Software tools for creating line drawings on a CRT

A software package that can be used for the generation of line drawings on CRTs is discussed. The pictures obtained are stored as sets of coordinates of line segments. Complex pictures consist of sets of subpictures. In this way, pictures, or parts of pictures, can be easily manipulated (e.g., rotated, translated, scaled) and can be displayed on different types of display systems.     

A CRT graphics system for experimental research

A graphics system to create, display, store, and print stimuli in a hardware-enhanced, microcomputer-driven CRT environment is described. This graphics system is compared with other computer-based systems, as well as with systems requiring the use of tachistoscopes.     

A CRT blanking circuit for video terminals

A simple and inexpensive modification for the ADDS 980 video terminal is described which permits the characters displayed on the CRT to be blanked and restored (under software control). This circuit is useful in applications, such as RT measurement, where precise definition of stimulus onset is important. The technique described is general enough to be applied to a variety of other video terminals.     

A note on luminance control in tachistoscopes

Procedures are described for measuring tachistoscopic luminance levels for brief light flashes and over longer intervals. Stability of luminance levels is discussed for both conditions.     

Straightness as a cue for luminance edge interpretation

In order to determine the reflectance of a surface, it is necessary to discount luminance changes produced by illumination variation, a process that requires the visual system to respond differently to luminance changes that are due to illumination and reflectance. It is known that various cues can be used in this process. By measuring the strength of lightness illusions, we find evidence that straightness is, used as a cue: When a boundary is straight rather than curved, it has a greater tendency to be discounted, as if it were an illumination edge. The strongest illusions occur when a boundary has high contrast and has multiple X-junctions that preserve a consistent contrast ratio.     

A vision research apparatus for broad luminance range displays

Lightness, the perceived gray shade of a surface, and the perception of self-luminous surfaces—that is, surfaces that appear to glow—have most often been studied with paper displays and computer-generated stimuli presented on CRT monitors. Although both methods are often effective, experiments that require a wide range of luminance values in the same display are often difficult to conduct with paper and computer displays alone. Also, color mode appearance is often an issue when surface color perception is the topic of research; CRT monitors are essentially light sources themselves and often appear in the luminous mode of color appearance. Here, we describe an apparatus in which the target is an undetected aperture whose luminance is adjustable. Whereas a typical CRT monitor offers a luminance range of about 100:1, much broader luminance ranges are possible with the described apparatus. Unlike a CRT monitor, the stimulus background will always appear in the surface mode of color perception, and the target(s) can appear as either surface colors or luminous colors. Apparatus modifications are possible, including the addition of a stereoscope or an embedded CRT for creating an adjustable region that is computer controlled.     

A vector graphic CRT display system

A vector graphic CRT display system for research in visual information processing is described. The vector graphic processor includes a 4K-byte display memory. The processor communicates with the controlling computer via a serial channel. The system is capable of exposure durations below 1 msec, and it allows on-line generation and modification of display files during an experiment. Brightness can be controlled separately for each vector of a picture. A graphic display editor in FORTRAN IV for interactive editing and debugging of display files has been developed.     

A vision research apparatus for broad luminance range displays.

Lightness, the perceived gray shade of a surface, and the perception of self-luminous surfaces--that is, surfaces that appear to glow--have most often been studied with paper displays and computer-generated stimuli presented on CRT monitors. Although both methods are often effective, experiments that require a wide range of luminance values in the same display are often difficult to conduct with paper and computer displays alone. Also, color mode appearance is often an issue when surface color perception is the topic of research; CRT monitors are essentially light sources themselves and often appear in the luminous mode of color appearance. Here, we describe an apparatus in which the target is an undetected aperture whose luminance is adjustable. Whereas a typical CRT monitor offers a luminance range of about 100:1, much broader luminance ranges are possible with the described apparatus. Unlike a CRT monitor, the stimulus background will always appear in the surface mode of color perception, and the target(s) can appear as either surface colors or luminous colors. Apparatus modifications are possible, including the addition of a stereoscope or an embedded CRT for creating an adjustable region that is computer controlled.     

