PIXEL INDEPENDENCE: MEASURING SPATIAL INTERACTIONS ON A CRT DISPLAY
Denis G. Pelli
Department of Psychology
New York University
6 Washington Place
New York, NY 10003
denis@psych.nyu.edu
February 1, 1996
Submitted for publication in Spatial Vision.
The standard working assumption of imaging on a CRT is that each pixel is imaged independently, through a point nonlinearity called the monitor's gamma function (relating screen luminance to input voltage), and then blurred by the point spread function of the beam spot on the phosphor. Thus the pixels may overlap (because of the blur), but are independent. However, this assumption is usually false. The independence assumption is important because it underlies all attempts to linearize CRTs by gamma correction. Trying to correct for a nonlinearity that affects multiple pixels would be difficult. However, one can avoid these effects by buying a monitor that minimizes them, or by careful stimulus design (see Pelli & Zhang, 1991). The rest of this note discusses the three main reasons for failure of pixel independence, and describes two practical tests to assess the degree of failure. We do not claim any originality in our analysis or suggestions, but are not aware of any published treatments of this important practical issue in working with CRTs.
To assess a monitor's pixel-independence: 1. compare the mean luminance of a full-contrast grating (alternating white & black pixels) oriented vertically (which critically tests the video bandwidth) vs horizontally (which doesn't). 2. measure the percent change of a gray spot in the middle of the screen when the rest of the screen changes from white to black.
There are three reasons for failure of the independence assumption: insufficient video bandwith, imperfect dc restoration, and inadequate high-voltage regulation. Some manufacturers (e.g. Vision Research Graphics, http://www.vrg.com/ ) are making research-grade video monitors that should have independent pixels, i.e. they have dc coupling (instead of dc restoration), high video bandwidth, and good high voltage regulation and capacity.
Insufficient video bandwidth has the effect of low-pass filtering the incoming video signal. This is undesirable because this voltage signal is nonlinearly related to luminance, by the monitor's gamma function. Alternating black and white pixels are the most critical test. Comparing the luminance of vertical and horizontal gratings is a good way to measure this effect.
Inadequate high-voltage regulation is a common cause for failure of pixel independence. Setting a large fraction of the screen to maximum luminance (i.e. white) may pull enough beam current to run down the high voltage (nominally 15 kiloVolts). It may take several milliseconds to recover, and the beam will be dimmer until the voltage recovers. The high voltage will usually recover during the 1 ms or so blanking interval between frames, so this effect will be minimal at the top of the display (drawn first) and maximal at the bottom (drawn last).
Unfortunately most video displays are not DC coupled. Instead they are AC coupled for most of the time, and momentarily DC coupled to make zero volts produce black at the end of the synch interval. This is called "DC restoration". If the AC time constant were much longer than a frame, the DC restoration would be equivalent to DC coupling, but in practice the AC time constant is typically short relative to the length of the frame, so that the same input voltage will produce different screen luminances depending on what the average voltage has been since the last blanking interval.
Both of these effects will depend on an average (current or voltage) since the last blanking. After a big white patch, high voltage droop will make the bottom of the screen dimmer, while the AC coupling of DC restoration will make it brighter. We suspect, but have not confirmed, that the manufacturers of color monitors take more trouble to minimize these effects because hue errors are more noticeable than luminance errors.
One can measure the combined effect of both defects by measuring the luminance of a vertical gray strip (e.g. 5 cm wide) running down the middle of the display while alternately displaying white and black on the rest of the screen.
Luminance measurements are usually done with a photometer, but it might be useful to create software that accurately measured these pixel-independence failures by allowing the observer to null the luminance errors visually. For example, one might alternate the two conditions at 4 Hz, producing flicker, and allow the observer to control a luminance offset that is applied to one of the two conditions, to null the flicker. Care must be taken not to involve visual nonlinearities. When looking at the fine grating alternating between vertical and horizontal the observer should stand far enough way to be unable to resolve the gratings. When judging the artifactual flicker of the nominally steady gray strip in the middle of a white-black flickering field, the observer should occlude most of the display, exposing only a 2 cm patch of the gray strip.
REFERENCES
Pelli, D. G. and Zhang, L. (1991) Accurate control of contrast on microcomputer displays. Vision Research, 31, 1337-1350.
