I total = k s I ambient + I i ( i = 1 7 8 Lighting in OpenGL - PDF document

University of British Columbia News: Homework CPSC 314 Computer Graphics � homework correction: questions 13-16 should Jan-Apr 2005 use: Tamara Munzner � unit square has points A=(0,0,0,1), B=(0,1,0,1), C=(0,1,1,1), D=(0,0,1,1) in world coordinates Lighting and Shading � homework clarification: question 1 � C_i is down one-half unit and sideways one Week 5, Wed Feb 2 unit. http://www.ugrad.cs.ubc.ca/~cs314/Vjan2005 2 News: Project Handin Review: Reflectance � when handing after the deadline, handin has this � specular : perfect mirror with no scattering unfriendly warning message � gloss : mixed, partial specularity � Checking that handin was successful ... /cs/csbox/user FAILED to find user a1b2. Your files � diffuse : all directions with equal energy DO NOT appear to be handed in successfully � Do you want to cancel? + + = � don’t panic � go ahead and complete the handin, do not cancel! specular + glossy + diffuse = � your submission will be put in the LATE directory reflectance distribution 3 4 Review: Reflection Equations Clarification: Calculating The R Vector N l n P � P = N cos � = projection of L onto N I diffuse = k d I light (n • l) L R � why is P = N cos � , not L cos � ? � � � N and R and L are unit length � difference between I specular = k s I light ( v • r ) n shiny � length of projection of u onto v u � scalar: |u| cos � � � in this case length of u is 1 v � cos � u cos scalar length! � � projection of u onto v � vector in direction of v, with scale factor � scale depends on angle between u and v, length of u 2 ( N ( N · L )) – L = R � v |u| cos � � in this case length of u is 1 � v cos � 5 6

Review: Reflection Equations 2 Review: Lighting � lighting models n n h h � Blinn improvement v v � ambient I specular = k s I light ( h • n ) n shiny l l � normals don’t matter � Lambert/diffuse h = ( l + v )/2 � angle between surface normal and light � full Phong lighting model � Phong/specular � combine ambient, diffuse, specular components � surface normal, light, and viewpoint # lights k d ( n • l i ) + k s ( v • r i ) n shiny ) � I total = k s I ambient + I i ( i = 1 7 8 Lighting in OpenGL Lighting in OpenGL !"#$!%&'()*#+#,*-./01*#+234,56.01789+"$!%&+:!971;< � light source: amount of RGB light emitted !"#$!%&'()*#+#,*-./01*#+=,>>?@501A$'+"$!%&+:!971;< � value represents percentage of full intensity !"#$!%&'()*#+#,*-./01*#+@B5C?#2D01EFGH+"$!%&+:!971;< e.g., (1.0,0.5,0.5) !"#$!%&'()*#+#,*-./01*#+BI@,.,I601FJE$&$JK;< !"5K79"G)*#+#,*-./;< � every light source emits ambient, diffuse, and specular light !"37&G:$7"'()1*#+>DI6.01*#+234,56.01789$GK&+:!971;< !"37&G:$7"'()1*#+>DI6.01*#+=,>>?@501A$''LEG+:!971;< � materials: amount of RGB light reflected !"37&G:$7"'()1*#+>DI6.01*#+@B5C?#2D01EFGHL"7:+:!971;< � value represents percentage reflected !"37&G:$7"'()1*#+>DI6.01*#+@-,6,65@@01K1;< e.g., (0.0,1.0,0.5) � warning: glMaterial is expensive and tricky � interaction: multiply components � use cheap and simple glColor when possible � red light (1,0,0) x green surface (0,1,0) = black (0,0,0) � see OpenGL Pitfall #14 from Kilgard’s list http://www.opengl.org/resources/features/KilgardTechniques/oglpitfall/ 9 10 Lighting vs. Shading Applying Illumination � lighting � we now have an illumination model for a point � process of computing the luminous intensity on a surface (i.e., outgoing light) at a particular 3-D point, � if surface defined as mesh of polygonal facets, usually on a surface which points should we use? � shading � fairly expensive calculation � the process of assigning colors to pixels � several possible answers, each with different implications for visual quality of result � (why the distinction?) 11 12

