a 23 in monitor. In other words, I want my text to be readable on a tablet, but I do not want it to be too big on a monitor.

On the other hand, I can understand that some UI elements could be specified in pixels, as in the Page layout guidelines.

However, in XAML font size is specified in pixels which is device dependent (to my understanding). Hence the fonts size will look tiny on a monitor with a higher resolution! See this post for more details. The answer in that post says "this way, you are getting a consistent font size". I can't see how I am getting a consistent size when it changes when the resolution changes?!?

Should I load different font size programmatically depending on the resolution of the device? I see here that Windows does some scaling adjustment depending on the DPI. Will this adjustment be enough for my users to have a great experience on a tablet and on, say, a 20 inch monitor (or should I programmatically change the font size depending on the device resolution)?

Bonus question: WHY are the Font Guidelines written using points when the software tools do not use points (like, what were they thinking)?

Answer 1

"What were they thinking" is extensively covered in this blog post.

You'll also see described how scaling for pixel density is automatic:

For those who buy these higher pixel-density screens, we want to ensure that their apps, text, and images will look both beautiful and usable on these devices. Early on, we explored continuous scaling to the pixel density, which would maintain the size of an object in inches, but we found that most apps use bitmap images, which could look blurry when scaled up or down to an unpredictable size. Instead, Windows 8 uses predictable scale percentages to ensure that Windows will look great on these devices. There are three scale percentages in Windows 8:

100% when no scaling is applied
140% for HD tablets
180% for quad-XGA tablets

The percentages are optimized for real devices in the ecosystem. 140% and 180% may seem like odd scale percentage choices, but they make sense when you think about how this will work on real hardware.

For example, the 140% scale is optimized for 1920x1080 HD tablets, which has 140% of the baseline tablet resolution of 1366x768. These optimized scale factors maintain consistent layouts between the baseline tablet and the HD tablet, because the effective resolution is the same on the two devices. Each scale percentage was chosen to ensure that a layout that was designed for 100% 1366x768 tablets has content that is the same physical size and the same layout on 140% HD tablets or 180% quad-XGA tablets.