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Shading and Illumination

Shading and Illumination. OpenGL Shading. Without Shading. With Shading. Physics. Bidirectional R eflectance D istribution F unction (BRDF). n: Surface normal L: Radiance (“intensity along a ray”). Light Events. Reflection. Refraction. Scattering. Bidirectional Reflectance

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Shading and Illumination

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  1. Shading and Illumination

  2. OpenGL Shading Without Shading With Shading

  3. Physics

  4. Bidirectional Reflectance Distribution Function (BRDF) n: Surface normal L: Radiance (“intensity along a ray”)

  5. Light Events Reflection Refraction Scattering Bidirectional Reflectance Distribution Function (BRDF) Bidirectional transmittance Distribution Function (BTDF) Scattering Function Bidirectional Scattering Distribution Function (BSDF)

  6. Rendering Equation • Given any point x in a large environment, let L(x, ω) be the radiance along a ray, we have: Le: Emitted intensity (light source) Ω: A hemisphere over x

  7. Light Transport • The rendering equation describes a light transport problem: • Each point has an integral equation. • Light can be reflected multiple times, so those integral equations are highly related. • Solution: global illumination. • Methods: ray tracing, radiosity.

  8. Example: Caustics

  9. Example: Color Bleeding

  10. OpenGL Illumination • Unfortunately, OpenGL doesn’t support global illumination. • Instead, it only considers local illumination: light can only be reflected once. • Even for local illumination, OpenGL is not physically based. It only provides an approximation. • To accurately get multiple reflections, you need to better rendering algorithms. • You can approximate some global illumination effects using GPU programmable shaders.

  11. OpenGL Illumination • Light sources • Surface material properties • Surface normal • Incoming (light) and outgoing (eye) directions

  12. Point Light Source GLfloat light_position[] = {1.0, 1.0, 1.0}; glLightfv(GL_LIGHT0, GL_POSITION, light_position);

  13. Directional Light Source GLfloat light_position[] = {1.0, 1.0, 1.0, 0.0}; glLightfv(GL_LIGHT0, GL_POSITION, light_position);

  14. Spot Light Source GLfloat light_position[] = {1.0, 1.0, 1.0}; glLightfv(GL_LIGHT0, GL_POSITION, light_position); Glfloat spot_position[] = {1.0, 1.0, 1.0}; glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, spot_position); GlfloatEXPONENT=10; glLightfv(GL_LIGHT0, GL_SPOT_EXPONENT, EXPONENT); Glfloat cutoff=30; glLightfv(GL_LIGHT0, GL_SPOT_CUTOFF, cutoff);

  15. OpenGL Light Intensity • OpenGL separates surface reflection into three components: • Therefore, you should define three light intensities. Each of them contributes to a reflection component: • Ambient • Diffuse • Specular Ambient Diffuse Specular Total

  16. OpenGL Light Source //To specify the light position and intensity GlfloatLightPosition []={100, 0.0, 100}; GlfloatLightAmbient []={0.2, 0.2, 0.2}; GlfloatLightDiffuse []={1.0, 1.0, 1.0}; GlfloatLightSpecular []={0.4, 0.4, 0.4}; glLightfv(GL_LIGHT1, GL_POSITION, LightPosition); glLightfv(GL_LIGHT1, GL_AMBIENT, LightAmbient); glLightfv(GL_LIGHT1, GL_DIFFUSE, LightDiffuse); glLightfv(GL_LIGHT1, GL_SPECULAR, LightSpecular);

  17. OpenGL Light Source • For a point light source, the light intensity drops when the light moves away from the source. So OpenGL multiplies the intensity with an attenuation factor: glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 2.0); glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0); glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0.5);

  18. Default Values No ambient Unit diffuse Unit specular Directional light along z No spot light No attenuation

  19. Multiple Light Sources //Specify the position and properties for GL_LIGHT0 glEnable(GL_LIGHT0); //Specify the position and properties for GL_LIGHT1 glEnable(GL_LIGHT1); //Create other light sources here. glEnable(GL_LIGHTING); //You can turn on and off lights like this: glDisable(GL_LIGHT0); glEnable(GL_LIGHT0); glDisable(GL_LIGHTING); glEnable(GL_LIGHTING);

  20. OpenGL Light Source • The light source is subject to the MODELVIEW matrix as well. • You can create the light source in the eye space, so it always attached to the camera. • You can create the light source after camera motion, so it is fixed in the world space. • You can even apply extra transformation/animation to light source.

  21. OpenGL Material • OpenGL separates surface reflection into three components: • Therefore, you should define three material properties. Each of them contributes to a reflection component: • Ambient • Diffuse • Specular Ambient Diffuse Specular Total

  22. Ambient Reflection Ambient Light Ambient Radiance (Pixel Intensity) Ambient Surface Material Coefficient • The object will have a uniform color. • It looks similar to the result without lighting. • The color is not directly assigned. It’s controlled by the light source and surface reflection coefficient.

  23. Diffuse Reflection • Diffuse reflection assumes that the light is equally reflected in every direction. • In other words, if the light source and the object has fixed positions in the world space, the camera motion doesn’t affect the appearance of the object. • The reflection only depends on the incoming direction. It is independent of the outgoing (viewing) direction. • Real objects with only a diffuse reflection property are called: Lambertian Surfaces, for example, clay.

