CS452/552; EE465/505. Lighting & Shading


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1 CS452/552; EE465/505 Lighting & Shading
2 Outline! More on Lighting and Shading Read: Angel Chapter 6 Lab2: due tonight use ASDW to move a 2D shape around; 1 to center
3 Local Illumination! Approximate model! Local interaction between light, surface & viewer! Phong model: fast, supported in OpenGL/WebGL! Pixar Renderman (offline) Rendered without global illumination Rendered with global illumination wikipedia.org
4 Recap: Phong Reflection Model! Identify the important vectors in the diagram below: l n v r θ θ n the normal of the plane/vertex l the incoming light vector r the reflection vector v the viewpoint (camera)
5 INTERACTION BETWEEN MATERIAL AND LIGHTS! Final color of an object is comprised of many things: The base object color (called a material ) The light color Example: a purple light on a white surface Any textures we apply (later)! Materials and lights have four individual components Diffuse color (c d and l d ) Specular color (c s and l s ) Ambient color (c a and l a ) Emit color (c e and l e ) c d * l d = [c d.r *l d.r, c d.g *l d.g, c d.b *l d.b ] // R, G, B COPYRIGHT 2015 JEFF CHASTINE 5
6 FINAL COLOR!To determine the final color (excluding textures) we sum up all components: + final_color diffuse final_color specular final_color ambient final_color emit final_color COPYRIGHT 2015 JEFF CHASTINE 6
7 How OpenGL Simulates Lights! Phong lighting model The model is a balance between simple computation and physical realism a simple reflection model that can be used with real time graphics hardware The model uses Light positions and intensities Surface orientation (normals) Material properties (reflectivity) Viewer location Computed at the vertices for each source and each color component
8 COMMON KINDS OF LIGHTS Point light Directional Light Spot Light Area Light Interesting fact: Lights cannot be seen! Only their effects We can light per vertex (fast) or per fragment (slower) JEFF CHASTINE 8
9 POINT LIGHTS JEFF CHASTINE Here, per fragment lighting used 9
10 POINT LIGHTS JEFF CHASTINE Here, per vertex lighting used 10
11 DIRECTIONAL LIGHTS Are infinitely far away position in NO WAY matters Have only direction All objects are lit evenly Sometimes called a Sun JEFF CHASTINE 11
12 SPOTLIGHTS Point light source Conical in shape JEFF CHASTINE 12
13 SPOTLIGHTS Point light source Conical in shape Have: An inner and outer cone angle Umbra areas that are fully in shadow Penumbra areas that are in partial shadow Note: There s an ambient light JEFF CHASTINE 13
14 AREA LIGHTS A surface lights objects Has a position and direction Provides for a smoother drop off than point Larger surface == smoother shadows Expensive to calculate JEFF CHASTINE 14
15 Light Sources! In the Phong Model, we add the results from each light source! Each light source has separate diffuse, specular, and ambient terms to allow for maximum flexibility even though this form does not have a physical justification! Separate red, green and blue components! Hence, 9 coefficients for each point source I dr, I dg, I db, I sr, I sg, I sb, I ar, I ag, I ab
16 Material Properties! Material properties match light source properties Nine absorbtion coefficients k dr, k dg, k db, k sr, k sg, k sb, k ar, k ag, k ab Shininess coefficient α
17 Adding up the Components For each light source and each color component, the Phong model can be written (without the distance terms) as I =k d I d l n + k s I s (v r ) α + k a I a For each color component we add contributions from all sources
18 Modified Phong Model! The specular term in the Phong model is problematic because it requires the calculation of a new reflection vector and view vector for each vertex! Blinn suggested an approximation using the halfway vector that is more efficient
19 The Halfway Vector h is the normalized vector halfway between l and v h = ( l + v )/ l + v
20 Using the halfway vector! Replace (v r ) α by (n h ) β! β is chosen to match shininess! Note that halfway angle is half of angle between r and v if vectors are coplanar! Resulting model is known as the modified Phong or PhongBlinn lighting model Specified in OpenGL standard
21 Example Only differences in these teapots are the parameters in the modified Phong model
22 Computation of Vectors! l and v are specified by the application! Can compute r from l and n! Problem is determining n! For simple surfaces it can be determined but how we determine n differs depending on underlying representation of surface! OpenGL leaves determination of normal to application Exception for GLU quadrics and Bezier surfaces was deprecated
23 Computing Reflection Direction! Angle of incidence = angle of reflection! Normal, light direction and reflection direction are coplaner! Want all three to be unit length r = 2(l n)n l
24 Plane Normals! Equation of plane: ax+by+cz+d = 0! From Chapter 4 we know that plane is determined by three points p 0, p 1, p 2 or normal n and p 0! Normal can be obtained by n = (p 2 p 0 ) (p 1 p 0 )
25 Normal to Sphere! Implicit function f(x,y.z)=0! Normal given by gradient! Sphere f(p)=p p1! n = [ f/ x, f/ y, f/ z] T =p
26 Parametric Form! For sphere x=x(u,v)=cos u sin v y=y(u,v)=cos u cos v z= z(u,v)=sin u! Tangent plane determined by vectors p/ u = [ x/ u, y/ u, z/ u]t p/ v = [ x/ v, y/ v, z/ v]t! Normal given by cross product n = p/ u p/ v
27 General Case! We can compute parametric normals for other simple cases Quadrics Parametric polynomial surfaces Bezier surface patches (Chapter 11)
