Hue, Saturation, and Brightness (HSB) • What they mean in terms of intensity distribution curves? • Hue is specified by the dominant wavelength color in the intensitydistribution curve • Saturation is the purity of a color (absence of other wavelengths). • The pure spectral colors are the most saturated • Brightness refers to the sensation of overall intensity of a color Brightness Bright white Grey Black Hue Saturation Desaturatated orange=saturated Brown (same) orange + white Orange Blue Blue Hue, Saturation and Brightness (HSB): One way to use 3 numbers to specify a color instead of using an intensity-distribution curve • Color tree (e.g. Fig. 9.5 in book) • Photoshop: uses H, S and B hue saturation lightness • Moving up the tree increases the lightness of a color • Moving around a circle of given radius changes the hue of a color • Moving along a radius of a circle changes the saturation (vividness) of a color • These three coordinates can be described in terms of three numbers Red, green and blue (RGB): RGB is another way to use 3 numbers to specify a color instead of using an intensity-distribution curve or HSB • • In addition to using Hue, Saturation and Brightness (HSB); Many (but not all) colors can be described in terms of the relative intensities of a light mixture of a certain wavelength red, wavelength green and wavelength blue lights • • • • • 650-nm red 530-nm green 460-nm blue These are called the additive primaries The mixing of the additive primaries is called additive mixing • Additive mixing is usually done by mixing primary color lights with different intensities but there are other ways to be discussed later • Demonstrate with Physics 2000 http://www.colorado.edu/physics/ 2000/tv/colortv.html 530-nm green cyan yellow 650-nm red magenta 460-nm blue Complementary additive colors • Definition of complementary color (for additive mixtures): • • The complement of a color is a second color. When the second color is additively mixed to the first, the result is white. • • • • • Blue & yellow are complementary B + Y = W. Green & magenta are complementary G+M=W Cyan and red are complementary C+R=W Magenta is not a wavelength color— it is not in the rainbow There is at most one wavelength complementary color for each wavelength color (Fig 9.9) yellow green white cyan magenta blue red Additive mixing of colored light primaries Blue added to green = cyan. Green added to red = yellow. Red added to blue = magenta. Complementary colored lights (additive mixing) Blue (primary) and yellow. Green (primary) and magenta. Red (primary) and cyan. Chromaticity diagrams: Yet another way to represent colors by (3) numbers • The chromaticity diagram is in many ways similar to a color tree • • • A chromaticity diagram has a fixed brightness or lightness for all colors Wavelength colors are on the horseshoe rim but non-wavelength colors like magenta are on the flat part of the rim Inside are the less saturated colors, including white at the interior less saturated colors saturated wavelength colors saturated non-wavelength colors Using the chromaticity diagram to identify colors • • The numbers that we use to identify a color are its x-value and y-value inside the diagram and a z-value to indicate its brightness or lightness x and y specify the chromaticity of a color • • • • Example: Apple pickers are told around the country that certain apples are best picked when they are a certaim red (see black dot) Since the chromaticity diagram is a world standard the company can tell its employees to pick when the apples have chromaticity • x = 0.57 • y = 0.28 The "purest" white is at x = 0.33 and y = 0.33 Chromaticity diagram can be related to colors in Photoshop Using the chromaticity diagram to understand the result of additive mixing of colors • An additive mixture of two wavelength colors lies along the line joining them • Example: The colors seen by mixing 700 nm red and 500 nm green lie along the line shown • Where along the line is the color of the mixture? • Answer depends on the relative intensities of the 700 nm red and the 500 nm green. • Here is what you get when the green is much more intense than the red (a green) • Here is what you get when the red is much more intense than the green (a red) • Here is what you get when the red is slightly more intense than the green (a yellow) Note — this works for adding two colors in middle also! Using the chromaticity diagram to understand complementary colors • The complement to any wavelength color on the edge of the chromaticity diagram is obtained by drawing a straight line from that