Topic 3 Light, shading and materials.ppt
- 1. Topic 3 Lighting and Shading Dr. Collins Oduor Kenyatta University Lighting and Shading
- 2. 2 Topics Introduction Geometric Model in Lighting Colored Surfaces and Lights Shading and Graphics Pipeline Flat Shading and Smooth Shading
- 3. 3 Introduction Lighting Process of computing the luminous intensity reflected from a specific 3-D point Shading Process of assigning colors to pixels Shading model dictates how light is scattered or reflected from a surface We will begin with achromatic light then colored lights
- 4. 4 Introduction Two types of light sources Point light source Ambient light Light interacts surfaces in different ways Absorbed by surface Reflected by surface Transmitted into the interior What absorbs all of the incident achromatic light?
- 5. 5 Introduction Types of reflection of incident light Diffuse scattering Some of the incident light penetrates the surface slightly and is re-radiated uniformly in all directions Scattering light interacts strongly with surface color is affected by nature of surface material Specular reflections Incident light does not penetrate the surface Reflected directly from the surface More mirror like and highly directional Highlight, shiny, plastic like
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- 7. 7 Shading and Graphics Pipeline Vertices are sent down the pipeline along with their associated normals All shading calculations are done on vertices VM clip projection matrix viewport matrix shading is applied here v1, m1 v2, m2 v0, m0
- 8. 8 Shading and Graphics Pipeline Lights are objects and the positions of light sources are also transformed by the modelview matrix After all quantities are expressed in camera coordinates, colors are attached to vertices using the formula If an object is clipped, normals of newly generated vertices are calculated by interpolation
- 9. 9 Flat Shading and Smooth Shading Polygonal face in 3D space Individual face Underlying surface approximated Shading methods Flat shading Smooth shading Gouraud shading Phong shading
- 10. 10 Comparison of Shading Methods
- 11. 11 Flat Shading Entire face is drawn with the same color Lateral inhibition When there is a discontinuity across an object, the eye manufactures a Mach band at the discontinuity and a vivid edge is seen Specular highlights are rendered poorly Either no highlight at all Or highlight on the entire face
- 12. 12 Smooth Shading Smooth shading computes colors at more points on each face to de-emphasize edges between adjacent faces Use linear interpolation Gouraud shading Interpolate vertex colors Phong shading Interpolate vertex normals Interpolate normal for each pixel
- 13. 13 Gouraud Shading Used by OpenGL Example colora: by interpolating color3 and color4 colorb: by interpolating color1 and color2 Colors of pixels on the horizontal line segment is obtained by interpolating colora and colorb Does not picture highlights well color1 color2 color3 color4 colora colorb
- 14. 14 Phong Shading Compute normal at each pixel by interpolating the normals at the vertices Apply the shading model to to every point to find the color Example ma: by interpolating m3 and m4 mb: by interpolating m1 and m2 Normals of pixels on the horizontal line segment is obtained by interpolating ma and mb Colors of the pixels are then computed m1 m2 m3 m4 ma mb
- 15. 15 Phong Shading Very smooth appearance Highlights are approximated better Principle drawback Heavy computation slow speed Not supported by OpenGL Can be approximated using texture mapping Phong shading Phong model Don’t be confused!!
- 16. 16 Introduction Lighting Process of computing the luminous intensity reflected from a specific 3-D point Shading Process of assigning colors to pixels Shading model dictates how light is scattered or reflected from a surface We will begin with achromatic light then colored lights
- 17. 17 Introduction Two types of light sources Point light source Ambient light Light interacts surfaces in different ways Absorbed by surface Reflected by surface Transmitted into the interior What absorbs all of the incident achromatic light?
- 18. 18 Introduction Types of reflection of incident light Diffuse scattering Some of the incident light penetrates the surface slightly and is re-radiated uniformly in all directions Scattering light interacts strongly with surface color is affected by nature of surface material Specular reflections Incident light does not penetrate the surface Reflected directly from the surface More mirror like and highly directional Highlight, shiny, plastic like
- 19. 19 Introduction Total light reflected from the surface in a certain direction is the sum of Diffuse components Specular components We calculate the size of each component that reaches the eye for each point of interest on surfaces
- 20. 20 Comparison of Shading Methods
- 21. 21 Flat Shading Entire face is drawn with the same color Lateral inhibition When there is a discontinuity across an object, the eye manufactures a Mach band at the discontinuity and a vivid edge is seen Specular highlights are rendered poorly Either no highlight at all Or highlight on the entire face
- 22. 22 Smooth Shading Smooth shading computes colors at more points on each face to de-emphasize edges between adjacent faces Use linear interpolation Gouraud shading Interpolate vertex colors Phong shading Interpolate vertex normals Interpolate normal for each pixel
- 23. 23 Gouraud Shading Used by OpenGL Example colora: by interpolating color3 and color4 colorb: by interpolating color1 and color2 Colors of pixels on the horizontal line segment is obtained by interpolating colora and colorb Does not picture highlights well color1 color2 color3 color4 colora colorb
- 24. 24 Phong Shading Compute normal at each pixel by interpolating the normals at the vertices Apply the shading model to to every point to find the color Example ma: by interpolating m3 and m4 mb: by interpolating m1 and m2 Normals of pixels on the horizontal line segment is obtained by interpolating ma and mb Colors of the pixels are then computed m1 m2 m3 m4 ma mb
- 25. 25 Phong Shading Very smooth appearance Highlights are approximated better Principle drawback Heavy computation slow speed Not supported by OpenGL Can be approximated using texture mapping Phong shading Phong model Don’t be confused!!
