OpenGL Illumination example. 2IV60 Computer graphics set 8: Illumination Models and Surface-Rendering Methods. Introduction 2.

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1 OpeG Illumiatio example 2I60 Computer graphics set 8: Illumiatio Models ad Surface-ederig Methods Jack va Wijk TU/e Glfloat lightpos[] = {2.0, 0.0, 3.0, 0.0}; Glfloat whitecolor[] = {1.0, 1.0, 1.0, 1.0}; Glfloat pikcolor[] = {1.0, 0.5, 0.5, 1.0}; glshademodel(g_smooth); // Use smooth shadig gleable(g_ightig); // Eable lightig gleable(g_ight0); // Eable light source #0 glightfv(g_ight0, G_POSITIO, lightpos); // positio S 0 glightfv(g_ight0, G_DIFFUSE, whitecolor); // set color S 0 glmaterialfv(g_fot, G_DIFFUSE, pikcolor); // set surface // color glbegi(g_tiages); glormal3fv(1); glertex3fv(v1); // draw triagle, give glormal3fv(2); glertex3fv(v2); // first ormal, followed glormal3fv(3); glertex3fv(v3); // by vertex gled(); What is goig o here? Itroductio 1 Illumiatio model: Give a poit o a surface, what is the perceived color ad itesity? Kow as ightig Model, or Shadig Model Surface rederig: Apply the Illumiatio model to color all pixels of the surface. Itroductio 2 Example: Illumiatio model gives color vertices, Surface is displayed via iterpolatio of these colors. H&B 17: H&B 17: Itroductio 3 Illumiatio: Physics: Material properties, light sources, relative positios, properties medium Psychology: Perceptio, what do we see Color! Ofte approximatig models H&B 17: ight sources 1 ight source: object that radiates eergy. Su, lamp, globe, sky Itesity I = (I red, I gree, I blue ) If I red = I gree = I blue : white light 1

2 ight sources 2 Simple model: poit light source - positio P ad itesity I - ight rays alog straight lies - Good approximatio for small light sources ight sources 3 Simpler yet: poit light source at ifiity - Directio ad itesity I - Sulight ight sources 4 Dampig: itesity of light decreases with distace Eergy is distributed over area sphere, hece I l = I / d 2, d with d distace to light source. I practice ofte too agressive, hece I l = I / (a 0 +a 1 d+a 2 d 2 ) If light source at ifiity: o dampig with distace ight sources 5 Directed light source, spotlight: ight is primarily sed i directio of light. If cosα > cosθ the Q is illumiated. Or : Q P If Q P Q l light > cosθ the Q is illumiated. l α θ l light coe light P ight sources 6 More subtle: et I decrease with icreasig agle α. Ofte used : I l = cos α I. The larger, the stroger the light decreases. Q P α θ l light coe light 2

3 Surface illumiatio 1 Whe light hits a surface, three thigs ca happe: absorptio reflectio trasmissio H&B 17-2: Surface illumiatio 2 Suppose, a light source radiates white light, cosistig of red, gree ad blue light. absorptio reflectio If oly red light is reflected, the we see a red surface. Surface illumiatio 3 Diffuse reflectio: ight is uiformly reflected i all directios Specular reflectio: ight is stroger reflected i oe directio. specular reflectio diffuse reflectio trasmissio H&B 17-2: H&B 17-2: Surface illumiatio 4 Ambiet light: light from the eviromet. Udirected light, models reflected light of other objects. Basic illumiatio model 1 Basic illumiatio model: Ambiet light; Poit light sources; I a I, l P of Ambiet reflectio; Diffuse reflectio; Specular reflectio. k k a d k, s s ka, kd, ks : reflectio coefficiets k p = ( kp, red, kp,gree, kp,blue ) H&B 17-2:

