Hold on a moment
Shedding some darkness on sample-and-hold displays.
By The Sony Tech Guy | July 27, 2011
This is a beautiful picture.
Ironic but true: CRT phosphors flash briefly and then go dark for most of the frame duration. Film projectors also go dark when the film gets pulled down to the next frame. Isn't the whole point of these displays to produce light? After more than a century of flickering images, the present generation of monitors is built on sample-and-hold technology, such as LCD and OLED. Today's monitors can show pictures that are absolutely unblinking. Is constant illumination display nirvana? Actually, no. The purpose of video displays is not simply to produce light. It's to trick the human visual system into perceiving a sequence of still images as continuous motion. And that's a good reason to embrace the darkness.
Vision in the real world
To appreciate how darkness works, consider natural vision. In life, we see motion as a smooth, continuous flow. If we could chart the progress of a moving object, with time on the horizontal axis and the object's position on the vertical axis, we'd see a smooth line (for constant velocity) or a smooth curve (for acceleration or deceleration).
As we track object motion in the real world, it generally appears smooth.
If we configure our new, sample-and-hold displays for a continuous, unblinking presentation, we actually create a new problem. As our visual system attempts to track moving objects based on a lifetime of experience with natural scenes, it expects continuous motion but the display shows step-wise motion.
When a sample-and-hold display generates an unblinking image, the motion is portrayed in discrete steps.
During the course of each step, the object's fixed screen location hardly ever matches the constantly moving location our brain expects to see. This difference between expected and actual locations confuses the human visual system. We perceive the result as motion blur.
The human visual system is tripped up by the differences between the expected and actual locations of objects on the screen. These differences, which we experience as motion blur, are shown as shaded areas in the diagram.
There are other types of motion blur. For example, we can create motion blur in the camera, where it may well be the deliberate choice of the director of photography. We can get motion blur in the display if the pixels are slow to transition from one frame to the next. But this specific motion blur is the result of the human visual system trying to make sense of a highly artificial presentation.
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