CS488 Vision and Light Luc R ENAMBOT 1
Outline • We talked about how to take 2D and 3D scenes and draw them on a 2D surface • We will be discussing how to make these images more interesting • Topics • light, illumination, • colour, shading, • occlusion 2
Plan • We are going to talk about light and colour, and the nature of human vision • In future weeks we will deal with the more sophisticated concepts of illumination and shading 3
Human Vision • Light is focused by the cornea and the lens onto the retina at the back of the eye • Vitreous humor - liquid inside the cornea is close to water, and has the same index of refraction as water. If we are under water the light is not refracted, but it is refracted if we are not in water 4
Reflection • Reflection is the change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated 5
Refraction • Refraction is the change in direction of a wave due to a change in its velocity • This is most commonly seen when a wave passes from one medium to another 6
Eye • Light passing through the center of the cornea and lens hits the fovea (or Macula). • Human eye has 2 types of photosensitive receptors • Cones • operate at higher illumination levels • provide better spacial resolution and contrast sensitivity • provide colour vision • Rods • operate at lower illumination levels 7
Cones • The cones are highly concentrated at the fovea and quickly taper off around the retina • For colour vision we have the greatest acuity at the fovea, or approximately at the center of out field of vision • Visual acuity drops off as we move away from the center of the field of view. However, we are very sensitive to motion on the periphery of our vision, so we can see movement even if we can't see what is moving 8
Rods • The rods are highly concentrated 10-20 degrees on both sides of the fovea, but almost none are at the fovea itself • Which is why if you are stargazing and want to see something dim you can not look directly at it 9
Optic Nerve • There is also the optic nerve which is 10-20 degrees away from the fovea which connects your eye to your brain • This is the blind spot where there are no cones and no rods. We can not see anything at this point though we are so used to this that we do not notice it unless we try to see the blind spot 10
Blind Spot 11
Test it ! http://serendip.brynmawr.edu/bb/blindspot1.html 12
Adaptation • What happens when we walk from a bright area into a dark area, say into a movie theatre? • When we are outside the rods are saturated from the brightness. The cones which operate better at high illumination levels provide all the stimulous • When we walk into the darkened theatre the cones don't have enough illumination to do much good, and the rods take time to desaturate before they can be useful in the new lower illumination environment 13
Sensitivity • It takes about 20 minutes for the rods to become very sensitive, so dark adjust for about 20 minutes before going stargazing • Since the cones do not operate well at low light intensities, we can not see colour in dim light as only the rods are capable of giving us information • The rods are also more sensitive to the blue end of the spectrum so it is especially hard to see red in the dark (it appears black) 14
Illusion • Computer graphics is based on illusion • From the R, G, and B dots of a LCD screens to creating an illusion of 3D on a flat screen 15
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Achromatic Light • No color, only grayscale with 0 as black, and 1 as white • Quantity of light or intensity of light is the only parameter • To human beings, brightness (perceived intensity) has a logarithmic scale, not a linear scale 20
Achromatic Light • LCDs can not get completely black due to backlight • CRTs can not get completely black due to light reflection within the tube • Minimum values range from 0.005 to 0.025 of the maximum intensity • Ration of maximum to minimum intensity is the dynamic range 21
Logarithmic Scale • For (n+1) intensities and a minimum intensity of Io and a maximum intensity of 1, the ratio of succeeding intensities is • r = ( 1 1 n − j ) I j = ( I o ) n n I o • For 10 intensities from 0.1 to 1 r = 1.292 and the 10 intensities are: 0.1 0.13 0.17 0.21 0.28 0.36 0.46 0.59 0.77 1.0 • whereas a linear scale would be: 0.1 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.0 22