4 Basic illumiatio model 2 Ambiet light: eviromet light. Udirected light, models reflected light of other objects. Basic illumiatio model 3 Perfect diffuse reflector: light is reflected uiformly i all directios. I = k amb a I a φ da/cos φ Itesity = eergy projected area. da da/ cosφ cos φ Basic illumiatio model 4 Perfect diffuse reflector: light is reflected uiformly i all directios.. φ φ da/cos φ ambert s law: eflected eergy is proportioal with cos φ, where φ deotes the agle betwee the ormal ad a vector to the light source. Basic illumiatio model 5 Perfect diffuse reflector: light is reflected uiformly i all directios. P source P surf Graphics model diffuse reflectio : kd Il ( ) Il,diff = 0 with 0 kd 1 ad Psource Psurf = Psource Psurf if > 0 if 0 Basic illumiatio model 6 Perfect specular reflector: light is oly reflected i oe directio. Agle of icidece is agle of reflectio. Basic illumiatio model 7 Imperfect specular reflector: light is distributed i the directio of the agle of reflectio, depedet o the roughess of the surface. θ θ θ θ θ θ glad ruw 4

5 Basic illumiatio model 8 Phog model: empirical model for specular reflectio Basic illumiatio model 9 Phog model: empirical model for specular reflectio θ θ φ s Il, spec = W ( θ ) Il cos φ, with W ( θ ) = ks, s smoothess (1 = ruw,100 = glad), θ :agle betwee ad, φ : agle betwee ad, :directio reflected ray of light :directio viewer θ θ φ I l, spec ksil ( ) = 0 s if > 0 ad > 0 if 0 or 0 Basic illumiatio model 10 Phog model: calculatig the vectors + = (2 ) hece = (2 ). = P P view surf Pview Psurf Basic illumiatio model 11 Phog model: variat with halfway vector H. Use α istead of φ. α H φ + H = + I l, spec = k I ( H) s l If light source ad viewer far away: H ± costat. s I = I = k I I = k I Basic illumiatio model 12 All together: amb a a a a + I + k I (max(0, )) + k I (max(0, H)) + dif d l l= 1 + I k I (max(0, )) + k I (max(0, H)) d l spec Multiple light sources : s l s l s s Basic illumiatio model 13 Color (reprise): ight itesity I ad reflectio coefficiets k: (r,g,b) triplets So for istace: I k I (max(0, )) dif, = d, l, Plastic: k d is colored (r,g,b), k s is grey (w,w,w) Metal: k d ad k s same color Basic model: simple but effective. It ca be doe much better though 5

6 Trasparacy 1 Trasparat object: - reflected ad trasmitted light - refractio - scatterig H&B 17-4: Sell s law of refractio: θ i θ i θ r T Trasparacy 2 ηi siθ r = siθi, η :idex of refractio ηr η i ηi T = cosθi cosθ r η r ηr Derivatio : Use Sell's law, T. T = 1, T = cosθr, T = α + β, ad solve for β ad α H&B 17-4: Trasparacy 3 Thi surface: - double refractio - shift of light ray H&B 17-4: Trasparacy 3 ery thi surface: - Discard shift Simple model: I = (1 k ) I 0 k 1 refl k : trasparacy t 1 k :opacity t t t + k I t tras Poor result for silhouette edges H&B 17-4: Atmospheric effects 1 Atmospheric effects: - dust, smoke, vapor - colors are dimmed - objects less well visible H&B 10-5:

7 Atmospheric effects 2 Atteuatio by atmosphere : f atmo Simpler: Perceiveditesity: I = f ( d) I + [1 f ( d)] I atmo obj ρd ( d) = e,with d : distace ρ :atteuatio factor atmo f atmo atmo d d ( d) = d d max = [ ] H&B 10-5: mi mi ederig polygos 1 Basic illumiatio model: Ca be used per poit, but that s somewhat expesive More efficiet: Illumiatio model gives color for some poits; Surface is filled i usig iterpolatio of these colors. ederig polygos 2 Costat-itesity rederig aka flat surface rederig: Determie color for ceter of polygo; Fill the polygo with a costat color. Ok if: Object cosists of plaar faces, ad ight sources are far away, ad Eye poit is far away, or Polygos are about a pixel i size. ederig polygos 2 Costat-itesity rederig aka flat surface rederig: Determie color for ceter of polygo; Fill the polygo with a costat color. Highlights ot visible, Facetted appearace, icreased by Mach badig effect. Mach badig Huma perceptio: edges are give emphasis, cotrast is icreased ear edges. ederig polygos 2 Gouraud surface rederig: Determie average ormal o vertices; Determie color for vertices; Iterpolate the colors per polygo (icremetally). Agel (2000) k 1 = = k= 1 k k 7

