Sunday, October 26, 2014

The (brief) Return of Tiled Displays

I've probably told the beginning of this story hundreds of times: in the late 1990's, a team of researchers led by Prof. Bob Massof at Johns Hopkins University, set out to achieve a seemingly-impossible task: to design a head-mounted display that was had both super-wide field of view as well as super-high resolutions.

The state-of-the-art display at that time was the eMagin SVGA OLED (800x600 resolution) that was just coming into production. One of the most popular mobile phones at the time was the Nokia 5110 which had an 84x47 black and white LCD display. The first HDTV broadcast in the US was in 1996 and displays were very large and heavy.

The solution was to use a tiled approach: bring together many small micro-displays, arrange them in a circular fashion so that they all point at the center of rotation of the eye, put an array of small "magnifying glasses", one magnifier in front of each display, and carefully align them to produce a nearly seamless image. The lenses were needed both to allow the eye to focus on an object so close to the eye as well as to eliminate the physical edges and borders between adjacent displays, magnifying each display so as to create an optical overlap. Keep in mind this optical overlap - it is going to be important later in this post.

See the images below for an illustration and a photo of an actual tiled module.

Concept of tiled HMD

Display module with 6 tiled SVGA displays

The performance was quite amazing. The original model had 16 800x600 displays per eye (4 rows of 4 displays each), so about 7.7M pixels. Commercial models of this, the piSight and xSight would go on to deliver >150 degrees field of view and 6 million pixels per eye. The image overlap between adjacent screens was about 30%.

There were a few downsides to this design: careful calibration was required to get both the geometry and colors nicely aligned across displays; a lot of computing or FPGA power was needed to reformat an image so that it displays inside the HMD because each display shows a somewhat different perspective of the 3D scene and because images in adjacent screens had content overlap. The result: some people loved it, some people not so much, and these products have generally been superseded by newer products that use either high-resolution microdisplays or high-resolution smartphone displays, both offering about 2 million pixels or more per eye using a single display.

A couple of months ago, I was invited to speak on a panel at the Siggraph 2014 show in Vancouver. While at Siggraph, I had the opportunity to visit some of the talks and technical exhibits and was delighted to see tiled display technology being used again.

The first example was a demonstration called Hapto MIRAGE from Tachilab in Keio University, See their demo video here

Hapto MIRAGE used three active-shutter LCD displays arranged so that their optical axis were intersecting. Large Fresnel lenses were placed in front of each display to magnify the image and take care of removing the seams. This was far from a wearable unit, but the concept seems identical to the original Johns Hopkins University work.

Another example was actually from 2013, though it was discussed again in an HMD session: the near-eye light field displays from NVIDIA:

In this demo, a small OLED was used and an array of very small lenses was placed in front of it. Small lenses allow for very small focal length, which makes it easy to place near the eye in a very lightweight package. The demo is very compelling but there is a catch: the overlap between the images displayed in each micro-lens is very large - approx 80% if I remember correctly. This means that the effective resolution of the display is reduced dramatically because so many pixels are overlapping. Thus, one might start with a 1280x720p micro display but end up with effective resolution of about 320x240, thus somewhat limiting the practical use of this particular configuration.

Are tiled displays making a comeback? Probably not, given high-resolution smartphone displays (including curved displays). But, it was certainly fun to see other implementations of the same fundamental principles developed at Johns Hopkins some 15 years ago.

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