6 Perceptual issues Thanks to Colin Ware, John Stasko, Robert - - PowerPoint PPT Presentation
Elective in Software and Services (Complementi di software e servizi per la societ dell'informazione) Section Inf nfor ormat ation V on Visual sualizat ation on Numbers of credit : 3 Gius usep eppe pe S Sant antucci 6
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Elective in Software and Services (Complementi di software e servizi per la società dell'informazione) Section Inf
Numbers of credit : 3
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to 700 nm (red)
eye as a pure color.
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A Incandescent light source D50 and D65 statistical representations of average day light B standard CIE definition of direct sunlight C standard CIE definition of average daylight
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visual system, and how light interacts with the object
– Refracted and then transmitted through it (if the object is transparent), – Absorbed by it (transforming in to heat) – Scattered inside the object if it is not completely transparent, because of the collision of photons with the molecules of the object (like sunlight scattering in the atmosphere) – Reflected
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– or is a camera like the eye?
Rods work only in low light Black and white Disabled with day light Cones work with high light Colors About 100.000 cones in the fovea 180 cones per degree
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able to focus both of them
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Contrast Spatial Freq.
This allows for producing monitors with high difference in luminance that is not perceived (center till 30 % brighter than borders)
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1 3 5 1 3 5 D i s t a n c e f r
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e a ( d e g . ) 1 8 6 4 2
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– But the iphone 4 with its Retina display?
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Cones in the fovea follow non regular patterns
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mitigate the problem in a cost-effective way than simply increasing the pixel number
colors
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– A 16x16 dots matrix can implement a pixel with 256 gray levels
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described with a Difference Of Gaussians (DOG) function
the center
border
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standardized the V(λ) function (averaging 200 people)
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luminous source
– E.g., nighttime instruments, stars, ship lights, etc.
technique is quite popular:
– Subject are experimentally asked to indicate when a perceived sensation is twice then a reference one – Most physical sensation follow a simple power low:
– S is the sensation, a is a constant and the stimulus intensity I is raised to a power n
Brighness = Luminance0.333
Brighness = Luminance0.5
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about 2.5
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from 1.4 to 3.0)
L=Vγ
Brightness = Luminance0.333 law resulting in a display characterized by a linear relationship between voltage and perceived brightness
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perceiving lightness (as well as colors) in a constant way:
– A black object in a sunny day reflects about 1000 candelas per square – The same object in an office light reflects 50 candelas per square
– A white paper in the same office reflects less light than the black
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rooms
% of light comes from the environment light and not from the monitor
adding a constant to the gamma equation: L=A+Vγ
relationship between Luminance and V is obtained with a lower gamma
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sensible to different wavelengths LMS (Long, Medium, Short)
generate colors using three primaries (e.g., RGB)
crazy in designing a monitor with 12 colored phosphors
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suffer from some color vision deficiency
receptors
systems usable by both trichromats and dichromats
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G + B + R
with a mixture of no more than three light (called primaries) C Ξ rR+gG+bB
equation refers to a perceptual match
distributions look the same (metamers) if they stimulate the three cones in the same way
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color space that includes the visible color gamut (in gray)
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difficult to understand they defined a transformation from X, Y, and Z to a new coordinate system x,y, and z that, under the constraint that x+y+z=1, i.e., normalizing with respect to the amount of light, can be represented in a two dimensional space
“pure”colors
point” give the color saturation
RGB space
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colors arranged in three pairs
– Black-White (luminance channel) – Red-Green – Yellow-Blue
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white black green yellow green blue brown pink purple
grey red yellow
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Rapid Visual Segm entation Color helps us to determ ine type
white black green yellow green blue brown pink purple
grey red yellow
Only about six categories
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12 Colors for labeling
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– No need to focus attention
– Eye movements = at least 200ms – Some processing can be done very quickly Implies low-level processing in parallel
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varies
regardless of the number of distractors
Preattentive
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– Orientation – Size / Length – Curvature – Spatial grouping
– Hue – Intensity
– Blinking – Direction of motion
– 2D position – Stereoscopic depth – Convex/concave for shading
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With conjunction encoding the red square is not pre-attentively identified.
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– Different using pre-attentive shapes
– Labeled with two colors
– Made different with pre-attentive enclosure
– Rendered through simple shapes and colors
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Interpretation of Bertin’s guidance regarding the suitability of various encoding methods to support common tasks
The marks are perceived as PROPORTIONAL to each other
Association Selection Order Quantity Size Value Texture Colour Orientation Shape
The marks can be perceived as SIMILAR The marks are perceived as DIFFERENT, forming families The marks are perceived as ORDERED
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The relative difficulty of assessing quantitative value as a function of encoding mechanism, as established by Cleveland and McGill Length Position Angle Slope Area Volume Colour Density Most accurate Least accurate
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Mackinlay’s guidance for the encoding of quantitative, ordinal and categorical data
Quantitative
Position Length Angle Slope Area Volume Density Shape
Ordinal
Position Density Colour saturatio Texture Connection Containment Length Angle Slope Area Volume Colour hue
Categorical
Position Colour hue Texture Connection Containment Density Colour saturatio Shape Length Angle Slope Area Volume
Treble Bass