color through white to the other edge of the diagram • Example: The complement to 700 nm red is 490 nm cyan • Example: The complement to green is magenta - a nonwavelength color Using the chromaticity diagram to find the dominant hue of a color in the interior of the diagram • To find the dominant hue of the color indicated by the black dot • Draw st. line from white through the point to get dominant wavelength, and hence, hue (547 nm green) • Works because additive mixture of white with a fully-saturated (wavelength) color gives the desaturated color of the original point What is partitive mixing? • Partitive mixing is another kind of additive color mixing but not achieved by superimposing colored lights! • Instead, it works by putting small patches of colors next to each other. • From a distance these colors mix just as though they were colored lights superimposed on each other • Examples: • Seurat pointillism • Color TV and computer screens (Physics 2000) • Photoshop example A colored filter subtracts colors by absorption. = Incident white light Cyan filter subtracts red Yellow filter subtracts blue Only green gets through A colored filter subtracts certain colors by absorption and transmits the rest = Incident white light Magenta filter subtracts green Cyan filter subtracts red Only blue gets through A colored filter subtracts colors by absorption. = Incident white light Magenta filter subtracts green Yellow filter subtracts blue Only red gets through What is the effect of combining (sandwiching) different colored filters together? • Rules for combining the subtractive primaries, cyan, yellow and magenta: • White light passed through a cyan filter plus a magenta filter appears blue • White light passed through a yellow filter plus a magenta filter appears red • White light passed through a yellow filter plus a cyan filter appears green • Why? cyan yellow magenta Colored surfaces subtract certain colors by absorbing them, while reflecting others White in Magenta out Magenta surface absorbs (subtracts) green. White in Green out Green surface absorbs (subtracts) red and blue (magenta). Green light on a magenta surface appears colorless because green is absorbed Green in Magenta surface absorbs (subtracts) green. Magenta light on a green surface appears colorless because magenta is absorbed Magenta in Green surface absorbs (subtracts) red and blue (magenta). When looking at a colored object in a colored light source what is the resulting color? • Rule: Multiply the intensity-distribution of the light source by the reflectance of the colored object to get the intensity distribution of the the illuminated object • Example: Look at a magenta shirt in reflected light from a Cool White fluorescent tube. • It appears grey (colorless) Confirm by multiplying the intensity distribution curve by the reflectance curve to get the new intensity distribution curve for the reflected light Cool white fluorescent bulb This number times Magenta shirt this number How the shirt appears in this light equals this number You multiply the two y-values at each x to get the new curve Halftone • Left: Halftone dots. • Right: How the human eye would see this sort of arrangement from a sufficient distance or when they are small. • Resolution: measured in lines per inch (lpi) or dots per inch (dpi); for example, Laser Printer (600dpi) Color halftoning Printer's ink Paper beneath Three examples of color halftoning with CMYK separations. From left to right: The cyan separation, the magenta separation, the yellow separation, the black separation, the combined halftone pattern and finally how the human eye would observe the combined halftone pattern from a sufficient distance. Demonstration Color Liquid Crystal Displays (LCDs) Chapter 10: We have three different kinds of cones whose responses are mainly at short, intermediate and long wavelengths • s-cones absorb short wavelength light best, with peak response at 450 nm (blue) • L-cones absorb long wavelength light best, with peak response at 580 nm (red) • i-cones absorb intermediate wavelengths best, with peak response at 540 nm (green) • Light at any wavelength in the visual spectrum from 400 to 700 nm will excite these 3 types of cones to a degree depending on the intensity at each wavelength. • Our perception of which color we are seeing (color sensation) is determined by how much S, i and L resonse occurs to light of a particular intensity distribution. Rule: To get the overall response of each type of cone, multiply the intensity of the light at each wavelength by the response of the cone at that