Copyright © 1997-05-13 Charles Poynton
This paper explores how the temporal characteristics of image capture and image display devices interact with eye tracking.
A display system with a large pixel count can be exploited only by having a wide viewing angle, such as the 30 degrees of HDTV. Eye tracking - which is insignificant for conventional television - becomes significant in HDTV. A fast-moving element might take as little as two seconds to traverse the width of a screen; for 1920 samples per picture width, this corresponds to 16 pixels per field time at 60 Hz. Artifacts due to temporal effects at the camera, and artifacts due to temporal effects at the display, can be expected to be revealed in HDTV displays.
Many emerging displays have pixels that emit a constant amount of light throughout a large fraction of the frame time - they have long duty cycles. We have little experience of motion portrayal on displays having long duty cycles. Computers cannot yet display full-screen motion, and although conventional television can display smooth motion, it is restricted to narrow viewing angles. But it's clear that wide-angle, long duty cycle displays will introduce substantial blur on objects that the eye is tracking, and thus will have poor motion portrayal.
Many emerging display technologies, such as plasma display panels (PDPs) and digital-micromirror devices (DMDs) are intrinsically bilevel: At any instant in time, light is either emitted or not at any pixel. Apparent grayscale reproduction can be achieved by pulse-width modulation (PWM). The PWM technique works well when image content is static. But when PWM is combined with eye tracking in scenes with rapid motion, a new class of artifacts is introduced.
Digital technologists have long speculated about displays where each pixel is updated independently. It is assumed that if updating is at least as frequent as the arrival of new frames, the display will be free of artifacts. I will prove using several very simple examples that artifacts will be introduced unless the updating process is spatially coherent.
An HTML version of the paper is available.