8 ederig polygos 3 Gouraud surface rederig: Much better result for curved surfaces Errors ear highlights iear iterpolatio still gives Mach badig Silhouettes are still ot smooth ederig polygos 4 Phog surface rederig: Determie average ormal per vertex; Iterpolate ormals per polygo (icremetally); Calculate color per pixel. Fast Phog surface rederig: ike Phog surface rederig, but use 2 d order approximatio of color over polygo: 2 2 I ( x, y) = ax + bxy + cy + dx + ey + f Gouraud Flat ederig polygos 5 Phog surface rederig: Eve better result for curved surfaces o errors at high lights o Mach badig Silhouettes remai coarse More expesive tha flat or Gouraud shadig ederig polygos 5 Flat Gouraud Phog OpeG Illumiatio Glfloat lightpos[] = {2.0, 0.0, 3.0, 0.0}; Glfloat whitecolor[] = {1.0, 1.0, 1.0, 1.0}; Glfloat pikcolor[] = {1.0, 0.5, 0.5, 1.0}; glshademodel(g_smooth); // Use smooth shadig gleable(g_ightig); // Eable lightig gleable(g_ight0); // Eable light source #0 glightfv(g_ight0, G_POSITIO, lightpos); // positio S 0 glightfv(g_ight0, G_DIFFUSE, whitecolor); // set color S 0 glmaterialfv(g_fot, G_DIFFUSE, pikcolor); // set surface // color glbegi(g_tiages); glormal3fv(1); glertex3fv(v1); // draw triagle, give glormal3fv(2); glertex3fv(v2); // first ormal, followed glormal3fv(3); glertex3fv(v3); // by vertex gled(); OpeG ight-sources 1 First, eable lightig i geeral: gleable(g_ightig); OpeG provides (at least) eight light-sources: lightame = G_IGHT0, G_IGHT1,, G_IGHT7 Eable the oe(s) you eed with: gleable(lightame); Set properties with glight*(lightame, lightproperty, propertyalue); * = i, f, iv, or fv (i: iteger, f: float, v vector) 8