wavelength and then add together all of the products for all of the wavenumbers in the intensity distribution L-cones i-cones s-cones Spectral response of cones in typical human eye Light color Brightness 460 nm blue 1 575 nm yellow 1.66 Mixture ( perceived as white) S-cone response 60 0 60 + 0 = 60 I-cone response 5 1.66 x 33 5+1.66 x 33 = 60 L-cone response 2 1.66 x 35 2+1.66 x 35 = 60 Examples of two different ways we see white • Our sensation of color depends on how much total s, i & L cone response occurs due to a light intensitydistribution • Multiply the intensity distribution curve by each response curve to determine how much total S, i, and L response occurs • We experience the sensation white when we have equal total s, i & L responses • There are many ways this can occur!! • E.g., when broadband light enters our eye • Another way to experience white is by viewing a mixture of blue and yellow • E.g., 460 nm blue of intensity 1 and 575 nm yellow of intensity 1.66 • The blue excites mainly s-cones but also a bit of i-cones and a bit of L-cones • The yellow excites i-cones and (slightly more) L-cones but no s-cones • The result is an equal response of s-cones, icones and L-cones (details) Spectral response of cones in typical human eye 1.66 1 0 460 nm blue of intensity 1 575 nm yellow of intensity 1.66 Light color Brightness S-cone response I-cone response L-cone response 530 nm green 1 negligible 41 How does a normal person see yellow when only 28red 650 nm red 2.15 negligible 2.15 x 2 2.15 x 9 and asgreen are superimposed? Mixture (perceived yellow ) lights negligible 41 +2.15 x 2 =45 28 +2.15 x 9 =47 575 nm yellow 1.35 negligible • Our sensation of yellow depends on a special s, i & L cone response • We experience the sensation yellow when 575 nm light reaches our eyes • What really gives us the sensation of yellow is the almost equal response of i and L cones together with no s-cones!! • Another way to experience yellow is by seeing overlapping red & green lights • E.g., 530 nm green of intensity 1 and 650 nm red of intensity 2.15 • The green excites mainly i-cones but also L-cones, while the red excites mainly Lcones but also i-cones • The total respone of s & i-cones due to the spectral green and red is the same as the total response due to spectral yellow • In general need 3 wavelength lights to mix to any color 1.35 response x 33 = 45of cones 1.35 x 35 = 47 Spectral in typical human eye 650 nm red 575 nm yellow of intensity of intensity 1.35 2.15 530 nm green 1 of intensity 1 2.15 0 We can verify color naming of hues in terms of the psychological primaries on the chromaticity diagram All of the hues can be named qualitatively by how much green, red, blue or yellow is "in" them • We don't need orange, purple or pink: • orange can be thought of as yellow-red • purple can be thought of as red-blue • pink has the same hue as red but differs only in lightness We can break up the diagram into 4 different regions by drawing two lines whose endpoints are the psychological primary hues • The endpoints of the yellow line are 580 nm "unique" yellow and 475 nm "unique" blue • One endpoint of the red line is 500 nm "unique" green and the other is "red" (not unique or spectral - really more like magenta) Greenness & yellowness Redness & yellowness What is meant by the opponent nature of red vs green (r-g) perception and of yellow vs blue (y-b) perception. • Viewing a progression of colors in the direction of the yellow line from 475 nm blue towards 580 nm yellow, we see more yellowness of each color and less blueness. • We call this perception our y-b channel • Yellow & blue are opponents • Moving parallel to the red line from 500 nm green towards nonspectral red we see more redness in each color and less greenness. • We call this perception our r-g channel • Red and green are opponents • The lines cross at white, where both y-b & r-g are neutralized Greenness & yellowness Redness & yellowness How might the three types of cones be "wired" to neural cells to account for our perception of hues in terms of two opponent pairs of psychological primaries r-g and y-b? • The 3 kinds of cones are related to r-g and y-b by the way they are connected to neural cells (such as ganglion cells) • Cones of each kind are attached to 3 different neural cells which control the two chromatic channels, y-b and r-g, and the white vs black channel called the achromatic channel (lightness) • "wiring" is the following: • When light falls on the L-cones they tell all 3 neural cells to increase the electrical signal they send to the brain • When light