8 Lighting & Materials

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Quentin Curtis
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1 CSc 155 Advanced Cmputer Graphics lluminatin Mdels 8 Lighting & Materials Real wrld lights have a frequency spectrum White light: all (visible) frequencies Clred light: restricted frequency distributin Phtns bunce arund bjects in the wrld Simplified mdel: 1. Glbal ient light Equal everywhere. Zer r mre explicit lights Usually at least ne is present Different types with varying prperties 3. Estimate the effects f the light(s) n different types f materials. ADS Lighting Mdel in 3D graphics Vertex Nrmals Light affects the appearance f a 3D scene by hw it reflects ff f the bjects in the scene. The sees a cmbinatin f three separate reflectins: Ambient, Diffuse and Specular Each is affected by material and light characteristics Ambient light explicit light surce L N = + diff + spec R Reflectin mdels require surface nrmal vectrs Every vertex needs a crrespnding nrmal The nrmal can be determined in varius ways: Built by a mdeling tl and laded int a buffer Cmputed in a prcedural mdel and laded int a buffer Cmputed n the fly as needed 3 4 Explicit Light Types Spt Effect Pint surce Directinal ( distant ) Spt Cutff angle dir Ф lcatin, directin, lcatin, directin,, cutffangle, fallffrate Sptlights add a fractin f their t the reflectin seen by the : Viewer 0% if the vertex lies utside the cne; a fractin based n angle Φ, and spt fallff, if vertex is inside cne. Spt directin Cutff angle Dˆ Ф Psitin Vertex directin 0, spteffect if max( Vˆ D ˆ, 0) cs( cutff ); max( Vˆ D ˆ, 0) fallffrate = spteffect * cmputedreflectin V therwise. Vertex 5 6

2 Light Clr Prperties Distance Attenuatin L cefficients: ntensity f crrespnding light cmpnent, L ar L ag L Ambient RGB ab Light i = L dr L dg L db Diffuse RGB L sr L sg L sb Specular RGB Light reduces ver distance traveled. Actual frmula: Prblems: 1 d Light-t-surface distances verwhelm surface-t- distances Lights mdeled slely by directin have n rigin, s can t measure distance 0 L Distance Attenuatin (cnt.) Typical Light class hierarchy Standard apprach : Omit attenuatin fr ient and directinal lights Prvide user-tunable parameters t cntrl attenuatin f pint-surce and spt: cnstatt, linearatt, quadraticatt Adjust cmputed reflectin fr each light: 1 cmputedreflectin kc kld kqd * <<singletn>> AmbientLight : [r g b a] DistantLight directin : Vectr3D <<abstract>> Light name : String ient : [r g b a] diffuse : [r g b a] specular : [r g b a] PsitinalLight psitin : Pint3D cnstantatt : Natural linearatt : Natural quadraticatt : Natural SptLight directin : Vectr3D cutffangle : {0..90} fallffexpnent : {0..18} 9 10 Material Prperties Additinal Material Prperties Materials have reflectance characteristics Shininess (glssiness) Ambient, diffuse, and specular reflectance Red, green, and blue reflectance Highly Glssy Less Glssy K ar K ag K ab Ambient reflectin Emissiveness (glw) Material i = K dr K dg K db Diffuse reflectin K sr K sg K sb 0 K 1 Specular reflectin Transparency Refractin ndex Refractin 11 1

3 Sme cmmn material parameters 1 Material Ambient r,g,b Diffuse r,g,b Specular r,g,b Shininess Black Plastic 0.0, 0.0, , 0.01, , 0.5, Brass Brnze 0.394, 0.35, , 0.175, , , , 0.484, Chrme 0.5, 0.5, , 0.40, 0.40 Cpper Gld Pewter Silver Plished Silver , , , , , , , 0.193, , 0.313, , 0.705, , , , , , , , 0.775, , 0.941, , 0.719, , , , , , , , , , , , , Material class (graphicslib3d) Material - name : String - ient : [r g b a] - diffuse : [r g b a] - specular : [r g b a] - emissin : [r g b a] - shininess : flat + getname() : String + getambient() : flat[4] + getdiffuse() : flat[4] + setname(string) : vid + setambient(flat []) : vid + setdiffuse(flat []) : vid + Material GOLD Predefined + Material SLVER static Materials + Material BRONZE 1 Hill, F.S., Cmputer Graphics Using OpenGL, nd Ed Wrld Lights Light psitin/directin usually defined as wrld values Must be transfrmed int values: Multiply psitins by MV Multiply directins by transpse(inverse(mv.rtatincmpnent)) Y w Ps e L E The ADS Reflectin Mdel Basic empirical reflectin mdel: diff spec Ps w L W Y e ntensity f ient reflectin ntensity f diffuse reflectin ntensity f specular reflectin X w Z w Light in Wrld Space X e Light in Eye Space Ambient Reflectin Diffuse Reflectin Lert s Csine Law : The amunt f light striking a surface is prprtinal t the csine f the angle f incidence L d L d L d = lightambientntensity * materialambientreflectin OR: Red = lightambntensity Red * matambreflectin Red Green = lightambntensity Green * matambreflectin Green Surface L N L N L N Blue = lightambntensity Blue * matambreflectin Blue = 0 ; maximum = 45 ; medium = 90 ; minimum 17 18

Diffuse Reflectin (cnt.) diff diff = lightdiffusentensity * matdiffusereflectin * cs() = lightdiffusentensity * matdiffusereflectin * ( Nˆ L ˆ ) must insure 0!

