Science Visualization

Science Visualization Color & Visual Perception

Visual Perception • Human Visual System: • is very fast • processes large amounts of data efficiently (sensory channel with the highest bandwidth) • permits perception of • colors, • textures, • movements, • objects, and • relations between objects and groups of objects SciVis 2013 - page 2

Light • The light perception is a subjective process - the brain reacts to external light stimulus extremely individual (reaction depends on several factors). • Three key factors of human vision: • Light • The light interaction with object’s material changes visual object’s characteristics and human vision system receives these changes, analyzes visual data and extracts objects characteristics. • Therefore, in different lighting, the same objects are perceived in a different way – without light people can not see anything. • Object characteristics • Person SciVis 2013 - page 3

Light • The light perception is a subjective process - the brain reacts to external light stimulus extremely individual (reaction depends on several factors). • Three key factors of human vision: • Light • Object characteristics • Object have their own characteristics, which result from the objects’ material nature. • color (red, green, etc.), roughness, transparency, physical properties, etc. • These characteristics directly affect light (the light wavelengths). • Person SciVis 2013 - page 4

Light • The light perception is a subjective process - the brain reacts to external light stimulus extremely individual (reaction depends on several factors). • Three key factors of human vision: • Light • Object characteristics • Person • Combination of physical, physiological and psychological condition of the person leads to large differences in the light perception • different observers perceive different ‘images’. • at various times we have different perception of our surroundings. SciVis 2013 - page 5

Light • Visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. • Primary properties of visible light are: • Intensity • propagation direction • frequency or wavelength spectrum • polarization. SciVis 2013 - page 6

Light • Its basic physical characteristics are: • intensity • direction of propagation • wavelength • polarization. • The most important characteristics of light for human visual perception are: • brightness (the intensity) • color (it's related to the wavelength) • polarization. SciVis 2013 - page 7

Polarization • It is a property of waves that can oscillate with more than one orientation. • Electromagnetic waves, such as light, and gravitational waves exhibit polarization; sound waves in a gas or liquid do not have polarization because the medium vibrates only along the direction in which the waves are travelling. • By convention, the polarization of light is described by specifying the orientation of the wave's electric field at a point in space over one period of the oscillation. SciVis 2013 - page 8

Polarization SciVis 2013 - page 9

Polarization • When light travels in free space, in most cases it propagates as a transverse wave – the polarization is perpendicular to the wave's direction of travel. • In this case, the electric field may be oriented in a single direction (linear polarization), or it may rotate as the wave travels (circular or elliptical polarization). • In the latter case, the field may rotate in either direction. • The direction in which the field rotates is the wave's chirality or handedness. SciVis 2013 - page 11

Polarization • Linear polarization • The two orthogonal (perpendicular) components are in phase. • The ratio of the strengths of the two components is constant, so the direction of the electric vector (the vector sum of these two components) is constant. • The direction of this line depends on the relative amplitudes of the two components. SciVis 2013 - page 13

Polarization • Circular polarization • The two orthogonal components have exactly the same amplitude and are exactly ninety degrees out of phase. • In this case one component is zero when the other component is at maximum or minimum amplitude. • There are two possible phase relationships that satisfy this requirement: the X component can be 90° ahead of the Y component or it can be 90° behind the Y component. • In this special case the electric vector traces out a circle in the plane, so this special case is called circular polarization. • The direction the field rotates in depends on which of the two phase relationships exists. SciVis 2013 - page 14

Polarization • Elliptical polarization • The two components are not in phase and either do not have the same amplitude or are not ninety degrees out of phase, though their phase offset and their amplitude ratio are constant. • The electric vector traces out an ellipse in the plane (the polarization ellipse). SciVis 2013 - page 15

Human Vision • The Eye • The human eye is a complex non-linear detector of electromagnetic waves ranges from 380 nm to 750 nm • Most people cannot fully encompass this range and therefore the most visible spectrum is narrower (from 400 nm to 700 nm). SciVis 2013 - page 16

Human Vision • The Retina • The retina is a light-sensitive layer at the back of the eye that covers about 65 percent of its interior surface. • Photosensitive cells called rods and cones in the retina convert incident light energy into signals that are carried to the brain by the optic nerve. • In the middle of the retina is a small dimple called the fovea or fovea centralis. • It is the center of the eye's sharpest vision and the location of most color perception. SciVis 2013 - page 17

Human Vision • Rods and Cones • The rods are more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. • The 6 to 7 million cones provide the eye's color sensitivity and they are much more concentrated in the central yellow spot known as the macula. • In the center of that region is the " fovea centralis ", a 0.3 mm diameter rod-free area with very thin, densely packed cones. SciVis 2013 - page 18

Human Vision • Rods and Cones • The experimental evidence suggests that among the cones there are three different types of color reception. • Response curves for the three types of cones have been determined. • Since the perception of color depends on the firing of these three types of nerve cells, it follows that visible color can be mapped in terms of three numbers called tristimulus values. • Color perception has been successfully modeled in terms of tristimulus values and mapped on the CIE chromaticity diagram. SciVis 2013 - page 19

