Assigning colour to pixels or fragments. Modelling Illumination. We shall see how it is done in a rasterization model. CS475/CS675 - Lecture 14

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1 - Computer Graphics Assigig colour to pixels or fragmets. Modellig Illumiatio Illumiatio Model : The Phog Model For a sigle light source total illumiatio at ay poit is give by: ecture 14: I =k a I a k d I d k s I s We shall see how it is doe i a rasterizatio model. where k a I a is the cotributio due to ambiet reflectio k d I d is the cotributio due to diffuse reflectio k s I s is the cotributio due to specular reflectio 2 Ambiet Illumiatio: I a 3 Diffuse Illumiatio: Diffuse Illumiatio: I d = I cos Represets the reflectio of all idirect illumiatio. Assumes Ideal Diffuse Surface that reflects light equally i all directio. Has the same value everywhere. Surface is very rough at microscopic level. For e.g., Chalk ad Clay. Is a approximatio to computig Global Illumiatio. I d = I cos Reflects light accordig to ambert's Cosie aw I d = I cos =I. : vector to the light source I : itesity of the light source From (14/08/2009) : surface ormal 4 5 6

2 Diffuse Illumiatio: I d =I cos Reflects light accordig to ambert's Cosie aw I d =I cos =I..V Specular Illumiatio: I s = I cos v = I R Specular Illumiatio: Ideal specular surface reflects oly alog oe directio. Reflected itesity is view depedet Mostly it is alog the reflected ray but as we move away some of the reflectio is slightly offset from the reflected ray due to microscopic surface irregularites. R R, 0 are i opposite directios the the dot product is egative. Use max. If ad to get the correct value. If r is distace to the light source ad I t is its true itesity the a distace based atteuatio ca be modelled by a iverse square falloff, i.e., I = I t / r 2 7 V 8 H V I local =k a I a are material costats defiig the amout of light that is reflected as ambiet, diffuse ad specular. They may be defied i as three values with R, G, B compoets. v 10 1 i m k d I d i k s I si Global Illumiatio Model I glo bal = I local k r I refected k t I trasmitted ka, kd, ks 9 ocal Illumiatio Model R I =k a I a k d I d k s I s The Bli-Phog Illumiatio Model cos The Phog Illumiatio Model.V I s = I cos v = I R 2 is called the coefficiet of shiiess ad I =I t / r V = V H V '. ) I s = I cos ϕ=i ( H v = = v = or v =

3 Surface Material Properties Colour For each object there ca be a 1 i m k d I d i k s I si Where ad how is colour of objects computed? Remember differetly coloured light is at differet wavelegth so: 1 i m 1 i m gleable(g_ight0); Property for the lights is defied usig: glightfv(geum light, Geum pame, Gfloat *param) light is the light eum like G_IGHT1 pame ca be G_AMBIET, G_DIFFUSE, G_SPECUAR, G_POSITIO, G_SPOT_CUTOFF, G_SPOT_DIRECTIO, G_SPOT_EXPOET, G_COSTAT_ATTEUATIO, G_IEAR_ATTEUATIO, ad G_QUADRATIC_ATTEUATIO Gfloat light_ambiet(0.0, 0.0, 0.0, 1.0); Gfloat light_diffuse(1.0, 1.0, 1.0, 1.0); Gfloat light_specular(0.0, 1.0, 0.0, 1.0); glmaterialfv(geum face, Geum pame, cost Gfloat* params); face ca be G_FROT, G_BACK or G_FROT_AD_BACK pame ca be The colour is computed at: I =k a I a glightfv(g_ight0, G_AMBIET, light_ambiet); glightfv(g_ight0, G_DIFFUSE, light_diffuse); glightfv(g_ight0, G_SPECUAR, light_specular); glightfv(g_ight0, G_POSITIO, light_positio); gleable(g_ight0); Costat o iterpolatio of itesity, oe itesity for whole object. o depth cues. Material properties ca be specified usig Gfloat light_positio(3.0, 4.0, 0.0, 1.0); Deprecated OG2.x cotet. See the shadig tutorial istead. For example: Deprecated OG2.x cotet. See the shadig tutorial istead. gleable(g_ightig); S i k d C d I di k s C s I si k r C r I r k t C t I t Every G implemetatio has at least 8 lights. k d C d I di k s C s I si k r C r I r k t C t I t Eablig lightig ad idividual lights Accoutig for shadows: I =k a C d I a OpeG uses the local Phog Illumiatio Model. I =k a I a Diffuse colour, Specular colour, Reflected colour ad Trasmitted colour I = k a C d I a G_AMBIET, G_DIFFUSE, G_SPECUAR, G_EMISSIO, G_SHIIESS, G_AMBIET_AD_DIFFUSE 1 i m k d I di k s I si Deprecated OG2.x cotet. See the shadig tutorial istead ecture

