Light effect

Light effect

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  1 Computer Graphics forScientists and Engineers Lighting in OpenGL
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  2 Graphics: Conceptual Model RealObject Real Object Human Eye Human Eye Display Device Graphics System Synthetic Model Synthetic Camera Real Light Synthetic Light Source
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  3 Lights, Surfaces, andImaging Proj. Plane Viewer Objects
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  4 Lights, Surfaces, andImaging Proj. Plane Viewer Light Source Objects
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  5 Modes of Interactionof Light with Materials Specular Surface Diffuse Surface Translucent Surface Note: Any given surface can have some of all three properties.
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  6 Illuminating Surfaces z x y I(θ,φ,λ) I(x, y,z,θ,φ,λ)Wecan define illumination function: To obtain total light, must integrate over total surface. Itotal (λ) = I(x,y,z,θ,φ,λ)∫∫∫ dxdydz
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  7 Simplified Model Exact modelcan easily get complicated! Three simplifications help. 1. We can consider four classes of light sources • ambient • point • spotlight • distant 2. Human color perception allows us to consider illumination function in terms of the three primary colors. 3. We can neglect (OpenGL caveats): • multiple reflections • obstruction of light path by objects
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  8 Light Sources Ambient light -no identifiable source or direction - hack for replacing true global illumination = (light bouncing off from other objects)
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  9 Ambient Light Simulates situationswhere light sources are designed to produce uniform lighting throughout a scene. Characterized by a scalar intensity, Ia , that is identical at every point in the scene. Ia = Iar Iag Iab         Although every surface receives the same illumination, each surface can reflect the light differently.
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  10 Point Sources An idealpoint source radiates equally in all directions. It can be characterized by: I(p0 ) = Ir (p0 ) Ig (p0 ) Ib (p0 )         Intensity of illumination from a point source is proportional to the inverse square of the distance between the point and the illuminated surface. I(p,p0 ) = 1 p−p0 2 I(p0 )
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  11 Light Sources Types oflight sources - glLightfv(GL_LIGHT0,GL_POSITION,light[]) - directional/parallel lights = real-life example: sun = infinitely far source: homogeneous co-ord. w=0 - point lights = same intensity in all directions - spot lights = limited set of directions: point+direction+cutoff angle             0 z y x             1 z y x
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  12 Spotlights Spotlights are pointsources for which the angle through which light has been emitted has been limited. θ l s I = k cose (θ) = k(l⋅s)e
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  13 Distant Sources Most shadingcalculations require the direction from the point on a surface to the light source. As light sources are moved to larger distances, this direction approaches a constant. Therefore the point source location will be replaced by a vector indicating the direction of the source. p0 = x y z 1             will be replaced by: p0 = x y z 0            
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  14 Reflection Rough Surface: Light isreflected equally in all directions. Diffuse Smooth Surface: Light is reflected at an angle near to the incident angle.Specular
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  15 Vectors Used by PhongLighting Model p is an arbitrary point on a surface. n is the unit normal to the surface at p. l is a unit vector in the direction of the light source. r is a unit vector in the direction of a perfectly (specular) reflection v is a unit vector in the direction of the viewer. n l r v p Viewer
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  16 Elements of thePhong Lighting Model Ri = Rira Riga Riba Rird Rigd Ribd Rirs Rigs Ribs           At each point p there is a reflection matrix for the ith light source: I = Ia + Id + Is = La Ra + Ld Rd + Ls Rs Assuming the calculations will be done separately for each primary, we can sum over light sources to get: Li = Lira Liga Liba Lird Ligd Libd Lirs Ligs Libs           At each point p there is an illumination matrix from the ith light source: red green blue ambient diffuse specular
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  17 Elements of thePhong Lighting Model Ambient Reflection Ia = ka La 0 ≤ ka ≤1 The intensity of ambient light is the same at every point on the surface. The percentage of light reflected is given by: Ra = ka Note that the ambient reflection terms can be different for red, green and blue.
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  18 Elements of thePhong Lighting Model Diffuse Reflection Intensity of illumination is dependent upon the incidence angle of light from the source: Adding a term for attenuation with distance from source: Id = k d a + bd + cd 2 ( l ⋅ n ) L d θ l n Rd ∝ cosθ = l⋅n Id = kd (l⋅n)Ld
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  19 Elements of thePhong Lighting Model Specular Reflection For a shiny surface, most light is reflected around vector r, corresponding to a reflected angle equal to the angle of incidence. If θ is the angle between v and r, Is = ks Ls cosα θ Adding a distance term, and expressing the cosine in terms of a dot product: Is = 1 a + bd + cd 2 k s L s (r ⋅ v )α n l r v p Viewer
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  20 The Complete LightingModel globalambglobalambm aassdd n i LkE LkLkLk cdbda I ++ +⋅+⋅ ++ = ∑ − = )])()(( 1 [ 2 1 0 α vrnl Distance (Attenuation) Ambient Diffuse Specular Emission Global Ambient n l r v p Viewer      () () () glMaterial glLight elglLightMod k L E
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  21 Calculation of Normals Normalexists at every point for mathematically defined smooth surfaces. Exists for each polygon in surface defined by flat polygons. What about the shared lines/points? Polygonal Case: p0 p1 p2 n n = (p1 − p0 ) × (p2 − p0 )
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  22 Calculation of Normals MathematicallyDefined Surface: Sphere Sphere is defined implicitly by equation: f ( x, y, z) = x2 + y2 + z2 − 1 = 0 n = ∂f ∂x ∂f ∂y ∂f ∂z               = 2x 2y 2z         = 2p
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  23 Specification of theNormal in OpenGL Specification of the current normal is modal and associated with vertices. glNormal3f(nx,ny,nz); glNormal3fv(pointer_to_normal_vector); You have to calculate the normals yourself.