Human Vision • Rods and Cones • The differentiation between light and dark vision is caused by the activity of the rods and cones in the retina, and their sensitivity to light. • Scotopic vision (dark adapted) • It vision is primarily rod vision. • Photopic vision (light adapted) • It vision is primarily cone vision. • The curves represent the spectral luminous efficacy for human vision. SciVis 2013 - page 20

Human Vision • Cones • Current understanding is that the 6 to 7 million cones can be divided into "red" cones (64%), "green" cones (32%), and "blue" cones (2%) based on measured response curves. • They provide the eye's color sensitivity. • The green and red cones are concentrated in the fovea centralis . • The "blue" cones have the highest sensitivity and are mostly found outside the fovea, leading to some distinctions in the eye's blue perception. Sensitivity of color’s changes SciVis 2013 - page 21

Human Vision • "Blue" Cone • The "blue" cones are identified by the peak of their light response curve at about 445 nm. • They are unique among the cones in that they constitute only about 2% of the total number and are found outside the fovea centralis where the green and red cones are concentrated. • Although they are much more light sensitive than the green and red cones, it is not enough to overcome their disadvantage in numbers. • The blue sensitivity of our final visual perception is comparable to that of red and green, suggesting that there is a somewhat selective "blue amplifier" somewhere in the visual processing in the brain. SciVis 2013 - page 22

Human Vision • "Blue" Cone • The visual perception of intensely blue objects is less distinct than the perception of objects of red and green. This reduced acuity is attributed to two effects. • First, the blue cones are outside the fovea, where the close-packed cones give the greatest resolution. All of our most distinct vision comes from focusing the light on the fovea. • Second, the refractive index for blue light is enough different from red and green that when they are in focus, the blue is slightly out of focus (chromatic aberration). SciVis 2013 - page 23

Human Vision • Rod • They are the most numerous of the photoreceptors, some 120 million. • They are the more sensitive than the cones. • However, they are not sensitive to color. • They are responsible for our dark-adapted, or scotopic, vision. • The rods are incredibly efficient photoreceptors. • More than one thousand times as sensitive as the cones, they can reportedly be triggered by individual photons under optimal conditions. • The optimum dark-adapted vision is obtained only after a considerable period of darkness, say 30 minutes or longer, because the rod adaption process is much slower than that of the cones. SciVis 2013 - page 24

Human Vision • Rod • The rod sensitivity is shifted toward shorter wavelengths compared to daylight vision, accounting for the growing apparent brightness of green leaves in twilight. • While the visual acuity or visual resolution is much better with the cones, the rods are better motion sensors. • Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive, enabling you to see dimmer objects in your peripheral vision. • You can detect motion better with your peripheral vision, since it is primarily rod vision. • They do not see red – the light response of the rods peaks sharply in the blue; they respond very little to red light. SciVis 2013 - page 25

Human Vision • Rod • They do not see red – the light response of the rods peaks sharply in the blue; they respond very little to red light. SciVis 2013 - page 26

Human Vision • Cones • The cones are less sensitive to light than the rods • The daylight vision (cone vision) adapts much more rapidly to changing light levels, adjusting to a change like coming indoors out of sunlight in a few seconds. • Like all neurons, the cones fire to produce an electrical impulse on the nerve fiber and then must reset to fire again. • The light adaption is thought to occur by adjusting this reset time. • The cones are responsible for all high resolution vision. • The eye moves continually to keep the light from the object of interest falling on the fovea centralis where the bulk of the cones reside. SciVis 2013 - page 27

Color vision • It is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they reflect, emit, or transmit. • Colors can be measured and quantified in various ways; • A human's perception of colors is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with the several types of cone photoreceptors in the eye. • In essence, different people may see the same illuminated object or light source in different ways. SciVis 2013 - page 28

Color vision • Two theories of color vision • Trichromatic theory • It defines the way the retina of the eye allows the visual system to detect color with three types of cones. • Opponent process theory • It accounts for mechanisms that receive and process information from cones. • Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level. SciVis 2013 - page 29

Opponent process • It is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. • The three types of cones: L (long), M (medium) and S (short). • There are three opponent channels: red vs. green, blue vs. yellow, and black vs. white. • B vs. W is achromatic and detects light-dark variation, or luminance. • Responses to one color of an opponent channel are antagonistic to those to the other color. • Opposite opponent colors are never perceived together – there is no "greenish red" or "yellowish blue". SciVis 2013 - page 30

Color • With colors you can set a mood, attract attention, or make a statement. • You can use color to energize, or to cool down. • By selecting the right color scheme, you can create an ambiance of elegance, warmth or tranquility, or you can convey an image of playful youthfulness. • Color can be your most powerful design element if you learn to use it effectively. • Colors affect us in numerous ways, both mentally and physically. SciVis 2013 - page 31

