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Shading (introduction to rendering)

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Rendering We know how to specify the geometry but how is the color calculated

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Rendering We know how to specify the geometry but how is the color calculated

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Rendering We know how to specify the geometry but how is the color calculated

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Rendering: simulation of light transport What makes up the final color of an object?

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Rendering: simulation of light transport Diffuse scattering o matt surfaces Specular reflection o shiny surfaces o highlight Transparency o glass, water o penetrate the surface

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Rendering: simulation of light transport How do we represent these observations in a mathematical framework

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Rendering: simulation of light transport Real time rendering is generally not concerned with using a "correct" lighting equation, just a series of hacks to make things look right with as little computational effort as possible

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Illumination Models Local o Direct illumination of surfaces by light sources Global o all light/surface interactions for entire environment

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Global illumination

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Local illumination Input: o a 3D object o Material and color of the object o Position and structure of the light source o Intensity of the light source Output: o Color and intensity of points of the given object A (modest) example of shading

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Representing 3D Objects Collection of triangles or mesh

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Dealing with color Three component intensity (red, green, blue) Luminance (intensity) of the source o Red component of source red component of image o Green component of source green component of image o Blue component of source blue component of image Three similar but independent calculations We focus on one scalar value only

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Diffuse reflection A perfect diffuse reflector (Lambertian surface) scatters the light equally in all directions Same appearance to all viewers Depends on o Material of the surface o The position of the light

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Normals o What direction is the surface facing?

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CrossProduct o n.x = a.y * b.z - a.z * b.y o n.y = a.z * b.x - a.x * b.z o n.z = a.x * b.y - a.y * b.x

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Normals o A = V2 V1 o B = V0 V1 o N = A x B

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Normals o For each triangle we can define a normal for the face o For each vertex we an define a normal by interpolating normals of attached faces

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Diffuse: Two important vectors To compute the intensity at P, we need o The unit normal vector N, o The unit vector L, from P to the light L N P

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Diffuse: Two important vectors To compute the intensity at P, we need o The unit normal vector N, o The unit vector L, from P to the light L N P What is the diffuse color at P?

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Lamberts cosine law I : diffuse reflection at P I d : intensity of the light from source coefficient of diffuse reflection

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Coefficient of diffuse reflection k d is usually determined by a trial and error approach Examples: Component Gold Black plastic Silver Red 0.75 0.01 0.5 Green 0.6 0.01 0.5 Blue 0.22 0.01 0.5 k d =0.05 k d =0.25 k d =0.5 k d =0.75 k d =1

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Specular reflection Diffusive reflection: no highlights, rough surface Specular reflection: highlights, shiny and smooth surfaces View dependent reflection

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Three important vectors To compute the intensity at P, we need o The unit normal vector N, o The unit vector L, from P to the light o The unit vector V, from P to the viewer L N V P

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Three important vectors To compute the intensity at P, we need o The unit normal vector N, o The unit vector L, from P to the light o The unit vector V, from P to the viewer L N V P What is the specular illumination at P?

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The shininess coefficient 0 90 o -90 o increasing n

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I : specular reflection at P I d : intensity of the light from source coefficient of specular reflection n: controls shininess The Phong model for specular reflection P N L R P N L R V

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Ambient light Physical rules are too simplified No indirect or global interaction of light A hack to overcome the problem: use ambient light

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Ambient light specification Not situated at any particular point Spreads uniformly in all directions I a : intensity of ambient light in the environment I : ambient light at a given point : coefficient of ambient light reflection k a =0 k a =0.5 k a =1

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A combined model (The Phong local illumination model) The final model = diffuse + specular + ambient

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How does it work in OpenGL

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Flat Shading Individual faces are visualized Same color for any point in the face

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Smooth Shading Visualize the underlying surface Each point on the face has its own color Two techniques Gouraud and Phong shading

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Shading GouraudPhongFlat Normal Per VertexInterpolated normal for each point across face Normal per face Color per vertex, interpolated across face Color calculated per pixel Color calculated per face Faster then phong Bad specular Costly (not really any more) Fast, good for distant objects N

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Clarification Phong reflection model or phong lighting refers to Phong Shading refers to filling a triangle by interpolating the normal and calculating the color at each point

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Gouraud Shading Specular highlight quality tied to detail of mesh Specular highlights can even be missed

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Incorporating a distance term a,b,c are control parameters Empirical formula:

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Multiple light sources The total reflection at p is the sum of all contributed intensities from all sources OpenGL allows us to define several light sources

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OpenGL 2.0 (programmable) pipeline

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More advanced rendering

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Questions?

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