Fundamentals of Computer Graphics Part 6 Shading prof.ing.Václav Skala, CSc. University of West Bohemia Plzeň, Czech Republic ©2002 Prepared with Angel,E.: Interactive Computer Graphics – A Top Down Approach with OpenGL, Addison Wesley, 2001

Shading – Light and Matter Our perception depends on: light directly – intensity, spectrum (color), position material of objects that “reflects” or “transmit” light, roughness, color of the surface Speed of computation – significant factor Fundamentals of Computer Graphics

Shading – Light and Matter Generally we do not need to compute all, but just those rays that contribute to the final image Methods: Global – ray tracing, radiosity – very slow Local – constant, Gouraud, Phong etc. – relatively fast Fundamentals of Computer Graphics

Shading – Light and Matter Interaction between light and materials can be classified as specular surfaces – ideal mirror diffuse surfaces – reflected light is ideally reflected to all directions uniformly translucent surfaces – allow some lights to penetrate the surface – refraction – glass, watter optical properties – Snell’s law Fundamentals of Computer Graphics

Shading – Light sources Light source – an object with a surface Each point (x,y,z) on the surface can emit light with characterization: direction of emission (,) intensity of energy emitted at each wavelength  illumination function I(x,y,z,,,) Basic light sources (sufficient for rendering the most simple scenes): ambient lighting point sources spotlights distance light Fundamentals of Computer Graphics

Shading – Light sources Light - an object with a surface Each point (x,y,z) on the surface can emit light with characterization: direction of emission (,) intensity of energy emitted at each wavelength  illumination function I(x,y,z,,,) Usually I = [ Ir , Ig , Ib ]T is handled as a scalar value Basic light sources - sufficient for rendering the most simple scenes: ambient lighting point sources spotlights distance light Fundamentals of Computer Graphics

Shading – Light sources Ambient light Ia- uniform light in the space (room etc.) Ideal point source – emits equally in all directions I(p0) Light received at a point p full shadow – umbra partial shadow – penumbra for non-point sources (d is distance) Fundamentals of Computer Graphics

Shading – Light sources Spotlights – very narrow angles of emission, if  = 180° -> point source distribution of light within the cone – usually cose() ; e determines how rapidly intensity drops off cos() = sT l s – vector that points from ps to a point s on a surface l – vector of the light direction Fundamentals of Computer Graphics

Shading – Distant light sources we replace location of light sources with their directions p0 = [ x , y , z , 0 ]T ( 0 is correct !) Phong Reflection Model I = Ia + Id + Is disadvantages linear model superposition Fundamentals of Computer Graphics

Ambient, Diffuse, Specular Reflections Ambient reflection 0  ka  1 Ia = ka La global ambient term or light Diffuse reflection characterized by rough surfaces perfectly diffuse surfaces – Lambertian surfaces Fundamentals of Computer Graphics

Ambient, Diffuse, Specular Reflections Lambert’s law: diffuse reflection 0  kd  1 if the influence of the distance is considered Fundamentals of Computer Graphics

Ambient, Diffuse, Specular Reflections Specular Reflection: 0  ks  1 as  ideal specular reflection   <100 , 500> metallic surfaces Fundamentals of Computer Graphics

Fundamentals of Computer Graphics Polygonal Shading How to display surfaces with shading? Flat (constant) shading glShadeModel(GL_FLAT); Mach bands Fundamentals of Computer Graphics

Fundamentals of Computer Graphics Polygonal Shading Interpolative and Gouraud shading Gouraud shading glShadeModel(GL_SMOOTH); normal in a vertex average normal vector intensity computation for a vertex intensity & color interpolation for a scan-line Fundamentals of Computer Graphics

Fundamentals of Computer Graphics Polygonal Shading Phong shading normal in a vertex interpolation of a normal normal interpolation along the scan-line intensity computation Phong shading is almost always done off-line Fundamentals of Computer Graphics

Light Sources in OpenGL Study 6.6 – 6.9 on your own Fundamentals of Computer Graphics

Global Rendering – Ray tracing global versus local lightings models Fundamentals of Computer Graphics

Global Rendering – Ray tracing Algorithm complexity: O(M2 N 2k) M – resolution of a screen N – number of objects k – number of levels of the tree Typical program: POV Ray – available free Fundamentals of Computer Graphics