Dynamic Range • The number of distinct intensities needed for smooth continuous tone depends on the dynamic range of the device. The human eye can not distinguish intensities with a ratio of less than 1.01. • so with 1/Io as the dynamic range, w set r = 1.01 and want to find n: 1 . 01 = ( 1 1 ) n I o 23
Dynamic Range • Through the magic of logarithms, we can find the following • For a CRT with dynamic range of 50 to 200 • 400-530 intensities are needed • For a photographic print with range 100 • 465 intensities are needed • For a photographic slide with range 1000 • 700 intensities are needed 24
Chromatic Color • Hue • distinguishes between colors • Saturation • how far is the color from a grey of equal intensity • vivid colors (bright red, royal blue) are highly saturated, further from grey • pastel colors (pink, sky blue) are lightly saturated, closer to grey • Lightness • perceived intensity of a reflecting object • Brightness • perceived intensity of a luminous object 25
Chromatic Color • Why do CRTs have red, green, and blue phosphors? • Currently believed there are three kinds of cones in the human eye, one attuned to red, one to green, and one to blue (Young and Helmholtz) • Light is electromagnetic energy with wavelengths from 400nm - 700nm • peak red response at 580nm (reddish-yellow) • peak green response at 545nm (greenish-yellow) • peak blue response at 440nm 26
Chromatic Color 27
Color Gamut • We can not generate all the colors that the eye can see using an RGB display • We also can not generate all the colors that the eye can see using photographic film (though it can display a larger part of the visible spectrum than a monitor) • Need for a standard 28
C.I.E Diagram • This is the CIE Chromaticity diagram developed in 1931 • If luminance is included this becomes a 3D cone, by normalizing we reduce the cone to a plane (the X + Y + Z = 1 plane) • Chromaticity depends on dominant wavelength (hue) and saturation only • As with homogeneous coordinates, different luminances are mapped onto the same point on the plane • So this plane DOES NOT contain all the possible colors as color also depends on luminance (lightness or brightness) 29
C.I.E Diagram • Y Unfortunately you can’t mix positive amounts of the three primary colors (red, green, and blue) to create all of the possible colors that G we can see • So for the diagram above there are three primary colors X, Y, and Z are defined so that you can add X and Y and Z together to get all of the possible colors • By definition X, Y, and Z are not be red, green, R and blue and the color matching functions (the curves) of X, Y, and Z are not the curves of X red, green, and blue. B • For a thorough explanation of the x,y axis, you should see section 13.2.2 (p579) 30
Y G W R B 31
Various Gamuts 32
C.I.E. Diagram • Given 2 points within the region the colors on the line between those 2 points are mixtures of those two colors. • Complementary colors exist on opposite sides of the white-light center, whose mixture yields the white-light center. • The triangle shows the colors we can make by adding R, G, and B • Different hardware has different triangles allowing us to compare the gamuts of different devices on the same diagram 33
Color Models • Various models exist • RGB • CMY • YIQ, YUV • HSV (HSB) 34
RGB • RGB has three primary (0,0,0) -> black colours Red Green Blue (1,1,1) -> white • Each of which ranges from (1,0,0) -> red 0 to 1 (0,1,0) -> green • Which are added together (0,0,1) -> blue to form the final color (1,1,0) -> yellow • Black is at the origin (0,0,0) (1,0,1) -> magenta (0,1,1) -> aqua • White (1,1,1) 35
Example 36
CMY • CMY has three primary colors Cyan Magenta Yellow each of which ranges from 0 to 1, which are subtracted from white to form the final color • cyan is white minus red, leaving only green and blue • magenta is white minus green, leaving only red and blue • yellow is white minus blue, leaving only red and green 37
CMY • White is at the origin (0,0,0) • (1,1,1) -> black • (0,0,0) -> white • (0,1,1) -> red (1,0,1) -> green (1,1,0) -> blue • (0,0,1) -> yellow (0,1,0) -> magenta (1,0,0) -> aqua 38
Color Conversion • To convert between RGB and CMY C R 1 = M G 1 − Y B 1 R C 1 = G M 1 − B Y 1 39
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