Why Do We Care About Lighting? Computer Graphics Lighting. The Surface Normal. Flat Shading (Per-face) Setting a Surface Normal in OpenGL

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1 Lightig Why Do We Care About Lightig? Lightig dis-ambiguates 3D scees This work is licesed uder a Creative Commos Attributio-NoCommercial- NoDerivatives 4.0 Iteratioal Licese Mike Bailey mjb@cs.oregostate.edu Without lightig Lightig.pptx With lightig The Surface Normal 3 Settig a Surface Normal i OpeGL 4 A surface ormal is a vector perpedicular to the surface. Sometimes surface ormals are defied or computed per-face. = ( P - P0 ) x ( P P0 ) Sometimes they are defied per-vertex to best approximate the uderlyig surface that the face is represetig. 0 0 glnormal3f( x, y, z ); glbegi( GL_TRIANGLES ); glvertex3f( x, y, z ); glvertex3f( x, y, z ); Per-face Settig a Surface Normal i OpeGL 5 Flat Shadig (Per-face) 6 glbegi(gl_triangles ); glnormal3f( x0, y0, z0 ); glnormal3f( x, y, z ); glvertex3f( x, y, z ); glnormal3f( x, y, z ); glvertex3f( x, y, z ); Per-vertex glshademodel( GL_FLAT ); glnormal3f( x, y, z ); glbegi(gl_triangles ); glvertex3f( x, y, z ); glvertex3f( x, y, z );

Smooth Shadig (Per-vertex) 7 8 glshademodel( GL_SMOOTH ); glbegi(gl_triangles ); glnormal3f( x0, y0, z0 ); glnormal3f( x, y, z ); glvertex3f( x, y, z ); glnormal3f( x, y, z ); glvertex3f( x, y, z );

ot, or if you are usig scalig trasformatios, you ca force OpeGL to do the uitizig for you with: gleable( GL_NORMALIZE ); P P I L R E Poit beig illumiated Light itesity Uit vector from poit to light

surface area. Ambiet = a costat Accouts for light boucig everywhere Diffuse = I*cosΘ. Diffuse = I*cosΘ Accouts for the agle betwee the icomig light ad the surface ormal 3.

2 Smooth Shadig (Per-vertex) 7 8 glshademodel( GL_SMOOTH ); glbegi(gl_triangles ); glnormal3f( x0, y0, z0 ); glnormal3f( x, y, z ); glvertex3f( x, y, z ); glnormal3f( x, y, z ); glvertex3f( x, y, z ); 9 OpeGL Surface Normals Need to be Uitized by Someoe The OpeGL built-i Lightig Model 0 glnormal3f( x, y, z ); R OpeGL expects the ormal vector to be a uit vector, that is: x + y + z = I L E If it is ot, or if you are usig scalig trasformatios, you ca force OpeGL to do the uitizig for you with: gleable( GL_NORMALIZE ); P P I L R E Poit beig illumiated Light itesity Uit vector from poit to light Uit vector surface ormal Perfect reflectio uit vector Uit vector to eye positio The OpeGL built-i Lightig Model Diffuse Lightig works because of spreadig out the same amout of light eergy across more surface area. Ambiet = a costat Accouts for light boucig everywhere Diffuse = I*cosΘ. Diffuse = I*cosΘ Accouts for the agle betwee the icomig light ad the surface ormal 3. Specular = I*cos S Accouts for the agle betwee the perfect reflector ad the eye. The expoet, S, accouts for surface shiiess Note that cosθ is just the dot product betwee uit vectors L ad Note that cos is just the dot product betwee uit vectors R ad E

The Specular Lightig equatio is a heuristic that approximates reflectio from a rough surface

Specular E = Poit Types of Light Sources 5 Poit Light at the Eye Lightig Examples 6 Directioal

Colored Objects 8 Spot Lights L R L G L B White Light What the light ca produce E R E G E B What