28 Lighting and Shading in WebGL! Introduce the WebGL shading methods Light and material functions on MV.js per vertex vs per fragment shading Where to carry out
29 WebGL lighting! Need Normals Material properties Lights Statebased shading functions have been deprecated (glnormal, glmaterial, gllight) Compute in application or in shaders
30 Normalization! Cosine terms in lighting calculations can be computed using dot product! Unit length vectors simplify calculation! Usually we want to set the magnitudes to have unit length but Length can be affected by transformations Note that scaling does not preserved length! GLSL has a normalization function
31 Normal for Triangle p 1 n plane n (p  p 0 ) = 0 p 2 n = (p 2  p 0 ) (p 1  p 0 ) p normalize n n/ n p 0 Note that righthand rule determines outward face
32 Specifying a Point Light Source! For each light source, we can set an RGBA for the diffuse, specular, and ambient components, and for the position var diffuse0 = vec4(1.0, 0.0, 0.0, 1.0); var ambient0 = vec4(1.0, 0.0, 0.0, 1.0); var specular0 = vec4(1.0, 0.0, 0.0, 1.0); var light0_pos = vec4(1.0, 2.0, 3,0, 1.0);
33 Distance and Direction! The source colors are specified in RGBA! The position is given in homogeneous coordinates If w =1.0, we are specifying a finite location If w =0.0, we are specifying a parallel source with the given direction vector! The coefficients in distance terms are usually quadratic (1/(a+b*d+c*d*d)) where d is the distance from the point being rendered to the light source
34 Spotlights! Derive from point source Direction Cutoff Attenuation Proportional to cos α φ θ φ θ
35 Global Ambient Light! Ambient light depends on color of light sources A red light in a white room will cause a red ambient term that disappears when the light is turned off! A global ambient term that is often helpful for testing
36 Moving Light Sources! Light sources are geometric objects whose positions or directions are affected by the modelview matrix! Depending on where we place the position (direction) setting function, we can Move the light source(s) with the object(s) Fix the object(s) and move the light source(s) Fix the light source(s) and move the object(s) Move the light source(s) and object(s) independently
37 Light Properties var lightposition = vec4(1.0, 1.0, 1.0, 0.0 ); var lightambient = vec4(0.2, 0.2, 0.2, 1.0 ); var lightdiffuse = vec4( 1.0, 1.0, 1.0, 1.0 ); var lightspecular = vec4( 1.0, 1.0, 1.0, 1.0 );
38 Material Properties! Material properties should match the terms in the light model! Reflectivities! w component gives opacity var materialambient = vec4( 1.0, 0.0, 1.0, 1.0 ); var materialdiffuse = vec4( 1.0, 0.8, 0.0, 1.0); var materialspecular = vec4( 1.0, 0.8, 0.0, 1.0 ); var materialshininess = 100.0;
39 Using MV.js for Products var ambientproduct = mult(lightambient, materialambient); var diffuseproduct = mult(lightdiffuse, materialdiffuse); var specularproduct = mult(lightspecular, materialspecular); gl.uniform4fv(gl.getuniformlocation(program, "ambientproduct"), flatten(ambientproduct)); gl.uniform4fv(gl.getuniformlocation(program, "diffuseproduct"), flatten(diffuseproduct) ); gl.uniform4fv(gl.getuniformlocation(program, "specularproduct"), flatten(specularproduct) ); gl.uniform4fv(gl.getuniformlocation(program, "lightposition"), flatten(lightposition) ); gl.uniform1f(gl.getuniformlocation(program, "shininess"),materialshininess);
40 Adding Normals for Quads function quad(a, b, c, d) { var t1 = subtract(vertices[b], vertices[a]); var t2 = subtract(vertices[c], vertices[b]); var normal = cross(t1, t2); var normal = vec3(normal); normal = normalize(normal); pointsarray.push(vertices[a]); normalsarray.push(normal);...
41 Front and Back Faces! Every face has a front and back! For many objects, we never see the back face so we don t care how or if it s rendered! If it matters, we can handle in shader back faces not visible back faces visible
42 Emissive Term! We can simulate a light source in WebGL by giving a material an emissive component! This component is unaffected by any sources or transformations
43 Transparency! Material properties are specified as RGBA values! The A value can be used to make the surface translucent! The default is that all surfaces are opaque! Later we will enable blending and use this feature! However with the HTML5 canvas, A<1 will mute colors
44 Adding Lighting to Cube // vertex shader attribute vec4 vposition; attribute vec3 vnormal; varying vec4 fcolor; uniform vec4 ambientproduct, diffuseproduct, specularproduct; uniform mat4 modelviewmatrix; uniform mat4 projectionmatrix; uniform vec4 lightposition; uniform float shininess;
45 Adding Lighting to Cube void main() { // Transform vertex position into eye coords vec3 pos = (modelviewmatrix * vposition).xyz; vec3 light = lightposition.xyz; vec3 L = normalize( light  pos ); vec3 E = normalize( pos ); vec3 H = normalize( L + E ); vec4 NN = vec4(vnormal,0); // Transform vertex normal into eye coordinates vec3 N = normalize( (modelviewmatrix*nn).xyz);
46 Adding Lighting to Cube // Compute terms in the illumination equation vec4 ambient = ambientproduct; float Kd = max( dot(l, N), 0.0 ); vec4 diffuse = Kd*diffuseProduct; float Ks = pow( max(dot(n, H), 0.0), shininess ); vec4 specular = Ks * specularproduct; if(dot(l, N)<0.0) {specular=vec4(0.0,0.0,0.0,1.0);} gl_position = projectionmatrix * modelviewmatrix * vposition; fcolor = ambient + diffuse +specular; fcolor.a = 1.0;}
47 Polygonal Shading! Now we know vertex colors can be determined either via OpenGL/WebGL lighting, or by setting directly if lighting disabled! How do we shade the interior of the triangle?
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