University of Texas at Austin CS 378 – Game Technology Don Fussell CS 378: Computer Game Technology Basic Rendering Pipeline and Shading Spring 2012Rendering ! Recall the standard graphics pipeline: University of Texas at Austin CS 378 – Game Technology Don Fussell 2Normal Vectors ! The intensity of a surface depends on its orientation with respect to the light and the viewer ! CDs are an extreme example ! The surface normal vector describes the orientation of the surface at a point ! Mathematically: Vector that is perpendicular to the tangent plane of the surface ! What’s the problem with this definition? ! Just “the normal vector” or “the normal” ! Will use N to denote University of Texas at Austin CS 378 – Game Technology Don Fussell 3Local Shading Models ! Local shading models provide a way to determine the intensity and color of a point on a surface ! The models are local because they don’t consider other objects at all ! We use them because they are fast and simple to compute ! They do not require knowledge of the entire scene, only the current piece of surface University of Texas at Austin CS 378 – Game Technology Don Fussell 4Local Shading Models (Watt 6.2) ! What they capture: ! Direct illumination from light sources ! Diffuse and Specular components ! (Very) Approximate effects of global lighting ! What they don’t do: ! Shadows ! Mirrors ! Refraction ! Lots of other stuff … University of Texas at Austin CS 378 – Game Technology Don Fussell 5“Standard” Lighting Model ! Consists of several simple terms linearly combined: ! Diffuse component for the amount of incoming light reflected equally in all directions ! Specular component for the amount of light reflected in a mirror-like fashion ! Ambient term to approximate light arriving via other surfaces University of Texas at Austin CS 378 – Game Technology Don Fussell 6Diffuse Illumination ! Incoming light, Ii, from direction L, is reflected equally in all directions ! No dependence on viewing direction ! Amount of light reflected depends on: ! Angle of surface with respect to light source ! Actually, determines how much light is collected by the surface, to then be reflected ! Diffuse reflectance coefficient of the surface, kd ! Don’t want to illuminate back side. Use University of Texas at Austin CS 378 – Game Technology Don Fussell 7 )( NL •idIk)0,ma x( NL •idIkDiffuse Example University of Texas at Austin CS 378 – Game Technology Don Fussell 8 Where is the light source?Specular Reflection (Phong Model) ! Incoming light is reflected primarily in the mirror direction R ! Perceived intensity depends on the relationship between the viewing direction V and the mirror direction R ! Bright spot is called a specular highlight ! Intensity controlled by: ! The specular reflectance coefficient ks ! The parameter n controls the apparent size of the specular highlight ! Higher n, smaller highlight University of Texas at Austin CS 378 – Game Technology Don Fussell 9 ksIi(R • V)nL!R!V!Specular Example University of Texas at Austin CS 378 – Game Technology Don Fussell 10Putting It Together ! Global ambient intensity, Ia: ! Gross approximation to light bouncing around of all other surfaces ! Modulated by ambient reflectance ka ! Emitted term Ie – no reflected light, comes from object ! Just sum all the terms ! If there are multiple lights, sum contributions from each light ! Several variations, and approximations … University of Texas at Austin CS 378 – Game Technology Don Fussell 11 I = Ie+ kaIa+ Iikd(Li•N) + ks(Ri•N)n( )lights i∑Flat shading ! Compute shading at a representative point and apply to whole polygon ! OpenGL uses one of the vertices ! Advantages: ! Fast - one shading value per polygon ! Disadvantages: ! Inaccurate ! Discontinuities at polygon boundaries University of Texas at Austin CS 378 – Game Technology Don Fussell 12Gourand Shading ! Shade each vertex with it’s own location and normal ! Linearly interpolate across the face ! Advantages: ! Fast - incremental calculations when rasterizing ! Much smoother - use one normal per shared vertex to get continuity between faces ! Disadvantages: ! Specular highlights get lost University of Texas at Austin CS 378 – Game Technology Don Fussell 13Phong Interpolation ! Interpolate normals across faces ! Shade each pixel ! Advantages: ! High quality, narrow specular highlights ! Disadvantages: ! Expensive ! Still an approximation for most surfaces ! Not to be confused with Phong’s shading model University of Texas at Austin CS 378 – Game Technology Don Fussell 14University of Texas at Austin CS 378 – Game Technology Don Fussell
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