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Illumination and Shading. Mengxia Zhu Fall 2007. Why?. What light source is used and how the object response to the light makes difference Lighting gives you a 3D view to an object A unlit sphere looks no different from a 2D disk
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Illumination and Shading Mengxia Zhu Fall 2007 .
Why? • What light source is used and how the object response to the light makes difference • Lighting gives you a 3D view to an object • A unlit sphere looks no different from a 2D disk • To get realistic pictures, the color computation of pixels must include lighting calculations
Local and Global Illumination • Local illumination: a surface point receives light directly from all light sources in the scene. Consider in isolation • Global illumination: a point receives light after light rays interact with other objects in the scene. • Shadow • Refract through transparent object • Reflection and transmission rays
Illumination Vs. Shading • Illumination (lighting) model: To model the interaction of light with surfaces to determine the final color & brightness of the surface • Shading model: applies the illumination models at a set of points and colors the whole image.
Local illumination • Only consider the light, the observer position, and the object material properties V L
Types of Light • Ambient • Light that’s been scattered so much by the environment that its direction is impossible to determine - it seems to come from all directions • Diffuse • Light that comes from one direction, but it gets scattered equally in all directions • Specular • Light comes from a particular direction, and its tends to bounce off the surface in a preferred direction
OpenGL Lighting Model • OpenGL approximates lighting as if light can be broken into red, green, and blue components • The RGB values for lights mean different than for materials • For light, the numbers correspond to a percentage of full intensity for each color • For materials, the numbers correspond to the reflected proportions of those colors
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 • 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 • Ambient, omni-directional light • So the amount of light that the surface reflect is therefore • Ka ambient coefficient Iamb =
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 • The brightness of the surface is independent of the observer position (since the light is reflected in all direction equally) L
Lambert’s Law • Lambert’s law: the radiant energy for a given light source is: Id * cos(q) Id : the intensity of the light source • is the angle between the surface normal (N) and light vector (L) If N and L are normalized, cos(q) = • Surface’s material property: assuming that the surface can reflect Kd (0<Kd<1), diffuse reflection coefficient) amount of diffuse light.
The Diffuse Component • Brightness is inversely proportional to the area of the object illuminated (dot product of light vector and surface normal) • greatest when N and L are parallel • smallest when N and L are orthogonal • In calculations, max {N.L, 0} is used to avoid negative values • The total diffuse reflection = ambient + diffuse
Specular Light • These are the bright spots on objects (such as polished metal, apple ...) • Light reflected from the surface unequally to all directions.
Phong’s Model for Specular • Brightness depends on the angle between reflection vector (R) and viewer vector (V), i.e, on direction of viewer
Summing the Components Ambient + Diffuse + Specular = final
Phong Illumination Curves • Specular exponents are much larger than 1, and Values of 100 are not uncommon. • The specular reflection exponent n is 1 for a slightly glossy surface and infinity for a perfect mirror. : glossiness, rate of falloff
2N(N•L) N(N•L) L L L R = 2N(N•L) - L Project L onto N Double length of vector Subtract L f f f f f f Reflected Ray N L How to calculate R? R + L = 2(N*L) N R = 2(N*L) N - L R a V
N L H V Half Vector • An alternative way of computing phong lighting is: • H (halfway vector): halfway between V and L: (V+L)
Putting It All Together • Single Light:
Attenuation • Attenuation factor • Light attenuates with distance from the source • where d = distance between the light and object • kc = constant attenuation • A light source does not give an infinite amount of light • kl = linear term • kq = quadratic term • Models the theoretical attenuation from a point source • The intensity becomes
Spotlight Effect • When the vertex lies inside the cone of illumination produced by spotlight, its contribution to the light intensity is Where D gives the spotlight’s direction. The intensity is maximum in the center of cone and is attenuated toward the edge of the cone s is 1 if the source is not spotlight m is exponent determining the concentration of the light The intensity of light source is
