Basics of Stimuli John H. Krantz Hanover College

Outline  The nature of the beast  Basic Perceptual Issues  Spatial  Temporal  Color  Luminance

The General Beast

Screen Mosaic Triad Arrangement

Screen Mosaic Color Strip Arrangement

Interlaced Projection

Progressive Projection

The Electron Beam

Seeing the Flicker If can’t get movie by clicking on the picture, click here.click here

Spatial Issues Spatial Inhomogeneity Aliasing Edge Effects and Contrast

Spatial Inhomogeneity  The same output value leads to different luminance outputs at different screen locations.  Thus, you get different outputs at different screen locations.  In general, draw to center if critical

CRT % Change from Minimum

How to Handle Spatial Inhomogeneity  Keep stimuli in central region

Aliasing  Technical definition:  When an image contains frequencies beyond the range of the sampling matrix, the wrap-around and occur as lower frequencies, distorting the image

Aliasing  Description of aliasing  Generally try to draw and image that has too fine of detail or sharp edges  Causes “jaggies”  And pixel artifacts  That is you can notice the pixels

Aliasing and Pixel Effects

Craik-Cornsweet

Craik-Cornsweet: Described The figure above is an exaggerated map indicating the light levels across the image on the previous slide. Note how the center and edges have identical luminance. That can be seen by sitting far enough away from the screen

Minimal Contours

Minimal Contours Described There are two circles below. Both circles have the same luminance (intensity level) at the center.Click on your mouse and This one changes abruptly watch as the edges are blurred to the level at the center.and the circle disappears.

Anti-aliasing  Solution to aliasing  Bandlimiting or anti-aliasing  Essentially blur the edges or image so fine detail is lost  In a lot of Software  Ideally use a gaussian filter  This is the shape of CRT electron beam which is why dots on screen never show up  Infante (1985); Silverstein et al. (1990)  Many graphics software and even some languages anti-alias (Java2D)

Temporal Issues Motion reproduction Flicker Stimulus Timing

Motion Reproduction  Frame rates of monitors far exceed what is needed for movement update  NTSC video: 30 updates per second  Movies: 24 updates per second  Most web video 10 to 15 frames/sec

Computer Video  Most Update rates are frames/sec 5 fps 10 fps 15 fps 5 fps 10 fps 15 fps Click on pictures to start each of the movies. If that does not work: click here for the left handclick here for the left hand movie, click here for the middle, and here for the right.click here for the middleand here for the right

Flicker  Critical threshold of flicker rates is about 60 Hz in the fovea  But gets higher for larger stimuli  Recommended flicker rates between 66 Hz and 120 Hz (Bridgeman, 1998)  Most monitors are adequate in this value  Notice difference between flicker and frame update rate.

Stimulus Duration  Pixels are not on continuously during a frame  In part this is necessary for clean motion  Typical CRT phosphors last about 4 msec. (Bridgeman, 1998)  On LCD and other technologies, persistence is longer  Makes motion less clean but flicker less noticeable

Differences in Persistence Click on pictures to start movies. If that does not work, click here for the crt click here for the crt and click here for the lcd.click here for the lcd

Determining Stimulus Duration  Possible errors:  Not knowing which frame the stimulus is first presented on  Assuming the stimulus is visible from the beginning of the display  Stimuli are not visible at beginning of frame but some time later  Delay depends upon where on frame stimulus is and frame rate  This is a constant error

Computer Video Architecture (Win95) Display Win32 Applications GDI Video OpenGL 3-D Games GDI DDI DCI DDI 3-D DDI

Effects on Timing  If no more than one frame delay:  Additional error of up to 17msec added to all conditions  No longer msec timing but really 60 Hz timing  If frame delay is larger effects are worse

Example Results from Myors (1999)

Luminance Issues Gamma

Gamma

Gamma Correction  Typically well described by: L=L 0 +a(V-V 0 )    is typically near 2.3; L, L 0, a,  are affected by contrast and brightness settings  Can’t be done on web  Make sure stimuli are not sensitive to distortions in gray scale  High contrast  Photo’s may be distorted slightly and impact results (Ruppertsberg et al., 2001)  Don’t require fine distinctions

Chromatic Issues Chromatic Issues The Trichromatic Theory The Trichromatic Theory Color Picker Color Picker The Color Gamut Glare Color Blindness

The CIE Color System  A set of Equations that allow predictions of matching.  Used in photo printing, TV and film.  Wyszecki & Stiles (1967)

The Color Gamut  The range of colors reproducible by any system

The Effect of Illuminance on Gamut In Dark With Lights on and 30% Luminance Level

Illuminance  Agostini & Bruno (1996)  Accuracy of the perceived stimulus is affected by the amount of illuminance.  D. F. Neri (1990)  Combination of light and gamut changed chromaticity  Request close windows and turn off unnecessary light  Do not use subtle color differences

Color Blindness Description Click on picture to start movie. If that does not work, click here.click here

Color Blindness Click on picture to start movie. If that does not work, click here.click here

Color Blindness Click on picture to start movie. If that does not work, click here.click here

Color Blindness Click on picture to start movie. If that does not work, click here.click here

How to handle  Screening questions?  Large sample and random assignment  Pre-test on color blind

Summary  Spatial  Aliasing  Edges  Temporal  Frame per second in video  Luminance  Gamma  Color  Glare  Color deficiencies