Lightness constancy through a veiling luminance

Observers were asked to select samples from a Munsell chart to match the lightness of seven identified surfaces in an outdoor scene they were shown. A separate group that was given the same task but viewed the same scene covered with a veiling luminance equal in intensity to the highest luminance in the scene selected almost the same matches. The same lightness constancy results were obtained using an abstract laboratory display to rule out memory color. These results challenge ratio and contrast theories because a veiling luminance, by adding a constant luminance to every poing in the image, dramatically alters luminance ratios. Lightness constancy was not obtained, however, when these three-dimensional real-world-type displays were replaced by a flat, Mondrian-type display consisting of surface grays from white to black, whether or not colored regions were present in the display; lightness matches were consistent with ratio predictions both with and without the veil.     

A new effect of luminance gradient on achromatic simultaneous contrast

A new effect in the domain of achromatic simultaneous contrast has been observed. A middle gray region placed at the center of an area filled by a linear achromatic gradient from black (outer part) to white (inner part) is perceived as being much darker than an identical middle gray region surrounded by a reversed gradient. By using a matching task in two experiments, it has been shown that this phenomenon is much stronger than the classical achromatic simultaneous contrast effect. The new effect is interpreted in terms of thealbedo hypothesis.     

Threshold exposure duration as a function of luminance for visual recognition

Threshold exposure duration was measured at set luminances for recognition at successively higher criteria of 1, 2, 3, 4, and 5 correct letters in nonsense words. Two sizes of letters were used and triads of Landolt “Cs” were compared with letters. Tachistoscopic procedures were the typical ascending seriesin the method of limits and pre- and postexposure light adaptation. Obtained log t vs log I functions were two-part straight line curves both describable by the equation, log t + log I^k = log C, but with k and C abruptly changing at a certain I. Reciprocity (k = 1) held only for low Is and the low criterion; as criterion was raised at low Is k became less negative (about ?.60 at five correct) for both sizesand for “Cs.” At higher Is, k shifted from ?.25 to ?.40 with increasing criterion for large letters, but variedaround ?.45 at all criteria for small. The lower the criterion, the lower the / and t at which k and C abruptly changed for large letters, but only t was lower for small. Changes in t with acuity requirement change, and from one criterion level to the next, were not uniform over all Is. Results may reflect obscuring procedural factors or basic properties of visual recognition processes. Some interpretations in terms of procedural variables are offered. It appears more likely that visual recognition involves central processes for which time is needed in excess of that for summation of luminances for mere detection by sensory processes, as expressed in Bloch s’ reciprocity law.     

A method for simulating non-normal distributions

A method of introducing a controlled degree of skew and kurtosis for Monte Carlo studies was derived. The form of such a transformation on normal deviates [X N(0, 1)] isY =a +bX +cX ² +dX ³. Analytic and empirical validation of the method is demonstrated.This work was done while the author was at the University of Illinois at Champaign-Urbana.     

Effects of absolute luminance and luminance contrast on visual discrimination in low mesopic environments

Recent research revealed considerable decline in visual perception under low luminance conditions. However, systematic studies on how visual performance is affected by absolute luminance and luminance contrast under low mesopic conditions (<0.5 cd/m²) is lacking. We examined performance in a simple visual discrimination task under low mesopic luminance conditions in three experiments in which we systematically varied base luminance and luminance contrast between stimulus and background. We further manipulated eccentricity of the stimuli because of known rods and cones gradients along the retina. We identified a “deficiency window” for performance as measured by d’ when luminance was < 0.06 cd/m² and luminance contrast as measured by the luminance ratio between stimulus and background was below < 1.7. We further calculated performance-based luminance as well as contrast efficiency functions for reaction times (RTs). These power functions demonstrate the contrast asymptote needed to decrease RTs and how such a decrease can be achieved given various combinations of absolute luminance and luminance contrast manipulations. Increased eccentricity resulted in slower RTs indicative of longer scan distances. Our data provide initial insights to performance-based efficiency functions in low mesopic environments that are currently lacking and to the physical mechanisms being utilized for visual perception in these extreme environments.