Applying Illumination Flat Shading � polygonal/triangular models � simplest approach calculates illumination at a single point for each polygon � each facet has a constant surface normal � if light is directional, diffuse reflectance is constant across the facet. � why? � obviously inaccurate for smooth surfaces 13 14 Flat Shading Approximations Improving Flat Shading � if an object really is faceted, � what if evaluate Phong lighting model at each pixel of the polygon? is this accurate? � better, but result still clearly faceted � no! � for point sources, the � for smoother-looking surfaces direction to light varies we introduce vertex normals at each across the facet vertex � usually different from facet normal � for specular reflectance, � used only for shading direction to eye varies � think of as a better approximation of the real surface across the facet that the polygons approximate 15 16 Vertex Normals Gouraud Shading � vertex normals may be � most common approach, and what OpenGL does � provided with the model � perform Phong lighting at the vertices � computed from first principles � linearly interpolate the resulting colors over faces � along edges � approximated by � along scanlines edge: mix of c 1 , c 2 C 1 averaging the normals of the facets that does this eliminate the facets? share the vertex C 3 C 2 interior: mix of c1, c2, c3 edge: mix of c1, c3 17 18

Gouraud Shading Artifacts Gouraud Shading Artifacts � Mach bands � often appears dull, chalky � eye enhances discontinuity in first derivative � lacks accurate specular component � very disturbing, especially for highlights � if included, will be averaged over entire polygon C 1 C 1 C 3 C 3 C 2 this vertex shading spread C 2 this interior shading missed! over too much area 19 20 Gouraud Shading Artifacts Gouraud Shading Artifacts � Mach bands � perspective transformations � affine combinations only invariant under affine, C 1 not under perspective transformations � thus, perspective projection alters the linear C 4 interpolation! C 3 Image plane C 2 Discontinuity in rate of color change occurs here Z – into the scene 21 22 Gouraud Shading Artifacts Phong Shading � perspective transformation problem � linearly interpolating surface normal across the � colors slightly “swim” on the surface as objects facet, applying Phong lighting model at every move relative to the camera pixel � usually ignored since often only small difference � same input as Gouraud shading � usually smaller than changes from lighting � pro: much smoother results variations � con: considerably more expensive � to do it right � not the same as Phong lighting � either shading in object space � common confusion � or correction for perspective foreshortening � Phong lighting: empirical model to calculate illumination at a point on a surface � expensive – thus hardly ever done for colors 23 24

Phong Shading Phong Shading Difficulties � linearly interpolate the vertex normals � computationally expensive � compute lighting equations at each pixel � per-pixel vector normalization and lighting computation! � can use specular component � floating point operations required # lights ( n shiny ) � ( ) + k s v � r i ( ) � lighting after perspective projection I total = k a I ambient + I i k d n � l i N 1 i = 1 � messes up the angles between vectors remember: normals used in diffuse and specular terms � have to keep eye-space vectors around N 4 � no direct support in hardware N 3 discontinuity in normal’s rate of � but can be simulated with texture mapping change harder to detect N 2 25 26 Shading Artifacts: Silhouettes Shading Artifacts: Orientation � interpolation dependent on polygon orientation � polygonal silhouettes remain view dependence! � A Rotate -90 o B and color same point C B A D � D Gouraud Phong C Interpolate between Interpolate between CD and AD AB and AD 27 28 Shading Artifacts: Shared Vertices Shading Models Summary � flat shading � compute Phong lighting once for entire vertex B shared by two rectangles on the right, but not by the one on polygon C H the left D � Gouraud shading � compute Phong lighting at the vertices and first portion of the scanline interpolate lighting values across polygon B G is interpolated between DE and AC � Phong shading � compute averaged vertex normals second portion of the scanline is interpolated between BC and GH � interpolate normals across polygon and F E A perform Phong lighting across polygon a large discontinuity could arise 29 30

Shutterbug: Flat Shading Shutterbug: Gouraud Shading 31 32 Shutterbug: Phong Shading Non-Photorealistic Shading � draw silhouettes: if , e =edge-eye vector ( e � n 0 )(e � n 1 ) � 0 k w = 1 + n � l � cool-to-warm shading: , c = k w c w + (1 � k w ) c c 2 http://www.cs.utah.edu/~gooch/SIG98/paper/drawing.html 33 34