Illumination and Shading Jian Huang CS594 Fall 2001 This set of slides reference slides used at Ohio State for instruction by Prof Machiraju and Prof Han Wei Shen Illumination Vs Shading Illumination lighting model determine the color of a surface point by simulating some light attributes Shading model applies the illumination models at a set of points and colors the whole image Illumination Lighting Model To model the interaction of light with surfaces to determine the final color brightness of the surface Global illumination Local illumination Global Illumination Global Illumination models take into account the interaction of light from all the surfaces in the scene will cover under the Radiosity section Local illumination Only consider the light the observer position and the object material properties Basic Illumination Model Simple and fast method for calculating surface intensity at a given point Lighting calculation are based on The background lighting conditions The light source specification color position Optical properties of surfaces Glossy OR matte Opaque OR transparent control refection and absorption Ambient light background light The light that is the result from the light reflecting off other surfaces in the environment A general level of brightness for a scene that is independent of the light positions or surface directions ambient light Has no direction Each light source has an ambient light contribution Ia For a given surface we can specify how much ambient light the surface can reflect using an ambient reflection coefficient Ka 0 Ka 1 Ambient Light So the amount of light that the surface reflect is therefore Iamb Ka Ia Diffuse Light The illumination that a surface receives from a light source and reflects equally in all directions This type of reflection is called Lambertian Reflection thus Lambertian surfaces The brightness of the surface is indepenent of the observer position since the light is reflected in all direction equally Lambert s Law How much light the surface receives from a light source depends on the angle between its angle and the vector from the surface point to the light light vector Lambert s law the radiant energy Id from a small surface da for a given light source is Id IL cos IL the intensity of the light source is the angle between the surface normal N and light vector L The Diffuse Component Surface s material property assuming that the surface can reflect Kd 0 Kd 1 diffuse reflection coefficient amount of diffuse light Idiff Kd IL cos If N and L are normalized cos N L Idiff Kd IL N L The total diffuse reflection ambient diffuse Idiff Ka Ia Kd IL N L Examples Sphere diffusely lighted from various angles Specular Light These are the bright spots on objects such as polished metal apple Light reflected from the surface unequally to all directions The result of near total reflection of the incident light in a concentrated region around the specular reflection angle Phong s Model for Specular How much reflection light you can see depends on where you are Phong Illumination Curves Specular exponents are much larger than 1 Values of 100 are not uncommon n glossiness rate of falloff Specular Highlights Shiny surfaces change appearance when viewpoint is changed Specularities are caused by microscopically smooth surfaces A mirror is a perfect specular reflector Reflected Ray N L How to calculate R R L 2 N L N R V R 2 N L N L 2N N L L N N L Project L onto N L L Double length of vector R 2N N L L Subtract L Half Vector An alternative way of computing phong lighting is Is ks Is N H n H halfway vector halfway between V and L V L 2 L Fuzzier highlight N H V Phong Illumination Moving Light Change n Putting It All Together Single Light white light source Multiple Light Source IL light intensity For multiple light sources Repeat the diffuse and specular calculations for each light source Add the components from all light sources The ambient term contributes only once The different reflectance coefficients can differ Simple metal ks and kd share material color Simple plastic ks is white Remember when cosine is negative lighting term is zero OpenGL Materials GLfloat white8 8 8 8 1 white2 2 2 2 1 black 0 0 0 GLfloat mat shininess 50 Phong exponent glMaterialfv GL FRONT AND BACK GL AMBIENT black glMaterialfv GL FRONT AND BACK GL DIFFUSE white8 glMaterialfv GL FRONT AND BACK GL SPECULAR white2 glMaterialfv GL FRONT AND BACK GL SHININESS mat shininess OpenGL Lighting GLfloat white 1 1 1 1 GLfloat light0 position 1 1 5 0 directional light w 0 glLightfv GL LIGHT0 GL POSITION light0 position glLightfv GL LIGHT0 GL DIFFUSE white glLightfv GL LIGHT0 GL SPECULAR white glEnable GL LIGHT0 glEnable GL NORMALIZE normalize normal vectors glLightModeli GL LIGHT MODEL TWO SIDE GL TRUE two sided lighting glEnable GL LIGHTING Shading Models for Polygons Constant Shading flat shading Compute illumination at any one point on the surface Use face or one normal from a pair of edges Good for far away light and viewer or if facets approximate surface well Per Pixel Shading Compute illumination at every point on the surface Interpolated Shading Compute illumination at vertices and interpolate color Constant Shading Compute illumination only at one point on the surface Okay to use if all of the following are true The object is not a curved smooth surface e g a polyhedron object The light source is very far away so N L does not change much across a polygon The eye is very far away so V R does not change much across a polygon The surface is quite small close to pixel size Un lit Flat Shading Mach Band Polygon Mesh Shading Shading each polygonal facet individually will not generate an illusion of smooth curved surface Reason polygons will have different colors along the boundary unfortunately human perception helps to even accentuate the discontinuity mach band effect Mach Banding Intensity change is exagerated Dark facet looks darker and lighter looks even more lighter Smooth Shading Need to have per vertex normals Gouraud Shading Interpolate color across triangles Fast supported by most of the graphics accelerator cards Phong Shading Interpolate normals across triangles More accurate but slow Not widely supported by hardware Gouraud Shading Normals are computed at the polygon vertices If we only have per face normals the normal at each vertex is the average of the normals of its adjacent faces Intensity interpolation linearly interpolate the pixel intensity color across a polygon
View Full Document
Unlocking...