9 OpeG ight-sources 2 Positio light-source: Glfloat sulightpos[] = {2.0, 0.0, 3.0, 0.0}; Glfloat lamplightpos[] = {2.0, 0.0, 3.0, 1.0}; glightfv(g_ight1, G_POSITIO, sulightpos); glightfv(g_ight2, G_POSITIO, lamplightpos); Fourth coordiate = 0: source at ifiity Fourth coordiate = 1: local source Specified i world-coordiates, accordig to the curret Modeliew specificatio just like geometry. Hece, take care whe you specify the positio. ight from above looks more atural OpeG ight-sources 3 Color light-source: Glfloat greycolor[] = {0.3, 0.3, 0.3, 1.0}; Glfloat pikcolor[] = {1.0, 0.7, 0.7, 1.0}; Glfloat whitecolor[] = {1.0, 1.0, 1.0, 1.0}; glightfv(g_ight1, G_AMBIET, greycolor); glightfv(g_ight1, G_DIFFUSE, pikcolor); glightfv(g_ight1, G_SPECUA, whitecolor); OpeG light-source has three color properties, depedet o reflectio surface. ot realistic, ca be used for special effects. If you do t have ambiet light, thigs ofte appear black. Colors are always 4-vectors here: Fourth coordiate is alpha. Most cases: set it to 1.0. More settigs: See book OpeG Global ightig Global parameters: glightmodel*(paramame, paramalue); * = i, f, iv, or fv (i: iteger, f: float, v vector) Global ambiet light: Glfloat globalambiet[] = {0.3, 0.3, 0.3, 1.0}; glightmodelfv(g_ight_mode_ambiet, globalambiet); More precise specular reflectio, take view positio ito accout: glightmodeli(g_ight_mode_oca_iewe, G_TUE); Two-sided lightig: glightmodeli(g_ight_mode_two_side, G_TUE); OpeG Surface properties 1 Surface reflectio parameters: glmaterial*(surfface, surfproperty, propertyalue); * = i, f, iv, or fv (i: iteger, f: float, v vector) surfface = G_FOT, G_BACK, or G_FOT_AD_BACK Glfloat emissiocolor[] = {0.2, 0.3, 0.1, 1.0}; Glfloat diffusecolor[] = {0.6, 0.3, 0.1, 1.0}; Glfloat specularcolor[] = {0.1, 0.1, 0.1, 1.0}; glmaterialfv(g_fot, G_EMISSIO, emissiocolor); glmaterialfv(g_fot, G_DIFFUSE, diffusecolor); glmaterialfv(g_fot, G_SPECUA, specularcolor); glmaterialf(g_fot, G_SHIIESS, 25.0f); OpeG Surface properties 2 If colors are chaged ofte (for istace, per vertex): gleable(g_coo_mateia); glcolormaterial(g_fot_ad_back, G_AMBIET_AD_DIFFUSE); glbegi( ); for i =... for j =... glcolor3f(red(i,j), gree(i,j), blue(i,j)); glertex3f(x(i,j), y(i,j), z(i,j)); gled( ); OpeG Surface properties 3 Trasparet surfaces: First, draw all opaque surfaces; ext, draw trasparet surfaces, back to frot *, usig somethig like: glcolor4f(, G, B, A); // A: alpha, for istace 0.40 gleable(g_bed); glbledfuc(g_oe_mius_sc_apha, G_SC_APHA);... Draw trasparet surfaces. gldisable(g_bed); * OpeG caot automatically hadle trasparecy, because of the z-buffer algorithm used for hidde surface removal. More o this later. 9

10 OpeG Surface properties 4 Color Bledig (see also H&B: ): Source: Destiatio: the ew graphics object to be draw; the curret image built up. ( S, G S, B S, A S ): Source color + alpha ( D, G D, B D, A D ): Destiatio color + alpha (S, S G, S B, S A ): Source bledig factors (D, D G, D B, D A ): Destiatio bledig factors Compoets of Source ad Destiatio are weighted ad added: (S S + D D, S G G S + D G G D, S B B S + D B B D, S A A S + D A A D ) is stored i the curret image. OpeG Surface properties 5 ( S, G S, B S, A S ): Source color + alpha ( D, G D, B D, A D ): Destiatio color + alpha (S, S G, S B, S A ): Source bledig factors (D, D G, D B, D A ): Destiatio bledig factors glbledfuc(sfactor, dfactor): specify the bledig factors. glbledfuc(g_oe_mius_sc_apha, G_SC_APHA); // Use alpha of source as trasparecy glbledfuc(g_sc_apha, G_OE_MIUS_SC_APHA); // Use alpha of source as opacity More optios available for special effects. OpeG Surface-ederig 1 glshademodel(m): specify the rederig method m = G_FAT or m = G_SMOOTH (Gouraud, default) glormal*(x, y, z) : specify the ormal vector OpeG Surface-ederig 2 glshademodel(m): specify the rederig method m = G_FAT or m = G_SMOOTH (Gouraud, default) glormal*(x, y, z) : specify the ormal vector Flat versio: glormal3fv(); glbegi(g_tiages); glertex3fv(1); glertex3fv(2); glertex3fv(3); gled(); Smooth versio: glbegi(g_tiages); glormal3fv(1); glertex3fv(1); glormal3fv(2); glertex3fv(2); glormal3fv(3); glertex3fv(3); gled(); gleable(g_omaize): et OpeG ormalize the ormals for you. Ad, also take care of effects of scalig, shearig, etc. ext ow that we kow how to reder curved surfaces, let s study how to defie these 10