3 The Specular Lightig equatio is a heuristic that approximates reflectio from a rough surface Specular = I*cos S 3 Ambiet + The Three Elemets of Lightig 4 S shiiess /S roughess Diffuse + R Specular E = Poit Types of Light Sources 5 Poit Light at the Eye Lightig Examples 6 Directioal (Parallel, Su) Poit Light at the Origi Spotlight Lightig Examples 7 Colored Lights Shiig o Colored Objects 8 Spot Lights L R L G L B White Light What the light ca produce E R E G E B What the eye sees M R M G M B What the material ca reflect Gree Light E R = L R * M R E G = L G * M G E B = L B * M B 3

4 Too May Lightig Optios 9 Ways to Simplify Too May Lightig Optios 0 If there is oe light ad oe material, the followig thigs ca be set idepedetly: Global scee ambiet red, gree, blue Light positio: x, y, z Light ambiet red, gree, blue Light diffuse red, gree, blue Light specular red, gree, blue Material reactio to ambiet red, gree, blue Material reactio to diffuse red, gree, blue Material reactio to specular red, gree, blue Material specular shiiess. Set the ambiet light globally usig, for example, gllightmodelfv( GL_LIGHT_MODEL_AMBIENT, MulArray3(.3f, White ) ) I.e., set it to some low itesity of white.. Set the light s ambiet compoet to zero. 3. Set the light s diffuse ad specular compoets to the full color of the light. 4. Set each material s ambiet ad diffuse to the full color of the object. 5. Set each material s specular compoet to some fractio of white. This makes for 5 thigs that ca be set for just oe light ad oe material! While may combiatios are possible, some make more sese tha others. float White[ ] =.,.,.,. ; // utility to create a array from 3 separate values: Settig the Material Characteristics float * Array3( float a, float b, float c ) static float array[4]; array[0] = a; array[] = b; array[] = c; array[3] =.; retur array; // utility to create a array from a multiplier ad a array: float * MulArray3( float factor, float array0[3] ) static float array[4]; glmaterialfv( GL_BACK, GL_AMBIENT, MulArray3(.4, White ) ); glmaterialfv( GL_BACK, GL_DIFFUSE, MulArray3(., White ) ); glmaterialfv( GL_BACK, GL_SPECULAR, Array3( 0., 0., 0. ) ); glmaterialf ( GL_BACK, GL_SHININESS, 5. ); glmaterialfv( GL_BACK, GL_EMISSION, Array3( 0., 0., 0. ) ); glmaterialfv( GL_FRONT, GL_AMBIENT, MulArray3(., rgb ) ); glmaterialfv( GL_FRONT, GL_DIFFUSE, MulArray3(., rgb ) ); glmaterialfv( GL_FRONT, GL_SPECULAR, MulArray3(.7, White ) ); glmaterialf ( GL_FRONT, GL_SHININESS, 8. ); glmaterialfv( GL_FRONT, GL_EMISSION, Array3( 0., 0., 0. ) ); array[0] = factor * array0[0]; array[] = factor * array0[]; array[] = factor * array0[]; array[3] =.; retur array; Settig the Light Characteristics 3 Settig the Light Positio 4 gllightmodelfv( GL_LIGHT_MODEL_AMBIENT, MulArray3(., White ) ); gllightmodeli ( GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE ); gllightfv( GL_LIGHT0, GL_AMBIENT, Array3( 0., 0., 0. ) ); gllightfv( GL_LIGHT0, GL_DIFFUSE, LightColor ); gllightfv( GL_LIGHT0, GL_SPECULAR, LightColor ); glloadidetity( ); The light positio gets trasformed by the value of the ModelView matrix at the momet the light positio is set. // if we do this, the the light will be wrt the scee at XLIGHT, YLIGHT, ZLIGHT: gllightfv( GL_LIGHT0, GL_POSITION, Array3(XLIGHT, YLIGHT, ZLIGHT) ); You ca have multiple lights gllightf ( GL_LIGHT0, GL_CONSTANT_ATTENUATION,. ); gllightf ( GL_LIGHT0, GL_LINEAR_ATTENUATION, 0. ); gllightf ( GL_LIGHT0, GL_QUADRATIC_ATTENUATION, 0. ); // this is here because we are goig to do object (ad thus ormal) scalig: gleable( GL_NORMALIZE ); // traslate the object ito the viewig volume: glulookat( XEYE, YEYE, ZEYE, 0., 0., 0., 0.,., 0. ); // if we do this, the the light will be wrt the eye at XLIGHT, YLIGHT, ZLIGHT: Atteuatio = where d is the distace from the light to the poit beig lit // gllightfv( GL_LIGHT0, GL_POSITION, Array3(XLIGHT, YLIGHT, ZLIGHT) ); 4