falls on the i-cones they tell the r-g channel cell to decrease (inhibit) its signal but tell the other cells to increase their signal • When light falls on the s-cones they tell the y-b channel cell to decrease (inhibit) its signal but tell the other cells to increse their signal s-cone ++ neural cell for y-b chromatic channel i-cone L-cone + + + ++ neural cell for r-g chromatic channel Electrical signal to brain neural cell for w-blk achromatic channel How can this "wiring" work to produce the chromatic channels? • The neural cell for the y-b chromatic channel has its signal s-cone i-cone L-cone • inhibited when (bluE) light excites the s-cone INTERPRETED AS BLUE • enhanced when light excites the i & L cones INTERPRETED AS YELLOW • The neural cell for the r-g chromatic channel has its signal • • inhibited when (green) light falls on the ++ + + i-cone INTERPRETED AS GREEN neural cell neural cell • enhanced when light excites the s and for y-b for r-g L cone chromatic chromatic INTERPRETED AS MAGENTA channel channel (Psychological red) The neural cell for the achromatic channel has its signal enhanced when light excites Electrical signal to brain any of the cones + ++ neural cell for w-blk achromatic channel Systematic description of color-blindness (no need to memorize terminology) • Monochromacy (can match any colored light with any 1 spectral light by adjusting • intensity) • Either has no cones (rod monochromat) or has only 1 of the 3 types of cones working (cone monochromat). • Sees ony whites, greys, blacks, no hues • Dichromacy (can match any colored light with 2 spectral lights of different intensities of (rather than the normal 3) • L-cone function lacking = protanopia • i-cone function lacking = deuteranopia • s-cone function lacking = tritanopia • no y-b channel but all 3 cones OK = tetartanopia Anomalous trichromacy (can match any colored light with 3 spectral lights of different intensities as in normal vision, but still have color perception problems) • Protanomaly • Shifted L-cone response curve • Deuteranomaly (most common) • Shifted i-cone response curve • Confusion between red and green. • Tritanomaly • Yellow-blue problems: probably defective s-cones • Neuteranomaly • ineffective r-g channel Receptive field of a double-opponent cell of the r-g type • 2 different ways to INCREASE the signal the ganglion cell sends to brain • Red light falling on cones in center of receptive field attached to ganglion cell • Green light on surround • 2 different ways to decrease the signal the ganglion cell sends to the brain • Red light on surround • Green light on center • Electrical signal to brain from ganglion cell is at ambient level when no light is on center or surround • When signal to brain is INCREASEDwe interpret that as red • When signal to brain is decreased we interpret that as green signal to brain We can summarize this by just showing the center & surround of the receptive field and indicating the effect of red (R) and green (G) on each • A double-opponent cell differs from a single opponent cell • In both of them R in the center increases the signal • In a single-opponent cell G in surround would inhibit signal, whereas in double-opponent cell G enhances • In a double-opponent cell • R in center enhances signal (ganglion cell signals red) • G in surround enhances signal (ganglion cell signals red) • R in surround inhibits signal (ganglion cell signals green) • G in center inhibits signal (ganglion cell signals green) Fictional cell real cell Here is an illustration of the effect of red or green light falling in various combinations on the center or surround of a double-opponent r-g cell Strongest signal (interpreted as red) Weakest signal (interpreted as green) Note, you would still "see" red if the center were grey! Note, you would still "see" green if the center were grey! No change in signal (color not noticed) No change in signal (color not noticed) y-b double-opponent receptive fields and cells work the same way Strongest signal (interpreted as yellow) Weakest signal (interpreted as blue) Note, you would still "see" yellow if the center were grey! Note, you would still "see" blue if the center were grey! No change in signal (color not noticed) No change in signal (color not noticed) b+yy+b- Here is an optical illusion which can be explained by double-opponent retinal fields and cells • Look at the grey squares in your peripheral vision • Does the grey square surrounded by yellow appear to take on a tint? • What color is it? • Repeat for the grey squares surrounded by • Blue • Green • Red (pink) Color