MORPHONOME Image Psychophysics Software
and Calibrator for Macintosh Systems

Christopher W. Tyler and Brennan McBride
cwt@skivs.ski.org

Smith-Kettlewell Eye Research Institute
2232 Webster Street, San Francisco Ca 94115 USA

Introduction
The core function of noninvasive psychophysical testing is the
measurement of contrast sensitivity for spatial patterns specified with
some mathematical luminance profile and a controlled time course.
Desk-top computers would appear to be ideally suited for such a task
and yet most of the software for such computers requires special-
purpose hardware modifications and software that requires
programming to achieve the desired experimental configuration. In
particular, there seems to be no convenient software that take
advantage of the graphics sophistication of the Macintosh computer.
We have therefore developed a software package for
psychophysical testing of image detectability for Macintosh
computers that is menu driven for ease of use. The challenge in this
enterprise was to develop stimuli with sufficient resolution of fine
contrast variations to allow measurement of human contrast
sensitivity, which can be as low as a 0.2% difference between light
and dark regions of the stimulus. This resolution is not possible with
conventional stimulus profiles on a standard 3 x 8-bit Macintosh
display, whose smallest luminance step at mid-luminance is about
1.5% due to the accelerating phosphor nonlinearity.
A technique that allows increased luminance resolution on any
digital computer with a color monitor is the Rbit-stealingS procedure
introduced by Tyler et al. (1990), in which the color values within
each pixel are jittered to provide a wider range of luminance
combinations from which to chose. This approach amounts to
relaxing the accuracy of the chromatic signal to provide greater
accuracy on the luminance signal. Under a wide range of conditions,
sensitivity to chromatic contrast is lower than that for luminance
contrast. A trade of bit-resolution between these two modalities can
therefore allow increased luminance precision without detectable loss
of color fidelity under many conditions.
The bit-stealing algorithm requires calibration of the output
luminance levels to high accuracy for each of the 256 steps of the
look-up table for each color gun. We have therefore developed an
automated calibration system with Rplug-and-playS operation through
the ADB (Macintosh keyboard and mouse) bus that provides a full
calibration in about 30 minutes. This calibrator provides an exact
specification of every look-up table value without curve-fitting or
approximation formulas.

 

Features of the Basic Image Psychophysics Software.

The MorphonomeTM software package for image psychophysics
allows generation and presentation of a wide spectrum of 2-D grey
scale images for psychophysical testing of the limits of human
perception with the Macintosh II family of computers.

The images are generated under parametric control to permit a wide
variety of published and novel experiments in the detection, masking,
discrimination and perception of contrast variations.

Repetitive spatial patterns are based on sinusoidal, square and ramp
luminance profiles in linear, radial, spoke, 2D checkerboard and
hexagonal configurations. 2D Gaussian and 2D difference of
Gaussian profiles are also available. Spatial frequency, orientation
and contrast are freely selectable.

Patterns may be windowed with square, circular, disk/annulus,
Gaussian fade, derivative of Gaussian, and Gaussian ring envelopes.
The Gaussian-based envelopes have variable ellipticity and
orientation. Multiple patch combinations of these patterns are also
possible. Center/surround and split-field configurations between any
of the pattern varieties are available.

The luminance scale is fully linearizable to 99.9% output contrast
(with monitor calibration). A parametrically variable sample of
"gamma" functions is provide to allow perceptual equalization of the
luminance scale for specific applications that require such departures
from physical linearity.

Output contrast can be varied in increments of as little as 0.1%. An
invariant base contrast can be set to any level. Patterns may be
balanced or unidirectionally biased from the mean luminance.
Contrast and its modulation are controlled by a look-up table
animation, which may be split into two or four separate functions
(with a corresponding loss in the number of available luminance
levels for each independent image).

A variety of temporal presentation profiles is available, with
durations down to 15 msec and a full range of flicker rates.
Adjustment, yes/no or two-alternative forced-choice psychophysical
procedures are selectable for the measurement of threshold sensitivity
levels.

 

Recent Enhancements: Motion, Color, Noise

A prototype version of the software is now available with the ability
to present stimuli with the attributes of motion, color and visual
noise. Either test or mask stimuli may be in these forms, which may
be modulated by any of the window functions in a similar fashion to
the luminance stimuli. Full flexibility of the psychophysical
paradigms in these dimensions has not yet been accomplished,
however.

Moving stimuli are generated with a two-phase motion algorithm
suggested by Mulligan (personal communication). Two stimuli are
interlaced on the screen in the two fields of a 1-pixel checkerboard.
When the images consist of a sinusoidal pair in spatial-temporal
quadrature, a drifting modulation is generated within any of the
available contrast modulation envelopes. For a Gaussian envelope,
this procedure generates the Rstationary moving Gabor patchS in
widespread use for studies of motion perception.

Chromatic stimuli on either the red-green and yellow-blue axes are
selectable, with adjustment to equiluminance for the human observer.
Full control of stimulus chromaticity is, however, not yet available.

Stimulus modulations of one- and two-dimensional pixel noise, in
the form of either static (changing from trial-to-trial) or modulatable
dynamic noise are available for use as a masking stimulus or as a test
stimulus.

Technical Requirements.

The MorphonomeTM software was written in C and is compatible
with Systems 6.07 and 7. It runs on the Macintosh LC, II, Centris,
Performa, Quadra and PowerMac families of computers. A Floating
Point Coprocessor (FPC) is required in those Macs on which it is not a
standard component. (A non-FPC version of the software is also
available for those computers without one, although it may then run
too slowly for many applications except on the PowerMac.)
The bit-stealing display software requires a color monitor and
24 bit graphics card for its operation. Any size of Macintosh monitor
is supported. The program will run with monochrome monitors, but
the enhanced gray-scale resolution provided by the bit-stealing will
not operate. It also will run on reduced bit-range color monitors, such
as those in the color Powerbooks. (Since it relies on look-up table
animation, the program will not work with the "millions" mode
selected for the graphics card).
With System 7, operation of the software is subject to system
interrupts. It is important to ensure that no interruption events, such
as network access, can occur during psychophysical testing.
The application is normally assigned 2Mbyte of application
memory, which can be reduced if there are memory constraints or
increased for use with large stimulus sizes. Reducing memory will
constrain the number and size of stimuli attainable, but should not be
a problem on standard size monitors.

 

Features of the Smith-Kettlewell Macintosh LightMouseTM

We have developed a LightMouseTM Macintosh calibrator with
Rplug-and-playS operation at Smith-Kettlewell in conjunction with Dr.
Anthony M. Norcia, Steven Chung (hardware engineer) and Kirk
Swenson (software engineer).

A photocell with a CIE photopic filter is attached to the monitor
screen and sends a signal to the LightMouseTM A/D converter. The
digital signal is input through the ADB (keyboard and mouse) bus to
allow use with any of Macintosh II and PowerMac family of
computers.

The look-up table levels for each color gun are stepped through with
sufficient time for the monitor output to settle before each reading is
taken. The luminance of each gun is stepped first up and then down,
to average over any warm-up or drift effects.

The output, calibrated in candelas per meter2, is placed in a
calibration table ready for use by the MorphonomeTM software or any
other application designed to recognize it.

 

Conflict-of-Interest Statement. These products are distributed at
cost on a non-profit basis through the Smith-Kettlewell Eye Research
Institute for the benefit of the vision community.

 

Reference.
Tyler C.W., Chan H., Liu L., McBride B. & Kontsevich L.L. (1992) Bit-
stealing: How to get 1786 or more grey levels from an 8-bit color
monitor. Proc. SPIE 1666, 351-364.


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