5 // perform the rotatios ad scalig about the origi: glrotatef( Xrot,., 0., 0. ); glrotatef( Yrot, 0.,., 0. ); glscalef( Scale, Scale, Scale ); // if we do this, the the light will be wrt to the object at XLIGHT, YLIGHT, ZLIGHT: // gllightfv( GL_LIGHT0, GL_POSITION, Array3(XLIGHT, YLIGHT, ZLIGHT) ); // specify the shadig model: glshademodel( GL_SMOOTH ); // eable lightig: gleable( GL_LIGHTING ); gleable( GL_LIGHT0 ); // draw the objects:... You ca eable ad disable lightig at all. (This toggles betwee usig the lightig equatios say ad what glcolor3f( ) says You ca eable ad disable each light idepedetly It is usually good form to disable the lightig after you are doe usig it 5 Sidebar: Why are Light Positios 4-elemet arrays where the 4 th elemet is.0? Homogeeous Coordiates! float * Array3( float a, float b, float c ) static float array[4]; array[0] = a; array[] = b; array[] = c; array[3] =.; retur array; We usually thik of a 3D poit as beig represeted by a triple: (x,y,z). Usig homogeeous coordiates, we add a 4 th umber: (x,y,z,w) Graphics systems take (x,y,z,w) ad the divide x, y, ad z by w before usig them. x y z X, Y, Z w w w Thus (,,3,), (,4,6,), (-,-,-3,-) all represet the same 3D poit. 6 This Lets us Represet Poits at Ifiity 7 Additioal Parameters for Spotlights 8 This is useful to be able specify a parallel light source by placig the light source locatio at ifiity. The poit (,,3,) represets the 3D poit (,,3) The poit (,,3,.5) represets the 3D poit (,4,6) The poit (,,3,.0) represets the poit (00,00,300) So, (,,3,0) represets a poit at ifiity, but alog the ray from the origi through (,,3). gllightfv( GL_LIGHT0, GL_SPOT_DIRECTION, Array3(xdir,ydir,zdir) ) Specifies the spotlight-poitig directio. This gets trasformed by the curret value of the ModelView matrix. gllightf( GL_LIGHT0, GL_SPOT_EXPONENT, e ) Specifies the spotlight directioal itesity. This acts very much like the expoet i the specular lightig equatio. Poits-at-ifiity are used for parallel light sources ad some shadow algorithms gllightf( GL_LIGHT0, GL_SPOT_CUTOFF, deg ) Specifies the spotlight maximum spread agle. Shortcuts I Like void SetMaterial( float r, float g, float b, float shiiess ) glmaterialfv( GL_BACK, GL_EMISSION, Array3( 0., 0., 0. ) ); glmaterialfv( GL_BACK, GL_AMBIENT, MulArray3(.4f, White ) ); glmaterialfv( GL_BACK, GL_DIFFUSE, MulArray3(., White ) ); glmaterialfv( GL_BACK, GL_SPECULAR, Array3( 0., 0., 0. ) ); glmaterialf ( GL_BACK, GL_SHININESS,.f ); glmaterialfv( GL_FRONT, GL_EMISSION, Array3( 0., 0., 0. ) ); glmaterialfv( GL_FRONT, GL_AMBIENT, Array3( r, g, b ) ); glmaterialfv( GL_FRONT, GL_DIFFUSE, Array3( r, g, b ) ); glmaterialfv( GL_FRONT, GL_SPECULAR, MulArray3(.8f, White ) ); glmaterialf ( GL_FRONT, GL_SHININESS, shiiess ); Defiitio of GL_FRONT ad GL_BACK: Vertices are CCW whe GL_FRONT side viewed from the outside 0 GL_BACK side 9 Vertices are CW whe viewed from the outside Shortcuts I Like void SetPoitLight( it ilight, float x, float y, float z, float r, float g, float b ) gllightfv( ilight, GL_POSITION, Array3( x, y, z ) ); gllightfv( ilight, GL_AMBIENT, Array3( 0., 0., 0. ) ); gllightfv( ilight, GL_DIFFUSE, Array3( r, g, b ) ); gllightfv( ilight, GL_SPECULAR, Array3( r, g, b ) ); gllightf ( ilight, GL_CONSTANT_ATTENUATION,. ); gllightf ( ilight, GL_LINEAR_ATTENUATION, 0. ); gllightf ( ilight, GL_QUADRATIC_ATTENUATION, 0. ); gleable( ilight ); void SetSpotLight( it ilight, float x, float y, float z, float xdir, float ydir, float zdir, float r, float g, float b ) gllightfv( ilight, GL_POSITION, Array3( x, y, z ) ); gllightfv( ilight, GL_SPOT_DIRECTION, Array3(xdir,ydir,zdir) ); gllightf( ilight, GL_SPOT_EXPONENT,. ); gllightf( ilight, GL_SPOT_CUTOFF, 45. ); gllightfv( ilight, GL_AMBIENT, Array3( 0., 0., 0. ) ); gllightfv( ilight, GL_DIFFUSE, Array3( r, g, b ) ); gllightfv( ilight, GL_SPECULAR, Array3( r, g, b ) ); gllightf ( ilight, GL_CONSTANT_ATTENUATION,. ); gllightf ( ilight, GL_LINEAR_ATTENUATION, 0. ); gllightf ( ilight, GL_QUADRATIC_ATTENUATION, 0. ); gleable( ilight ); Computer Graphics 30 5

