# Visual cues The human headway

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1 Visual cues The human headway Overlapping objects Quantized scenes Lo sposalizio della Vergine Raffaello Sanzio Pinacoteca di Brera Perspective geometry Depth from shading Multi-presence Depth from texture Height in the field of view 1

2 The photometric track Michelangelo 1528

3 Local rendering Optical geometry of the light-source/eye-receiver system Notation: n is the normal to the surface at the incidence point î corresponds to the incidence angle â = î is the reflectance angle ô is the mirrored emergence angle û is the phase angle ê is the emergence angle. ê ô â n û î 3

4 The Lambertian model n cos ˆ Φ= elsewhere π π ι ˆ ι î 4

5 The specular model n cos m oˆ Φ= elsewhere π π oˆ ô â î 5

6 The Phong model a is the amount of incident light diffused according to a Lambertian model (isotropic) independent from the receiver s position b is the amount of incident light specularly reflected by the object, which depends on the phase angle, and m being the exponential specular reflection coefficient c accounts for the background illumination Φ= acos ˆ ι + bcos m oˆ + c Bùi Tường Phong, Illumination for computer generated images, Commun. ACM 18 (6) (1975)

7 Wire frame

8 Local rendering

9 The mirrored emergence angle o r e u cos(e)=no cos(u)=oi cos (o) = 2 cos (i) cos (e) - cos (u)

10 Reflectance maps Let us take a reference system where the optical axis of the acquisition system (the receiver) coincides with the z axis. The surface described by the function z = f(x, y) has the normal vector: ( z/ x, z/ y, -1) t. Calling p= z/ x and q= z/ y there is a one-to-one correspondence between the plane p, q (called gradient plane) and the normal directions to the surface. The three angles î, û and ê may be computed with the following formulas: 1 + pps + qqs cos ˆ ι = 1+ p + q 1+ p + q s s ˆ cos e = p + q 2 2 cos uˆ = p + q 2 2 s s 10

11 Sphere (m=10, c=0) a=1.0 b=

12 Specular sphere (b=0.9, c=0.1) m=1 m=3 m=6 m=1000 m=100 m=10

13 Reflectance maps a) reflectivity map for a Lambertian case having both camera and light source coincident with the z-axis (0, 0, -1); b) the specular case having the specularity index m=3; c) an intermediate case with a = b = 0.5, m=3 and c = 0 13

14 Reflectance maps a) Reflectivity map for a Lambertian model; having the camera on the z-axis (0, 0, -1) and the light source positioned at (1, 1, -1). The isoluminance patterns are quadrics labelled with their corresponding ratios, the incident light source corresponds to the bisector of the first octant in 3D space. b) Reflectivity map for a specular model having the specularity index is m = 3. 14

15 Photometric analysis Overlapped isoluminance patterns, for the Lambertian model, with three different positions of the light source used [(1,1,-1), (0,0,-1), (-1, 1,-1)] for determining the attitude of an object s facet. Φ 1 =0,95 Φ 2 =0,3 Φ 2 =0,1 15

16 Global rendering: ray tracing The two basic schemes of forward and backward ray tracing, this is last is computationally efficient but and cannot properly model diffuse reflections and all other changes in light intensity due to non-specular reflections Light Light Image Plane Image Plane 16

17 Global rendering: ray tracing A ray sent from the eye to the scene through the image plane may intersect an object; in this case secondary rays are sent out and three different lighting models can be considered: transmission. The secondary ray is sent in the direction of refraction, following the Descartes law; reflection. The secondary ray is sent in the direction of reflection, and the Phong model is applied; shadowing. The secondary ray is sent toward a light source, if intercepted by an object the point is shadowed, if it reaches the source the ray is not considered anymore. Figure. A background pixel corresponding to the ray P 1 and a pixel representing an object (ray P 2 ). The secondary rays triggered by the primary ray P 2 according to the three models: transmission (T 1 and T 2 ), reflection (R 1, R 2 and R 3 ), and tentative shadowing (S 1, S 2 : true shadows, and S 3 ). Opaque object Light R 3 S 3 R 2 S 2 P 2 P 1 T 1 R 1 Opaque object S 1 T 2 Transparent object Image plane Opaque object 17

18 Reflections and transparencies Created by David Derman CISC 440

19 Global rendering: radiosity The radiosity method has been developed to model the diffuse-diffuse interactions so gaining a more realistic visualization of surfaces. The diffused surfaces scatter light in all directions (i.e. in Lambertian way). Thus a scene is divided into patches - small flat polygons. For each patch the goal is to measure energies emitted from and reflected to respectively. The radiosity of the patch i is given by: Bi = Ei + ρi BF j ij n j= 1 Where represents the energy emitted by patch i, the reflectivity parameter of patch i, and the energy reflected to patch i from the n patches j around it, depending on the form factors. The form factor represents the fraction of light that reaches patch i from patch j. It depends on the distance and orientation of the two patches. A scene with n patches, follows a system of n equations for which the solution yields the radiosity of each patch. A i da i N i ϕ i N j ϕ j r A j da j 19

20 Example

21 A comparison by Ledah Casburn

22 . Phong model No objects interactions No shadows

23 ray tracing is there a corner? is the window in depth? is the floor properly rendered?