4 Faceted Oe itesity per polygo computed from the surface ormal ad light vector. (G_FAT) - ecture Gouraud iear iterpolatio of itesity across triagles to elimiate edge discotiuity. (G_SMOOTH) - ecture Shadows, texture mappig, reflectio mappig simulatig GI. Phog Iterpolatio of surface ormals. Still local illumiatio o GI. - ecture Faceted Fast Surface does ot look smooth if a piece wise liear approximatio to a flat surface is beig doe Mach Bad Efect accetuate the facets. Faceted glshademodel(g_fat);

5 Gouraud iearly iterpolate itesity alog sca lies: elimiates itesity discotiuities at polygo edges; still have gradiet discotiuities, mach badig is largely ameliorated, ot elimiated. must differetiate desired creases from tesselatio artifacts (edges of cube vs. edges o tesselated sphere). 1 Calculate approximate vertex ormals as a average of ormals of polygos meetig at that vertex. 5 eighborig polygos sharig vertices ad edges approximate smoothly curved surfaces ad will ot have greatly differig surface ormals hece this approximatio is reasoable. Calculate itesity at vertices. i 4 v 2 i i=1 i vec3 VertexPositio; iearly iterpolate itesity alog sca lies: elimiates itesity discotiuities at polygo edges; still have gradiet discotiuities, mach badig is largely ameliorated, ot elimiated. must differetiate desired creases from tesselatio artifacts (edges of cube vs. edges o tesselated sphere). Iterpolate itesity alog polygo edges. Iterpolate alog sca lies I a= I 1 3 I b= I 1 25 y s y2 y 1 y 2 ys y 3 y 1 y 3 I2 I 3 y2 y 1 y 2 y3 y1 y s y1 y3 I p =I a i vec2 VertexTex; out Data I1 Ia ys y 1 y s i vec3 Vertexormal; Ib Ip I2 x b x p x b x a sca lie data; struct ightifo vec3 Positio; vec3 a; vec3 d; vec3 s; ; I3 I b x p xa x b x a vec3 FrotColor; vec3 BackColor; vec2 TexCoord; 26 struct MaterialIfo vec3 Ka; vec3 Kd; vec3 Ks; float Shiiess; ; void geteyespace( out vec3 orm, out vec3 positio ) orm = ormalize( ormalmatrix * Vertexormal ); //Ambiet reflectivity //Diffuse reflectivity //Specular reflectivity //Specular shiiess factor glshademodel(g_smooth); retur ambiet + diffuse + spec; void mai() vec3 eyeorm; vec3 eyepositio; geteyespace( eyeorm, eyepositio ); positio = vec3( ModelViewMatrix * vec4( VertexPositio, 1 ) ); data.frotcolor = vec3(0); vec3 light( it lightidex, vec3 positio, vec3 orm ) vec3 s = ormalize( vec3( ight[lightidex].positio - positio ) ); vec3 v = ormalize( -positio.xyz ); 27 Gouraud spec = ight[lightidex].s * Material.Ks * pow( max( dot(r,v), 0.0 ), Material.Shiiess ); uiform mat4 ModelViewMatrix; uiform mat3 ormalmatrix; uiform mat4 MVP; if ( sdot > 0.0 ) uiform MaterialIfo Material; //ight Positio i eye-coords //Ambiet light itesity //Diffuse light itesity //Specular light itesity vec3 spec = vec3( 0.0 ); uiform ightifo ight[ightcout]; Gouraud : Vertex Shader #versio 430 y1 v = i =1 Gouraud data.backcolor = vec3(0); for( it i=0; i<ightcout; ++i ) data.frotcolor += light( i, eyepositio, eyeorm ); 28 vec3 r = reflect( -s, orm ); data.backcolor += vec3 ambiet = ight[lightidex].a * Material.Ka; float sdot = max( dot( s, orm ), 0.0 ); data.texcoord = VertexTex; vec3 diffuse = ight[lightidex].d * Material.Kd * sdot; light( i, eyepositio, -eyeorm ); gl_positio = MVP * vec4( VertexPositio, 1 ); 29 30

6 Gouraud Itegrates well with scalie rasterizatio. O a edge I / y is costat. vs. Faceted Gouraud Ca miss specular highlights because it iterpolates vertex colors istead of calculatig the itesity at every surface poit. Half Way vector Viewpoit Iterpolate ormals istead comes closer to actual surface ormal. Called Phog (ote: OT Phog Illumiatio Model) Ib Ia Gouraud Ib Iterpolate ormals alog sca lies. ormalize after iterpolatig (expesive!). ot available i plai OpeG doe as per pixel lightig o hardware. Still o Global Illumiatio most of the effects of Ray Tracig still missig. Ia Faceted Phog Gouraud Actual 32 33