Sidebar: Note that we are computig the light itesity at each vertex 3 first, ad the iterpolatig that itesity across the polygo secod That is, you are oly usig

You ca do a eve better job if you iterpolate the ormal across the polygo first, ad the compute the light itesity with the lightig model at each fragmet secod:

colors, 33 ot just the results of the lightig model Flat glshademodel( GL_SMOOTH ); Smooth Shadig ca also iterpolate vertex colors, ot just the results of the

6 Sidebar: Note that we are computig the light itesity at each vertex 3 first, ad the iterpolatig that itesity across the polygo secod That is, you are oly usig the lightig model at each vertex. You ca do a eve better job if you iterpolate the ormal across the polygo first, ad the compute the light itesity with the lightig model at each fragmet secod: But, for that you will eed the Shaders course (CS 457/557) 3 Per-vertex Per-fragmet Per-vertex Per-fragmet Sidebar: Smooth Shadig ca also iterpolate vertex colors, 33 ot just the results of the lightig model Flat glshademodel( GL_SMOOTH ); Smooth Shadig ca also iterpolate vertex colors, ot just the results of the lightig model This is especially useful whe usig colors for scietific visualizatio: 34 glbegi(gl_triangles ); glcolor3f( r0, g0, b0 ); glvertex3f( x, y, z ); Smooth Tricky Lightig Situatios 35 Tricky Lightig Situatios 36 Watch for these i movies! Notice the lightig i the fur! Feathers Hair Fur Disey 6

Computer Graphics Lighting

Computer Graphics Lighting Lighting 1 Mike Bailey mjb@cs.oregonstate.edu This work is licensed under a Creative Commons Attribution-NonCommercial- NoDerivatives 4.0 International License Lighting.pptx Why Do We Care About Lighting?