Human Vision and Human-Computer Interaction Much content from Jeff Johnson, UI Wizards, Inc.
are these guidelines grounded in perceptual psychology and how can we apply them intelligently?
Mach bands: When looking at the Mach Bands it appears as if the color bands curve inward or that each band is a gradient, however, each band is a solid color. 6
receptive-fields (center-surround structure of excitatory and inhibitory photoreceptors) lateral inhibition 7
Hermann Grid: When looking at the Hermann Grid (named after Ludimar Hermann) you might ask: 1) Why are there gray spots located at the intersections between black squares? 2) Why do those spots disappear when you look directly at them? 8
9
Simultaneous Contrast 10
Visual perception
Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. It is a difficult problem from a computational perspective and difficult to explain in terms of neural processing. First-order" motion perception is thought to be performed by relatively simple "motion sensors" in the vision system that have evolved to detect a change in luminance at one point on the retina and correlate it with a change in luminance at a neighboring point on the retina after a short delay. Sensors that work this way have been referred to as Reichardt detectors or motion-energy sensors. These sensors detect motion by spatio-temporal correlation and are plausible models for how the vision system may detect motion.
The visual angle is the angle subtended by an object. If two objects have the same visual angle, they project onto the same retinal area. Your thumbnail at arm's reach is about 2 deg of visual angle.
Visual acuity: a measure of spatial resolving power; the 'angular size' of the smallest detail one can resolve. A person with normal vision - 20/20 (or 6/6 in meters), can resolve a spatial pattern in which each element of the pattern subtends an angle of one minute of arc at the retina (1/60th of a degree) when viewed at 20 feet, or 6 meters away. This represents the minimum angle of resolution (MAR). A person with normal vision can see an object with a height of 1.77mm at 6 meters (20 feet).
Display Viewing Distances Visual acuity represents the distance beyond which some of the picture detail will no longer be resolved by one's vision system, as it will appear to blend with adjacent picture information. The pixel should have a size that is not smaller than 0.0698-inch when viewed from 20 feet away. Translating visual acuity limitations in terms of minimum image size when viewing takes places from 20 feet away would give: HDTV minimum image size of 134-inch for 1080p HDTV (1920 x 1080 pixel image.) corresponding to a maximum TV viewing distance that is 1.8 times the screen width for 1080p HDTV.
you would not be able to enjoy the full resolution of 1080p HDTV if you were to sit further away than 1.8 times the TV screen width from your HDTV. If you were to increase the TV viewing distance to 2.7 times the screen width, the picture detail from a 1080p HDTV will appear just the same as that from a cheaper 720p HDTV of the same screen size. The '2-to-5' Principle for optimum TV viewing distance: the nearest seating position should be limited to two times the screen width; the furthest seating position should be at maximum, five times the screen width.
contrast sensitivity function for medium frequency you need less contrast than for high or low frequency to detect the sinusoidal fluctuation
Contrast sensitivity describes the ability of the vision system to distinguish bright and dim components of a static image. Visual acuity can be defined as the angle with which one can resolve two points as being separate, given that the image is shown with 100% contrast and is projected onto the fovea of the retina. Thus, when an optometrist or ophthalmologist assesses a patient s visual acuity using a Snellen chart, the target image is displayed at high contrast (e.g. black letters on a white background). A subsequent contrast sensitivity exam may demonstrate difficulty with decreased contrast (e.g. grey letters on a white background).
Theories of Visual Perception Constructive theories: the process of seeing is active; view of the world constructed from information in the environment and previously stored knowledge. Ecological theories: perception involves the process of acquiring information from the environment; doesn t require construction or elaboration.
Visual perception How long did it take to recognize the Dalmation? Only after you knew what you were looking for? After recognizing the Dalmation, what else could you see? Interpretation of the scene is possible because we know what Dalmations, trees, etc. look like -- active construction of the image.
Constructivist approach Perception involves intervention of representations and memories not like the image a camera would produce -- instead, a model that is transformed, enhanced, distorted, and portions discarded ability to perceive objects on a screen is a result of prior knowledge and expectations + image on retina
Effect of context on perception When presented with ambiguous stimuli, our knowledge of the world helps us to make sense of it -- same with ambiguous info on computer screen Constructive process also involves decomposing images into recognizable entities: figure and background
Gestalt psychologists have argued that the act of perception creates a Gestalt, a figure or form that is not a property of an object observed but represents the organization of sensations by the brain. This is thought to be important for the performance of simple, everyday visual tasks such as the recognition of an object that is partially occluded. How the brain is capable of filling in the missing pieces is an important topic; one that has most often been carried out through the use of illusory contours.
Gestalt laws of perceptual organization Proximity - dots appear as groups rather than a random cluster of elements Similarity - tendency for elements of same shape or color to be seen as belonging together Closure - missing parts of the figure are filled in to complete it, so that it appears as a whole circle Continuity - the stimulus appears to be made of two lines of dots, traversing each other, rather than a random set of dots Symmetry - regions bounded by symmetrical borders tend to perceived as coherent figures
Figure and Ground Figure similar elements Ground contrasting, dissimilar elements
Figure and Ground White horses Black horses? Escher art often plays with figure/ground
Camouflage Figure so similar to ground that it tends to disappear
Similarity Things that share visual characteristics like shape, size, color, texture, orientation seen as belonging together
Similarity Larger circles seen as belonging together
Proximity/Contiguity Things that are closer are seen as belonging together See vertical vs. horizontal lines See two groups of two
Continuity Tend to see figures as continuous
Closure Tend to see complete figures, even when part of info is missing
Closure What do you see?
Area The smaller of two overlapping figures is perceived as figure while larger is perceived as ground
Area Can reverse effect with shading
Symmetry Whole figure is perceived rather than individual parts What do you see?