Color • The three distinct attributes of color are: • Saturation - a fully saturated color is one with no mixture of white. • A spectral color consisting of only one wavelength is fully saturated. • Brightness – the brightness of a colored surface depends upon the illuminance and upon its reflectivity. • The perceived brightness is not linearly proportional to the reflectivity. • It is found that equal surfaces with differing spectral characteristics but which emit the same number of lumens will be perceived to be equally bright. • Hue – it is related to wavelength for spectral colors. • It is convenient to arrange the saturated hues around a Newton Color Circle. SciVis 2013 - page 32

Tints / Shades / Tones • These terms are often used incorrectly, although they describe fairly simple color concepts. • If a color is made lighter by adding white, the result is called a tint (c). • If black is added, the darker version is called a shade (b). • If gray is added, the result is a different tone (a). SciVis 2013 - page 33

Color • According to the number of available wavelengths and their amplitudes, colors are divided into three main types: • Monochromatic colors are all the colors (tints, tones, and shades) of a single hue. • Achromatic (neutral) color meaning literally a color "without color. • Polychromatic is used to describe light that exhibits more than one color, which also means that it contains radiation of more than one wavelength. SciVis 2013 - page 34

Color • According to the number of available wavelengths and their amplitudes, colors are divided into three main types: Monochromatic Achromatic Polychromatic SciVis 2013 - page 35

The Color Wheel • The color wheel (color circle) is the basic tool for combining colors. • The first circular color diagram was designed by Sir Isaac Newton in 1666. • Color harmonies (color chords) • Traditionally, there are a number of color combinations that are considered especially pleasing. • They consist of two or more colors with a fixed relation in the color wheel. SciVis 2013 - page 36

The Color Wheel • Primary Colors • The are sets of colors that can be combined to make a useful range of colors. • For human applications, three primary colors are usually used, since human color vision is trichromatic. RYB model SciVis 2013 - page 37

The Color Wheel • The secondary colors • They are created by mixing two primary colors. RYB model SciVis 2013 - page 38

The Color Wheel • Tertiary colors • They are created by mixing primary and secondary colors. RYB model SciVis 2013 - page 39

Warm / Cool Colors • White, black and gray are considered to be neutral. • The color circle can be divided into warm and cool colors. • Warm colors are vivid and energetic, and tend to advance in space. • Cool colors give an impression of calm, and create a soothing impression. SciVis 2013 - page 40

Color Harmonies • Complementary color scheme • Colors that are opposite each other on the color wheel are considered to be complementary colors. • The high contrast of complementary colors creates a vibrant look especially when used at full saturation. This color scheme must be managed well so it is not jarring. • Complementary color schemes are tricky to use in large doses, but work well when you want something to stand out. • Complementary colors are really bad for text. SciVis 2013 - page 41

Color Harmonies • Analogous color scheme • Analogous color schemes use colors that are next to each other on the color wheel. They usually match well and create serene and comfortable designs. • Analogous color schemes are often found in nature and are harmonious and pleasing to the eye. • Make sure you have enough contrast when choosing an analogous color scheme. • Choose one color to dominate, a second to support. The third color is used (along with black, white or gray) as an accent. SciVis 2013 - page 42

Color Harmonies • Triadic color scheme • A triadic color scheme uses colors that are evenly spaced around the color wheel. • Triadic color schemes tend to be quite vibrant, even if you use pale or unsaturated versions of your hues. • To use a triadic harmony successfully, the colors should be carefully balanced • Let one color dominate and use the two others for accent. SciVis 2013 - page 43

Color Harmonies • Split-Complementary color scheme • The split-complementary color scheme is a variation of the complementary color scheme. In addition to the base color, it uses the two colors adjacent to its complement. • This color scheme has the same strong visual contrast as the complementary color scheme, but has less tension. • The split-complimentary color scheme is often a good choice for beginners, because it is difficult to mess up. SciVis 2013 - page 44

Color Harmonies • Rectangle (tetradic) color scheme • The rectangle or tetradic color scheme uses four colors arranged into two complementary pairs. • This rich color scheme offers plenty of possibilities for variation. • Tetradic color schemes works best if you let one color be dominant. • You should also pay attention to the balance between warm and cool colors in your design. SciVis 2013 - page 45

Color Harmonies • Square color scheme • The square color scheme is similar to the rectangle, but with all four colors spaced evenly around the color circle. • Square color schemes works best if you let one color be dominant. • You should also pay attention to the balance between warm and cool colors in your design. SciVis 2013 - page 46

Color models SciVis 2013 - page 47

CIE chromaticity diagram SciVis 2013 - page 48

CIE chromaticity diagram • CIE 1931 SciVis 2013 - page 49

CIE chromaticity diagram • CIE 1964 SciVis 2013 - page 50

Science Visualization