  24. Diffuse Reflection • Lambert’s law says that the outgoing radiance depends on the cosine of the incident angle. • Because the irradiance (photons per unit surface area) becomes smaller (as in the rendering function before). dA dA θi dA dA/cosθi

  25. Diffuse Reflection Diffuse Light Diffuse Radiance Diffuse Surface Material Coefficient θi Brighter Darker

  26. An Example Kdiffuse Kambient

  27. Specular Reflection • Some materials, such as plastic and metal, can have shiny highlight spots. • This is due to a glossy mirror-like reflection. • Materials have different shininess/glossiness.

  28. Specular Reflection N • For a mirror, the light is reflected into a single direction. • Specular reflection assumes that the glossy surface is not even, so the light is reflected into a small cone. • If your viewing direction is close to the mirror direction, you receive high radiance. θi θi N θi ϕ

  29. Specular Reflection • Given L, N and V, we first calculate the mirror reflection direction: • Then we have: V N L R θi ϕ ϕis the angle between V and R. nindicates surface shininess.

  30. An Example Kspecular Shininess n

  31. Total Reflection • Illumination from a single light source i: • Illumination from multiple light source:

  32. OpenGL Material • OpenGL defines material properties like this: • Face: GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK • Property: GL_AMBIENT, GL_DIFFUSE, … • Value: A vector array glMaterialfv("face”, "property”, “value”)

  33. For Example Glfloatmat_amb_diff[]={1.0, 0.5, 0.8, 10.0}; Glfloatmat_specular[]={1.0, 1.0, 1.0, 1.0}; Glfloatshiniess []={5.0}; glMaterialfv(GL_FRONT, GL_AMBIENT_AND_DIFFUSE, mat_amb_diff); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); glMaterialfv(GL_FRONT, GL_SHININESS, shininess); • Specular highlights are usually in the light source’s color. • Ambient and diffuse can be defined together or separately. • If you don’t mind dark shadow, just ignore ambient. • Shininess is from 0 (dull) to 128 (shiny).

  34. Light Model glLightModelfv(property, value); • Enable two-sided lighting • Property=GL_LIGHT_MODEL_TWO_SIDE • If value=0, only GL_FRONT is rendered; • If value=1, both GL_FRONT and GL_BACK are rendered • Global ambient color • Property=GL_LIGHT_MODEL_AMBIENT • Value is a color array • And more…

  35. Surface Normal • Normals are used in calculating reflection. Normals are also used to decide the surface front and back side. (x, y, z) It’s normal is normalized(x, y, z) V2 E20=V2-V0 V0 E10=V1-V0 V1

  36. Surface Normal • We can define a triangle normal for the whole triangle. The result is a surface made of triangles. for(inti=0; i<t_number; i++) { glNormal3f(…); glBegin(GL_TRIANGLES); glVertex3f(…); glVertex3f(…); glVertex3f(…); glEnd(); }

  37. Surface Normal • But the mesh represents a smooth surface. We define a normal for each vertex, by taking the normalized average of its adjacent triangle normals. for(inti=0; i<t_number; i++) { glBegin(GL_TRIANGLES); glNormal3f(…); //XN0 glVertex3f(…); //X0 glNormal3f(…); //XN1 glVertex3f(…); //X1 glNormal3f(…); //XN2 glVertex3f(…); //X2 glEnd(); }

  38. Lighting and Shading • Each pixel corresponds to a visible 3D point. • We need to calculate the pixel intensity for that 3D point, based on the reflection model. Given a 2D Pixel Location Given a 3D polygon Inverse Projection Projection Selection Ray A 2D polygon Intersection Test Rasterization A 3D point on a polygon A number of pixels Reflection Calculation Ray Tracing The Graphics Pipeline

  39. Lighting and Shading • OpenGL calculates reflection at each vertex. Given a 3D polygon Given a 3D primitive Lighting Projection A 3D polygon, with intensity info A 2D polygon, with 3D info Projection Rasterization A 2D polygon, with intensity info Pixels with 3D info Fragment Shader Raserization Pixels with intensities Pixels with intensities OpenGL + GPU (Phong) OpenGL

  40. Shading Given a 3D polygon • Now given a 2D polygon with the value on each vertex, how to interpolate the value for each pixel? • Flat shading • Gouraud Shading* • Phong Shading* Lighting A 3D polygon, with info Projection A 2D polygon, with info Interpolation/Raserization Pixels with intensities *: Both were proposed by researchers at the University of Utah in 1970s.

  41. Flat Shading • Flat shading uses one vertex’s intensity value for the whole polygon. glShadeModel(GL_FLAT) What’s the difference? Vertex Normal + Flat Shading Triangle Normal + Smooth Shading

  42. Flat Shading VS. Smooth Shading • Flat shading has this discontinuous artifact. • It is fast, since only needs one calculation per polygon. • Only useful when the polygons are very small. • Smooth shading avoid the discontinuous artifact. • Slightly slower. • More realistic.

  43. Smooth Shading • OpenGL uses a smooth Gouraud shading technique. • The basic idea is to calculate the color intensity at each vertex, and then interpolate over the polygon. • To use smooth shading: glShadeModel(GL_SMOOTH) Vertex Normal + Flat Shading Vertex Normal + Smooth Shading

  44. Review: Linear Interpolation • Given a line segment with two end points (P and Q) and a sample point R, we can interpolate a value function V over the line as: Vq Vr=? Q Vp R P

  45. OpenGL Interpolation • OpenGL does 2D interpolation in a scanline way. • The result is identical to the interpolation using barycentric coordinates. But the scanline method is faster. Why? For every scanline

  46. Gouraud Shading • Gouraud shading calculates the color per vertex. • It has artifacts too. • To solve it, use smaller polygons. dark dark dark or bright? dark dark

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