Displays are getting sharper all the time. Sharpness is very important, because things don’t just look better on high-resolution displays; they are measurably faster to use.
Low-resolution monitors (including all computer screens until now) have poor readability: people read about 25% slower from computer screens than from printed paper. Experimental 300 (PPI) displays (costing $30,000 (in 1998)) have been measured to have the same reading speed as print, so we will get better screens in the future.
The resolution of a display is measured in PPI, Pixels Per Inch, the number of pixels used to draw a one inch long, one pixel thick, line. A higher PPI means smaller pixels, and clearer images. You can calculate the PPI of your screen by
PPI ≅ sqrt(pixels_across^2 + pixels_down^2) / diagonal_inches
I am writing this article on an iMac with a 24″ screen, showing 1920 x 1200 pixels, so my PPI ≅ sqrt(1920*1920+1200*1200)/24 ≅ 93 PPI.
The resolution of a printer is measured in DPI, or Dots Per Inch. Unfortunately, DPI is often used as another term for PPI. That is technically inaccurate, but even Microsoft does it. SPI, Scans Per Inch, is another term that is used instead of PPI, but fortunately is correct when describing digital displays.
Bitmapped images are smaller on high-resolution displays, because the pixels the picture is made of are smaller. This can be a big problem for interface elements.
For example, today Apple’s interface guidelines say “The standard Help button is 20 pixels in diameter and should be placed at least 12 pixels from other interface elements.” On my iMac, a 20-pixel wide circle is about 0.21″ in diameter — roughly 1/3rd the diameter of a Dime. 
But on a 300 PPI display, it would be 0.07″ in diameter — roughly as thick as three pieces of mechanical pencil lead 
. As you can see, it’s way too small to comfortably click!
Just scaling bitmapped images, so a 20-pixel wide picture is always drawn 0.21″ wide, isn’t a good work around. It’s just emulating a low-definition, low-readability, display on a more expensive high-definition display. Another problem is that it looks terrible when everything else on the system is crystal clear.
It gets more ugly when the scaling-factor is higher, say for a display that matched the print fidelity of the tawdry magazines you see in the grocery check-out isle.
A resolution independent interface can be usably, and pleasantly, displayed on high-definition or standard-definition screens. There are two ways to build a resolution-independent display.
You can use vector images, which can be resized without problems. SVG and PDF are two popular formats. This sounds like an elegant “design once, show anywhere” solution. But in practice today building a vector image of the same quality as a bitmapped image can be very difficult, especially with complex images. This will change in time. Today designers create bitmap images by embellishing a vector image with bitmap-effects. As vector-imiging tools improve, they will be able to perform equivalent effects, but in a resolution-independent way.
Alternately, your app can carry-around a set of bitmapped images and draw the one that most-closly matches the current resolution. If you only have to support a handful of well-definied resolutions, this might be the best choice with today’s tools. The only caveat is that all signs point to vector-images being the future. Loading one vector image is also easier the selecting one of many bitmapped images to load. (And theoretically a vector image won’t have to be tweaked if we suddenly get one million PPI displays).
Already Solved
I think it’s important to note that Fonts have been facing an even more severe resolution-independence problem for decades, and have mostly solved it. The same font can be rendered well on a 1200 DPI printer, and a 72 PPI display. That’s a much bigger resolution disparity then a computer interface will ever have to deal with, because by the time a screen close to the resolution of a 1200 DPI printer is common, nobody will be using 72 PPI displays anymore (they aren’t even sold today).
Early fonts were bitmap fonts — a collection of bitmaps of what the font should look like at a particular scaling. But today vector fonts have replaced bitmapped fonts on the desktop, and even the iPhone. Vector fonts let us do amazing things with scaling (font specific stuff starts at 1:18). Vector fonts have been such an unqualified success, that it’s hard to imagine vector images not replacing bitmapped images for interface elements.
When Will It Happen
Year
Model
ResolutionSize DPI 1984
Original Macintosh512 x 342 9″ (8.5″) 72 1984 640 x 350 13″ (12.3″) 60 1994
Apple Multiple Scan 17 Display1024 x 768 17″ (16.1″) 80 2004 Apple Cinema HD display
2560 x 1600 30″ 100 I used the Tag studios Monitor DPI calculator to arrive at the DPI numbers in the above table. I couldn’t quite figure out what the actual displayable area of those early CRT monitors were, so I estimated about 5% non-displayable area based on the diagonal measurement.
Regardless, it’s sobering to consider that the resolution of computer displays has increased by less than a factor of two over the last twenty years. Sure, displays have gotten larger– much larger– but actual display resolution in terms of pixels per inch has only gone up by a factor of about 1.6.
I can’t think of any other piece of computer hardware that has improved so little since 1984.
It certainly doesn’t not look like we will get 300 PPI displays anytime soon! At the present rate, it will be decades before a $2000 display matches the clarity of a $20 printer.
However today’s smarphones and small-devices have much sharper screens then computers. The first generation iPhone is 160 PPI, the Kindle
UPDATE 2009-02-11: Wikipedia’s list of devices by PPI lists some very high PPI devices. The Sony Ericsson X1
phone has broken the 300 PPI “readability threshold”. The Fujitsu Lifebook U820 has a 270 PPI display. But the screen is only slightly bigger then the Osborne 1 “portable” computer of 1981. Still, if history is any indicator, once better technology is actually sold, it catches on very quickly.