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Illumination Models & Shading

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Illumination Models & Shading CS5620 Illumination Models Intro to Computer Graphics and Shading Illumination Models So Far … and Shading wireframe hidden surfaces Next step… 1 2 What’s Missing ? Lighting vs. Shading • Light ! • Lighting – Interaction between materials and light sources • Need to understand: – Physics – How lighting works Zbuffer(Scene) • Types of lights • Shading … • Types of surfaces – Determining the color of a pixel putColor(x,y,col(P)) – Computer graphics … end – How shading works • • Shading usually determined by Shading algorithms lighting 3 4 The Physics Local vs. Global Illumination Model Basic Illumination Model Light rays are emitted from light sources and • Local bounce (reflect) in the scene until they reach the – only direct and local interaction eye of objects with light • Global – all interactions and exchange of light – Can model more effects • Reflection, refraction, soft shadows 5 6 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 1 Computer Science Dept., Technion CS5620 Illumination Models Intro to Computer Graphics and Shading Local vs. Global A Single Interaction What determines pixel color? Type of light Object color Object surface material Object geometry 7 8 Light Sources Material Properties • Point: All light originates at a point – Rays hit planar surface at different incidence • Specular angles – Smooth surface • Directional: All light rays are parallel – Rays hit a planar surface at identical – Reflects light at well-defined angle incidence angles – May be modeled as point source at infinity – Also called parallel source • • Area: Light originates at finite area in space Diffuse – In between the point and parallel sources – Rough surface – Also called distributed source – Reflects light equally in all directions • Ambient: Background light – Comes equally from all directions 9 10 Light/Reflection Types Phong Reflection Model • Local illumination model • Looking for a value I(p) per point on the surface • Represents the total contribution of all lights in the scene • Takes into consideration – Position of the viewer – Position of the lights – Color of the lights – … 11 12 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 2 Computer Science Dept., Technion CS5620 Illumination Models Intro to Computer Graphics and Shading Some Notation Ambient Reflection • I – direction to light source • Assume non-directional light in the environment • n – normal direction • Object illuminated with same light everywhere – Looks like a silhouette • v – direction to COP (eye, camera) 퐼푎 = 푘푎퐿푎 푘푎 : fraction of ambient light reflected from surface • r – direction of reflected ray 퐿푎: ambient light intensity • Defines object’s color 13 14 Diffuse Reflection Diffuse Reflection • Dull surfaces such as solid matte plastic reflects • How does the light direction affect the illumination? incoming light uniformly in all directions • Larger angle with normal less illumination density • Called diffuse or Lambertian reflection Fully Partially illuminated illuminated 15 16 Diffuse Reflection Specular Reflection • Shiny objects (e.g. metallic) reflect light in 푘푑 : fraction of diffuse light preferred direction 푟 determined by surface reflected from surface 퐼푑 = 푘푑(푙 ⋅ 푛)퐿푑 normal 푛. 퐿푑: diffuse light intensity 푙 ⋅ 푛 = cos 휃 17 18 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 3 Computer Science Dept., Technion CS5620 Illumination Models Intro to Computer Graphics and Shading Specular Reflection Specular Reflection • Most objects are not ideal mirrors – also 퐼 = 푘 푟 ⋅ 푣 훼퐿 reflect in the immediate vicinity of 푟 푘푠 : fraction of specular light 푠 푠 푠 reflected from surface 퐿푠 • Approximate attenuation 퐿 : diffuse light intensity by cos훼 푠 (no real physical basis) 퐿푠 푟 ⋅ 푣 = cos 훼 : shininess coefficient 19 20 Specular Reflection Total Illumination • Exponent 훼 of cosine controls decay factor • 퐼 = 퐼푎 + 퐼푑 + 퐼푠 • Sum over all light sources • No physical basis, • May use different coefficients for RGB components but looks good • Beware of overflows 21 22 Triangle Shading Algorithms Flat Shading • Applied to piecewise linear • Given the lights and materials in the scene, polygonal models • Simple surface lighting how do we compute the color at a given point approximated over polygons on a triangle ? • Illumination value depends only on polygon normal each polygon is colored with a uniform intensity • Three main types • Looks non-smooth (worsened by – Flat shading (per polygon) Mach band effect) – Gouraud shading (per vertex) – Phong shading (per pixel) 23 24 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 4 Computer Science Dept., Technion CS5620 Illumination Models Intro to Computer Graphics and Shading Normal per Vertex Gouraud Shading • Compute illumination intensity at vertices using normals • Linearly interpolate intensity over polygon interior If a polyhedron is an approximation of smooth surface: • Assign to each vertex the normal of original surface at that point • If surface is not available use estimated normal (e.g. average of neighboring faces). 25 26 Gouraud Shading Phong Shading Linearly interpolate lighting intensities at the vertices over • Interpolate (at the vertices in image space) normal vectors interior pixels of the polygon, in the image plane instead of illumination intensities • Apply the illumination equation for each interior pixel with its c1 c12 t 12 c 1 (1 t 12 ) c 2 c13 t 13 c 1 (1 t 13 ) c 3 own (interpolated) normal n scanline Y=y 1 n12 t 12 n 1 (1 t 12 ) n 2 n13 t 13 n 1 (1 t 13 ) n 3 (,)xy scanline Y=y c 3 c2 n3 c( x , y ) t123 c 12 (1 t 123 ) c 13 n2 Question: Can Gouraud shading support specular lighting? n(,) x y t123 n 12 (1 t 123 ) n 13 ;(,) c x y Illum ((,)) n x y 27 28 Comparison Comparison phong flat gourard flat gourard phong 29 30 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 5 Computer Science Dept., Technion CS5620 Illumination Models Intro to Computer Graphics and Shading Gourard 31 Copyright C. Gotsman, G. Elber, M. Ben-Chen Page 6 Computer Science Dept., Technion .
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