- 26. •Lighting •Lighting models •Ambient •Diffuse •Specular •Surface Rendering Methods •Ray-Tracing
- 27. “Lighting” Two components: Lighting Model or Shading Model - how we calculate the intensity at a point on the surface Surface Rendering Method - How we calculate the intensity at each pixel
- 28. Jargon Illumination - the transport of light from a source to a point via direct and indirect paths Lighting - computing the luminous intensity for a specified 3D point, given a viewpoint Shading - assigning colors to pixels Illumination Models: Empirical - approximations to observed light properties Physically based - applying physics properties of light and its interactions with matter
- 29. The lighting problem… What are we trying to solve? Global illumination – the transport of light within a scene. What factors play a part in how an object is “lit”? Let’s examine different items here…
- 30. Two components Light Source Properties Color (Wavelength(s) of light) Shape Direction Object Properties Material Geometry Absorption
- 31. Light Source Properties Color We usually assume the light has one wavelength Shape point light source - approximate the light source as a 3D point in space. Light rays emanate in all directions. good for small light sources (compared to the scene) far away light sources
- 32. Distributed Lights Light Source Shape continued distributed light source - approximating the light source as a 3D object. Light rays usually emanate in specific directions good for larger light sources area light sources
- 33. Light Source Direction In computer graphics, we usually treat lights as rays emanating from a source. The direction of these rays can either be: Omni-directional (point light source) Directional (spotlights)
- 34. Ambient Term - Background Light The ambient term is a HACK! It represents the approximate contribution of the light to the general scene, regardless of location of light and object Indirect reflections that are too complex to completely and accurately compute Iambient = color
- 35. Diffuse Term Contribution that a light has on the surface, regardless of viewing direction. Diffuse surfaces, on a microscopic level, are very rough. This means that a ray of light coming in has an equal chance of being reflected in any direction. What are some ideal diffuse surfaces?
- 36. Different for shiny vs. dull objects
- 37. Snell’s Law is for IDEAL surfaces • Think about the amount of light reflected at different angles. N L R V
- 38. Combining the terms Ambient - the combination of light reflections from various surfaces to produce a uniform illumination. Background light. Diffuse - uniform light scattering of light rays on a surface. Proportional to the “amount of light” that hits the surface. Depends on the surface normal and light vector. Sepecular - light that gets reflected. Depends on the light ray, the viewing angle, and the surface normal.
- 39. Ambient + Diffuse + Specular
- 40. Lighting Equation Ilambient = light source l’s ambient component Ildiffuse = light source l’s diffuse component Ilspecular = light source l’s specular component kambient = surface material ambient reflectivity kdiffuse = surface material diffuse reflectivity kspecular = surface material specular reflectivity shininess = specular reflection parameter (1 -> dull, 100+ -> very shiny) 1 0 lights l shininess specular l diffuse l ambient l final shininess specular specular diffuse diffuse ambient ambient final H N k I L N k I k I I H N k I L N k I k I I specular diffuse ambient N L R V
- 41. Attenuation One factor we have yet to take into account is that a light source contributes a higher incident intensity to closer surfaces. The energy from a point light source falls off proportional to 1/d2 . What happens if we don’t do this?
- 42. What would attenuation do for: Actually, using only 1/d2 , makes it difficult to correctly light things. Think if d=1 and d=2. Why? Remember, we are approximating things. Lighting model is too simple AND most lights are not point sources. We use: 2 2 1 0 1 d a d a a d f
- 43. Shading When do we do the lighting equation? What is the cost to compute the lighting for a 3D point? 1 0 lights l shininess specular l diffuse l l ambient l final H N k I L N k I d f k I I specular diffuse ambient
- 44. Shading Shading is how we “color” a triangle. Constant Shading Gouraud Shading Phong Shading
- 45. Constant Shading Constant Intensity or Flat Shading One color for the entire triangle Fast Good for some objects What happens if triangles are small? Sudden intensity changes at borders
- 46. Gouraud Shading Intensity Interpolation Shading Calculate lighting at the vertices. Then interpolate the colors as you scan convert
- 47. Gouraud Shading Relatively fast, only do three calculations No sudden intensity changes What can it not do? What are some approaches to fix this? Question, what is the normal at a vertex?
- 48. Phong Shading Interpolate the normal, since that is the information that represents the “curvature” Linearly interpolate the vertex normals. For each pixel, as you scan convert, calculate the lighting per pixel. True “per pixel” lighting Not done by most hardware/libraries/etc
- 49. Shading Techniques Constant Shading Calculate one lighting calculation (pick a vertex) per triangle Color the entire triangle the same color Gouraud Shading Calculate three lighting calculations (the vertices) per triangle Linearly interpolate the colors as you scan convert Phong Shading While you scan convert, linearly interpolate the normals. With the interpolated normal at each pixel, calculate the lighting at each pixel