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  24 Shading in OpenGL: FlatShading OpenGL uses the normal of the first vertex of a single polygon to determine the color. Requested by: glShadeModel(GL_FLAT); Flat shading exaggerates the visual effect of the boundaries between polygons due to the Mach band effect.
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  25 Mach Band Effect (Mach,1865) Color Science, Concepts and Methods Wyszecki, Stiles
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  26 Mach Band Effect
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  27 Mach Band Effect
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  28 Shading in OpenGL: SmoothShading Requested by: glShadeModel(GL_SMOOTH); Lighting calculations will be done at each vertex using the material properties, and the vectors v and l calculated for that vertex. Bilinear interpolation is used to determine color values in the interior of the polygon. Gouraud Shading: Normal at a vertex is the normalized average of the normals of the polygons that share that vertex.
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  29 Gouraud Shading
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  30 Lighting Implementation inOpenGL
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  31 Lighted Sphere Demoand Source Code
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  32 Enable Depth Buffer ForHidden Surface Removal //before enabling while (1) { get_viewing_point_from_mouse_position(); glClear(GL_COLOR_BUFFER_BIT); draw_3d_object_A(); draw_3d_object_B(); } //after enabling glutInitDisplayMode (GLUT_DEPTH | .... ); glEnable(GL_DEPTH_TEST); ... while (1) { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); get_viewing_point_from_mouse_position(); draw_3d_object_A(); draw_3d_object_B(); }
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  33 Specifying Light Sourcesin OpenGL General form: glLightf(source, parameter, value); glLightfv(source, parameter, *array); source is one of at least eight lights: GL_LIGHTi Parameters: GL_AMBIENT contains four values that specify the ambient RGBA intensity of the light. Default is (0.0, 0.0, 0.0, 1.0). GL_DIFFUSE contains four values that specify the diffuse RGBA intensity of the light. Default is (1.0, 1.0, 1.0, 1.0). GL_SPECULAR contains four values that specify the secular RGBA intensity of the light. Default is (1.0, 1.0, 1.0, 1.0).
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  34 Specifying Light Sourcesin OpenGL General form: glLightf(source, parameter, value); glLightfv(source, parameter, *array); GL_POSITION specifies the position of the light in homogeneous object coordinates. If the w component is zero, the light is treated as a directional source. GL_SPOT_DIRECTION specifies the direction of the light in homogeneous object coordinates. Default is (0.0, 0.0, -1.0) GL_SPOT_EXPONENT spotlight exponent, default 0.0 GL_SPOT_CUTOFF spot cutoff angle in [0,90] or (default) 180. θ l s I = kcose (θ) = k(l⋅s)e
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  35 Specifying Light Sourcesin OpenGL General form: glLightf(source, parameter, value); glLightfv(source, parameter, *array); GL_CONSTANT_ATTENUATION constant atten. factor, default 1.0 GL_LINEAR_ATTENUATION linear atten. factor, default 0.0 GL_QUADRATIC_ATTENUATION quadratic atten. factor, default 0.0 Id = k d a + b d + c d 2 ( l ⋅ n ) L d n l r v p Viewer
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  36 Enabling the Lightsand Lighting Enabling a specific light source: glEnable(GL_LIGHTi ); Enabling the lighting model: glEnable (GL_LIGHTING);
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  37 Specifying Materials inOpenGL General form: glMaterialf(face, parameter,value); glMaterialfv(face, parameter,*array); face is GL_FRONT, GL_BACK, GL_FRONT_AND_BACK parameter is: GL_AMBIENT four values that specify the ambient RGBA reflectance of the material. (0.2,0.2,0.2,1.0) GL_DIFFUSE four values that specify the diffuse RGBA reflectance of the material. (0.8,0.8,0.8,1.0) GL_SPECULAR four values that specify the ambient RGBA reflectance of the material. (0.0,0.0,0.0,1.0) GL_SHININESS specifies the specular reflectance exponent of the material. 0.0
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  38 1. Ambient lightin a scene with 3 spheres. 2. Diffuse light hitting the surface of 3 spheres. Notice, the spheres look matte and almost plastic like. 3. The three spheres illuminated by specular light. Imagine an extremely shiny billiard ball and the sheen it creates Types Of Light And Its Effects
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  39 By S.Vanitha, Chennai