4 Diffuse Reflectin (cnt.) diff diff = lightdiffusentensity * matdiffusereflectin * cs() = lightdiffusentensity * matdiffusereflectin * ( Nˆ L ˆ ) must insure 0! ( use max(cs(), 0 ) ) Specular highlights are view-psitin dependent Fr a perfect reflectr, Specular Reflectin diff L spec N if 0, spec = lightspecularntensity * materialspecularreflectin else spec = 0 R φ V 19 0 Specular Reflectin (cnt.) Specular Reflectin (cnt.) Mst surfaces are nt perfect reflectrs Specular reflectin is spread abut the reflectin vectr View vectrs clse t reflectin vectr will see sme reflectin Specular reflectin reduces as φ increases L N R V 1 φ φ 1 V arccs( R V) Fallff can be mdeled (apprximated) using cs(φ) cs (φ) spec = lightspecularntensity * matspecularreflectin * cs(φ) = lightspecularntensity * matspecularreflectin * ( Rˆ Vˆ) must insure 0! ( use max( cs(φ), 0 ) ) 1 Specular Reflectin (cnt.) Different materials have different fallff (glssiness) values Specular Reflectin (cnt.) Different fallff rates can be mdeled with pwers f the csine functin: cs n (φ), n = 1,, 3, 5, 10, 0, 50, 100; -π/ φ π/ cs cs 3 cs Highly Glssy Less Glssy 100 cs Needed: a way t adjust the fallff functin based n material glssiness spec = lightspecularntensity * matspecularreflectin * cs n (φ) = lightspecularntensity * matspecularreflectin * R ˆ V ˆ n 3 4

Rendering Methds lluminatin & reflectin mdels shade vertices.

Raster Scan Lines V 3 V 3 Cnstant Per-Object Shading Cnstant Per-Plygn ( Faceted r Flat ) Shading Smth Shading a = ( 1 ) /

) Misses specular highlights unless at a vertex Has prblems with cncave plygns Nrmal averaging can prduce incrrect effects

Scan Line L Specular highlight 5 6 Ptentially LARGE discntinuity here Sudden change 3 4 7 8 Mach Banding Phng Smth Shading

5 Rendering Methds lluminatin & reflectin mdels shade vertices. We have t assign clrs t pixels Guraud Smth Shading Apprach: vertex shade (clr) interplatin Pssible appraches: a V 1 p 1 b Raster Scan Lines V 3 V 3 Cnstant Per-Object Shading Cnstant Per-Plygn ( Faceted r Flat ) Shading Smth Shading a = ( 1 ) / numscanlines p = ( b a ) / numpixels 5 6 Guraud Shading Drawbacks Guraud Shading Drawbacks (cnt.) Misses specular highlights unless at a vertex Has prblems with cncave plygns Nrmal averaging can prduce incrrect effects Shws plygnal silhuette edges Average nrmals are parallel Susceptible t Mach Banding Discntinuity here in rate f change 1 Scan Line L Specular highlight 5 6 Ptentially LARGE discntinuity here Sudden change Mach Banding Phng Smth Shading examples Apprach: vertex nrmal interplatin. Use interplated nrmals fr shading calculatins at each pixel N 1 s each band unifrmly gray acrss its width? Are there gray dts at the intersectins? N N 3 Scan lines 9 30

6 Cmparisn Guraud vs. Phng Finding reflectin vectr Blinn / Phng Shading is expensive But what is actually needed is a way t measure S instead cnsider vectr H halfway between & Angle = L R N H α φ V R V L 31 3 Using The Blinn Reflectin Vectr cmputing H L V cs N ˆ H ˆ L V H N L V R We can use cs in the reflectin mdel. Cmputatin can be dne in fragment shader, r in vertex shader (with sme tweaks) if necessary fr perfrmance. 33

Shading. Outline. Introduction Diffuse reflection Specular reflection Ambient light Refinements: Rendering Faces

Shading. Outline. Introduction Diffuse reflection Specular reflection Ambient light Refinements: Rendering Faces Shading Outline Intrductin Diffuse reflectin Specular reflectin Ambient light Refinements: Incrprating several light surces and distance Adding clr Rendering Faces 1 Shading T add realism Shading mdel