Putting All Together Multiple lighting calculation in RGB mode gives Vertex color
Adding Lighting to the Scene • Define normal vectors for each vertex of each object • Create, select, and position one or more light sources • Create and select a lighting model • Define material properties for the objects in the scene
Creating Light Sources • Properties of light sources are color, position, and direction • void glLight{if}(GLenum light, GLenum pname, TYPE param); • void glLight{if}v(GLenum light, GLenum pname, TYPE *param); • Creates the light specified by light that can be GL_LIGHT0, GL_LIGHT1, … or GL_LIGHT7 • Pname specifies the characteristics of the light being set • Param indicates the values to which the pname characteristic is set • glEnable(GL_LIGHT0);
Color for a Light Source • GLfloat light_ambient[] = {0.0,0.0,0.0,1.0}; • GLfloat light_diffuse[] = {1.0,1.0,1.0,1.0}; • GLfloat light_specular[] = {1.0,1.0,1.0,1.0}; • glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient); • glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse); • glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);
Position of Light Source • Positional light source • (x, y, z) values specify the location of the light • GLfloat light_position[] = {x, y, z, w}; • glLightfv(GL_LIGHT0, GL_POSITION, light_position); • Directional light source • (x, y, z) values specify the direction of the light located at the infinity • No attenuation • GLfloat light_position[] = {x, y, z, 0}; • glLightfv(GL_LIGHT0, GL_POSITION, light_position);
Attenuation • Attenuation factor for a positional light • Needs to specify three coefficients • glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, 2.0); • glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, 1.0); • glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 1.0); • Ambient, diffuse, and specular contributions are all attenuated
Spotlights • The shape of the light emitted is restricted to a cone • glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 45.0); • The cutoff parameter is set to 45 degrees • GLfloat spot_direction[] = {-1.0, -1.0, 0.0]; • glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, spot_direction); • specifies the spotlight’s direction which determines the axis of the cone of light • glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 2.0); • Controls how concentrated the light is
Multiple Lights • You can define up to eight light sources • Need to specify all the parameters defining the position and characteristics of the light • OpenGL performs calculations to determine how much light each vertex gets from each source • Increasing number of lights affects performance
Defining Material Properties • Specifying the ambient, diffuse, and specular colors, the shininess, and the color of any emitted light • void glMaterial{if}(GLenum face, GLenum pname, TYPE param); • void glMaterial{if}v(GLenum face, GLenum pname, TYPE *param); • Specifies a current material property for use in lighting calculations • Face can be GL_FRONT, GL_BACK, or GL_FRONT_AND_BACK • Pname identifies the particular material property being set • Param defines the desired values for that property
Reflectance • Diffuse and ambient reflection • Gives color • GLfloat mat_amb_diff[] = {0.1, 0.5,0.8,1.0}; • glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, mat_amb_diff); • Specular reflection • Produces highlights • GLfloat mat_specular[] = {1.0,1.0,1.0,1.0); • Glfloat low_shininess[] = {5.0}; • glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular); • glMaterialfv(GL_FRONT, GL_SHININESS, low_shininess);
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)
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
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 surface
Linear Interpolation • Calculate the value of a point based on the distances to the point’s two neighbor points • If v1 and v2 are known, then x = b/(a+b) * v1 + a/(a+b) * v2
Linear Interpolation in a Triangle • To determine the intensity (color) of point P in the triangle, • we will do: • determine the intensity of 4 by linearly interpolating between 1 and 2 • determine the intensity of 5 by linearly interpolating between 2 and 3 • determine the intensity of P by linear interpolating between 4 and 5
Phong Shading Model • Gouraud shading does not properly handle specular highlights, specially when the n parameter is large (small highlight). • Reason: colors are interpolated. • Solution: (Phong Shading Model) • 1. Compute averaged normal at vertices. • 2. Interpolate normals along edges and scan-lines. (component by component) • 3. Compute per-pixel illumination.
References • Slides from Karki at LSU • Jian Huang at UTK