constancy depends on doubleopponent processing • Color constancy means we see the proper colors of a picture or scene or object relatively correctly even though the overall illumination may change its color • This is because our double-opponent receptiive fields compare neighboring colors and are not very sensitive to an overall change in color • Color constancy developed in the evolution of mankind so that we could recognize colorful things in broad daylight, late afternoon, and early evening No change in signal (color not noticed) No change in signal (color not noticed) Illustration of how the three opponency channels work in your perception of the design below • Here are the enhanced edges resulting from your y-b chromatic channel • Note the edges that separate a yellowish from a bluish color are enhanced the most • Here are the enhanced edges resulting from your r-g chromatic channel • Note the edges that separate a reddish from a greenish color are enhanced the most • Here are the enhanced edges resulting from your wt-blk achromatic channel • Compare with the way a photocopy machine would see the design Chapter 13: What can a light wave do when it encounters matter? • Be TRANSMITTED laser aimed at water or glass • Be REFLECTED specular reflection of light by a mirror diffuse reflection of the light in this room off all the other students reflection is re-radiation of light by the electrons in the reflecting material • Be ABSORBED Cyan light shining on a red apple is absorbed by electrons in the apple • A light wave shining on molecules in the air or plastic or other “transparent” materials can be • SCATTERED Light ray moves over to the side in all directions rather than forward, backward or being absorbed. Intensity of the scattered light can depend on wavelength What is Rayleigh scattering? (or why is the sky blue) • The shorter the wavelength, the more light is scattered blue is scattered more than red. this is why the sky is blue and sunsets are red. (Fig. 13.1) Dust or smoke enhances red look of the sun by providing more scattering • Larger particles scatter red as well as blue and hence look white. Clouds; Milk; Colloidal suspensions Think of white light from sun as a mixture of R, G and B Blue is scattered the most so sky looks blue when we look away from the sun For same reason sun looks yellow (red + green) More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks red What is polarized light? • Light is polarized if the waveform and electric force field arrows remains in the same plane The (green) electric force arrows must always be perpendicular to the ray • This is a light ray traveling in the zdirection and polarized in the ydirection • Here is a light ray traveling in the same direction but polarized in the xdirection • We will visualize the polarization in the x-y plane, looking at rays head-on The green force arrows point up and down or left and right, stacked up behind one-another. Here is the convention for visualizing vertical and horizontal polarization Looking at ray "head-on" see green arrows up & down y y z z x x y x What is unpolarized light? • For unpolarized light the plane of polarization keeps jumping around But the electric force field arrows remain perpendicular to the ray (direction of travel of the wave) We visualize this in the x-y plane (looking into the ray) as shown at right y electric force arrows jump around while remaining perpendicular to the ray wave travels in z-direction z x • The many crossed double sided arrows are the symbol for unpolarized light • See Physics 2000 http://www.colorado.edu/physics/2000/index.pl y x When unpolarized light reflects off a horizontal surface (such as water or beach) near a special angle, the reflected light is polarized in the horizontal direction The special angle of incidence is where the refracted ray and reflected ray are perpendicular to each other This is called Brewster's angle To understand, imagine the electric force arrows of the incident unpolarized light to be decomposed into two perpendicular polarizations • the first polarization is horizontal (force arrows are parallel to the flat reflecting horizontal surface and perpendicular to the ray) • in the 2nd (Fig. 13.5), the arrows are perpendicular to both the ray and the horizontal force arrows – The second polarization cannot be sustained in the reflected ray because the force arrows would be parallel to that ray (impossible for a light ray) – Hence, only the horizontal polarization survives in the reflected ray Some material from Chapter #8 How do 3D movies use polaroid filters?