CS488. Vision and Light. Luc RENAMBOT

Transcription

1 CS488 Vision and Light Luc RENAMBOT 1

2 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

3 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

4 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

5 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

6 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

7 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

8 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

9 Rods The rods are highly concentrated 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

10 Optic Nerve There is also the optic nerve which is 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

11 Blind Spot 11

12 Test it! 12

13 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

14 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

15 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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20 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

21 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 to of the maximum intensity Ration of maximum to minimum intensity is the dynamic range 21

22 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 I o ) 1 n For 10 intensities from 0.1 to 1 r = and the 10 intensities are: whereas a linear scale would be: I j = (I o ) n j n

23 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 so with 1/Io as the dynamic range, w set r = 1.01 and want to find n: 1.01 = ( 1 I o ) 1 n 23

24 Dynamic Range Through the magic of logarithms, we can find the following For a CRT with dynamic range of 50 to intensities are needed For a photographic print with range intensities are needed For a photographic slide with range intensities are needed 24

25 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

26 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

27 Chromatic Color 27

28 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

29 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

30 C.I.E Diagram Unfortunately you can t mix positive amounts of the three primary colors (red, green, and blue) to create all of the possible colors that 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, and blue and the color matching functions (the curves) of X, Y, and Z are not the curves of red, green, and blue. For a thorough explanation of the x,y axis, you should see section (p579) Y G B R X 30

31 Y G W R B 31

32 Various Gamuts 32

33 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

34 Color Models Various models exist RGB CMY YIQ, YUV HSV (HSB) 34

35 RGB RGB has three primary colours Red Green Blue Each of which ranges from 0 to 1 Which are added together to form the final color Black is at the origin (0,0,0) White (1,1,1) (0,0,0) -> black (1,1,1) -> white (1,0,0) -> red (0,1,0) -> green (0,0,1) -> blue (1,1,0) -> yellow (1,0,1) -> magenta (0,1,1) -> aqua 35

36 Example 36

37 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

38 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

39 Color Conversion To convert between RGB and CMY C M = 1 1 R G Y 1 B R G = 1 1 C M B 1 Y 39

40 CMYK CMYK adds black as a fourth parameter to CMY K = min(c, M, Y) C = Ccmy - K M = Mcmy - K Y = Ycmy - K 40

41 YIQ Recoding of RGB for US television broadcasts Y = luminance, and is the component shown on black and white TVs I and Q represent the chrominance information Coding Y I Q = R G B

42 YUV etc Similar to YIQ NTSC now uses the YUV color space YPbPr color space used in analog component video and its digital child YCbCr used in digital video 42 YUV

43 HSV (HSB) Hue Saturation Value (Brightness) HSV etc HLS Hue, Lightness, Saturation There is a lot of research out there on how to effectively use color, and many, many examples of how not to use color 43

44 Color Blindness 8 percent of men 0.5 percent of women 44

45 Example Normal Protanopia Deuteranopia (Daltonism) Tritanopia 45

46 Appropriate Use of Color Color is a very powerful tool - and therefor a dangerous tool Aside from issues of color blindness and other physiological issues we looked at with the optical illusions, there is a lot of cultural baggage associated with color If color is important in the computer graphics that you are doing (e.g.. scientific visualizations) you need to learn about color or work with someone who knows about color Book: Interaction of Color by Josef Albers 46

47 2D Cues to Depth We determine depth through a combination of 2D and 3D cues, which is how many of the optical illusions at the beginning work, and how we can play 'Doom' and get a sense of 3D space and motion Some people do not have stereo vision so all of their depth cues are 2D, and they do very well with only these 2D cues 47

48 Depth Cues Overlap Apparent Size (size consistency) Differential Size Linear Perspective Motion Parallax Aerial Perspective Texture Shading and Lighting 48

49 Depth Cues Overlap: Closer objects cover parts of objects that are further away Apparent Size: As an object moves towards us it gets larger in our retina but we do not perceive it getting bigger. We understand that it isn't changing its size as it moves Differential Size: If we know that two objects are the same size and one appears to be smaller than the other then the smaller one is further away 49

50 Depth Cues Linear Perspective: Parallel lines converge to the vanishing point as they go off in the distance Motion Parallax: Objects that are closer will move 'more' or 'faster' than objects that are further away. Aerial Perspective: Fog/Smog/Dust/Dirt in the air make objects that are further away appear less distinct 50

51 Depth Cues Texture: You can only discern textures when an object is near, otherwise the surface appears uniform Shading and Lighting: for a lot of small reasons 51

52 Depth Cues The dominant cue at long distances is differential size The dominant cue at intermediate distances is motion parallax The dominant cue within 12" is stereo vision 52

53 Next Time... Visible-Surface Determination 53