24 .. radiosity there is a corner the window is in depth floor and walls are clearer and pink

25 Exemples

26 Award winning examples July-August 2000 Norbert Kern - trace 101 h 18 min (AA 0.1) Machine - 1,4 GHz Athlon C / 1 GB RAM

27 Award winning examples January-February 2002 Stephen M. Farrell - trace 2d 4h 43m 30s Machine GHz Thunderbird; 512 MB RAM

28 The betrothal of the Arnolfini Jan Van Eyck 1434, National Gallery, London

29 Examples

30 Intensity re-distribution and rendering An image histogram has in the abscissa the pixel grey (or color) value while the ordinate plots the number of occurrences of such value. Such histogram may be manipulated so as: to obtain a sharper image, to highlight some chromatic distributions, to correct over or under exposed images, to optimize the contrast distribution, etc.. The information conveyed by histogram is limited to the first order statistics, corresponding to the number of pixels having a given grey value, or color component. Working on the histogram, gross level repartitions can be extracted such as dark vs. bright parts dominant levels (e.g. local maxima) up to splitting the histogram into a conjunction of bounded repartitions. In order to distinguish automatically an object from its background a typical approach is: i) firstly produce the grey level histogram (this process applies on the RGB components as well); ii) find the best bi-gaussian fit to the histogram distribution; iii) compute the threshold T by minimizing the misplaced pixels (either background pixels in the foreground or vice-versa). The misplaced pixel are represented by the green (levels approximately between 25 and T) and magenta (levels between T and 40) values respectively 30

31 Original image Image: History of Saint Peter by Masaccio, around 1427, Church of the Holy Spirit, Florence Histogram

32 Extended dynamics To profit from the full grey level scale, note that very clear and dark pixels are absent, the pixel distribution may extended by stretching with the following formula: (, ) gij = 255 (, ) gij g max g g min min where g max, and g min correspond to the maximum and minimum grey levels respectively, and is the whole available grey level scale. 0 a b

33 Stretching of a range in grey scale 255 To enhance significant details in the image the whole dynamics can be concentrated on a given interval [a, b] of the grey scale; then a linear transformation inside the interval can be computed by: ( ) 0 if g i, j a gij (, ) a gij (, ) = 255 if a g( i, j) b b a 255 if g( i, j) b This process produces the maximal contrast on the interval [a,b] of the grey scale. Considering this chosen interval no specific information is particularly enhanced. 0 a b

34 Under-exposure To compensate or even create lighting effects as, for instance, underexposure, a non linear transformation of the grey levels can be performed by: (, ) = 255gij (, ) gij

35 Over-exposure To compensate or even create lighting effects as, for instance, overexposure, a non linear transformation of the grey levels can be performed by: (, ) gij = (, ) gij

36 Uniform distribution To obtain a uniform distribution of image contrast a technique known as equalization is employed. This technique consists in making the empiric grey level distribution as close as possible to a uniform distribution, in an adaptive way. The more uniform is a grey level distribution, the better contrasted is the associated image i.e. maximal entropy. g(i,j) h(s) s=0 g (i,j) = h(s) s=

37 Human eye color sensors: Types of Cones 37

38 Color representation C = G+ B M= B+ R Y = R+ G C + R = M + G = Y + B = White The models of color perception are firstly based on primary colors, Red, Green, Blue (RGB). Color-mixing works differently, depending whether a camera (receiver) or a TV set (emitter) is considered: this leads to the triplet of secondary colors Cyan, Magenta, Yellow (CMY) which forms a basis for pigments. The color triangle (originated from physiological experiments) represents the white color in the centre, and the locus of monochromatic colors on the border. The Hue (first chromatic variable) is given by the angle between a color p and the ray connecting the corner 700 nm. The Saturation, i.e. the ratio wp/wp, where p represents the pure color perceived for that given wavelength. The grey scale (Intensity) cannot be else than perpendicular to the triangle plane. This representation gave rise to a simple linear model and formulas to convert the R,G,B into the H,S,I one. 38

39 Color Qualities: Hue, Saturation and Brightness 39

40 R,G,B convertion into H,S,I to normalize the intensity, all variables are divided by the sum R+G+B, e.g. r = R/(R+G+B). The Intensity is measured by the distance to the black. possibly normalized for a given Imax equal to 1. The Hue, which is an angle, is given by its cosine: 1 ( R G) + ( R B) cos H = 2 ( R G) 2 + ( R B)( G B) 101 Magenta 100 Red 001 Blue 000 Black 011 Cyan 111 White 010 Green 110 Yellow r b g The Saturation is obtained by the Thales theorem: T : projection of W in the plane (r,g), whence T = (1/3, 1/3, 0) Q : projection of P on WT, whence Q = (1/3, 1/3, b) WP WQ WT QT 1 = = = = 3 b S WP' WT WT 1 3 R M H P Y B W C G r p p b W Q T g 40

41 Color distribution Examples of natural color images and their color distribution in RGB space (colourspace Software, available on: The color components are clearly visible in the pictures containing caps, case of monochromatic items; b) less apparent in a fine grained textured image of the baboon. 41

42 Spatial resolution 20 x x x x 300

43 Gray level resolution 2 colors 1 bit 4 colors 2 bit 16 colors 4 bit 256 colors 8 bit Dithering

44 Gray level dithering Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images.

45 Color level dithering

46 Color level resolution 256 colors 8 bit 16M colors 24 bit Dithering

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