Local Illumination and Shading

1. Local Illumination and Shading Computer Graphics – Lecture 3 Taku Komura tkomura@ed.ac.uk Institute for Perception, Action & Behaviour

2. Today is about calculating the color of objects • The incident light • Position of light source • Direction • Color The object • Reflectance Viewer • Position

3. Overview • Illumination (how to calculate the colour)‏ • Shading (how to colour the whole surface)? • BRDF (how to simulate the reflection of real objects)‏

4. Illumination • Simple 3 parameter model • The sum of 3 illumination terms: • Ambient : 'background' illumination • Specular : bright, shiny reflections • Diffuse : non-shiny illumination and shadows 'Virtual' camera = + + Rc Specular (highlights)‏ Ambient (colour)‏ Diffuse (directional)‏

5. Ambient Lighting • Light from the environment • light reflected or scattered from other objects • Coming uniformly from all directions • and then reflected equally to all directions • simple approximation to complex 'real-world' process • Result: globally uniform colour for object • I = resulting intensity • Ia= light intensity • ka = reflectance Example: sphere Object

6. Object Diffuse Lighting • Light reflected to all directions • considers the angle of incidence of light on surface • (angle between light and surface normal)‏ • Also known as Lambertian reflection • Result: lighting varies over surface with orientation to light Ln Infinite point light source Example: sphere (lit from left)‏ N  I No dependence on camera angle!

7. Object Specular Lighting • Direct reflections of light source off shiny object • specular intensity n = shiny reflectance of object • Result: specular highlight on object Ln N Infinite point light source R (Reflection)‏   α Ip: light intensity I : output color R: reflection vector S: direction towards the camera α : angle between R and S No dependence on object colour.

8. Specular Light

9. Combined Lighting Models • Summing it altogether : Phong Illumination Model = + + Rc Specular (highlights)‏ Ambient (colour)‏ Diffuse (directional)‏

10. Object When you implement it… Use dot product of the vectors instead of calculating the angles N R L   α V : Vector from the surface to the viewer : Normal vector at the colored point : Normalized reflection vector : Normalized vector from the coloured point towards the light source

11. Demo applets • http://www.cs.auckland.ac.nz/~richard/research-topics/PhongApplet/PhongDemoApplet.html

12. Local Illumination Model • Considers light sources and surface properties only. • Not considering the light reflected back onto other surfaces • Fast real-time interactive rendering. • Most real-time graphics (games, virtual environments) are based on local illumination models • Implementation - OpenGL, Direct3D

13. Attenuation • Haven’t considered light attenuation – the light gets weaker when the object is far away • Use 1/(s+k) where s relates to eye-object distance and k is some constant for scene. • Note: must apply 3 times – for each primary colour

14. Color • Finally color the pixel by the RGB color

15. Overview • Illumination (how to calculate the color)‏ • Shading (how to color the whole surface)?

16. How often do we do the computation of illumination? • At all the points on the surface? • But usually we have normals only at the vertices Depends on the shading model – Flat Shading (once per polygon)‏ (less computation needed) – Gouraud shading (once per vertex)‏ – Phong Shading (once per pixel)‏ (heavy computation needed)‏

17. Flat Shading • Compute the colour at the middle of the polygon • All points in the same polygon are coloured bythe same colour

18. Shading Polyhedra • Flat (facet) shading: • Works well for objects really made of flat faces. • Appearance depends on number of polygons for curved surface objects. • If polyhedral model is an approximation then need to smooth.

19. Mach banding. • Physiological effect exaggerates difference between closely coloured polygons. • Mach banding. • Exaggerates intensity change at an edge with a discontinuity in magnitude. • Our eyes are well adapted to seeing edges ! • Using a finer mesh is surprisingly ineffective.

20. Gouraud Shading (Smooth Shading) • Compute the color at each vertex first • Compute the color inside the polygon by interpolating thecolors of the vertices composing the polygon

21. Gouraud shading

22. Gouraud Shading. Find vertex normals by averaging face normals • - Use vertex normals with desired shading model, • Interpolate vertex intensities along edges. • Interpolate edge values across a scanline. B A Ibc Iac Iac interpolated A to C, Ibc interpolated B to C. C

23. Phong Shading • interpolating the normal vectors at the vertices • Do the computation of illumination at each point in thepolygon

24. Direction of maximum highlight Phong shading. • For specular reflection, highlight falls off with cosn • Gouraud shading linear interpolates – makes highlight too big. • Gouraud shading may well miss a highlight that occurs in the middle of the face.

25. Direction of maximum highlight Highlight on surface. Phong shading. • In Phong model of specular reflection, highlight falls off with cosn • Gouraud shading linear interpolates – makes highlight too big. • Gouraud shading may well miss a highlight that occurs in the middle of the face.

26. Phong example

27. Gouraud Shaded Floor Phong Shaded Floor • Gouraud shading is not good when the polygon count is low

28. A C B Problems with interpolation shading. • Problems with computing vertex normals. • A,B are shared by all polygons, but C is not shared by the polygon on the left. • Shading information calculated to the right of C will be different from that to the left. • Shading discontinuity. Solution 1: subdivide into triangles that that share all the vertices Solution 2 : introduce a ‘ghost’ vertex that shares C’s values

29. Problems with interpolation shading. • Problems with computing vertex normals. Face surface normals and averaged vertex normals shown. Unrepresentative so add small polygon strips along edges or test angle and threshold to defeat averaging.

30. A B A D B D C Problems with interpolation shading. • Rotational invariance? • When interpolating the colour inside a polygon, the edge colours are determined first, and then they are interpolated to compute the internal colour • If the number of edges is >= 4, the results of interpolation shading can change with orientation of the polygon. Point on left is interpolated between AD & AB , Point on right is interpolated between AB & BC C

31. Problems with interpolation shading. • Solution. • Decompose polygon into triangles before Gouraud shading.

32. Overview • Illumination (how to calculate the color)‏ • Shading (how to color the whole surface)? • BRDF (how to simulate the reflection of real objects)‏

33. What about real objects? • Phong Illumination model is popular but actually it is not accurate • True photorealism, requires more sophisticated and elaborate models of surface properties

34. Reflectance of objects • The way the light reflects depends on the physical characteristics of the surface • Its not uniform

35. Bidirectional Reflectance Distribution Function (BRDF)‏ • The reflectance of an object can be represented by a function of the incident and reflected angles • This function is called the Bidirectional Reflectance Distribution Function (BRDF)‏ • where E is the incoming irradianceand L is the reflected radiance

36. What affects the BRDF? • The way light reflecting over the surface • The smoothness/roughness of the surface • Single reflection : if smooth, specular • Multiple reflections : diffusive • The shadowing effect

37. Isotropic and Anisotropic BRDFs • Isotropic: • Can model by diffuse + specular reflection • Anisotripic • Brushed metal • Cannot model by diffuse + specular reflection • Reflectance changing systematically with respect to direction

38. Measuring the BRDF • Measured using a device called gonioreflectometer • Casting light from various directions to the object, and capturing the light reflected back

39. Summary • Illumination model • Phong model • Shading • Flat, • Gouraud, • Phong Shading • BRDF • Measuring

40. Recommended Reading • Foley et al. Chapter 16, sections 16.1.6 up to and including section 16.3.4. • Introductory text Chapter 14, sections 14.1.6 up to and including section 14.2.6.