visual appearance chapter 4 tomas akenine-möller department of computer engineering chalmers...

Visual AppearanceVisual AppearanceChapter 4Chapter 4

Tomas Akenine-Möller

Department of Computer Engineering

Chalmers University of Technology

Tomas Akenine-Mőller © 2002

Overview of today’s lectureOverview of today’s lecture

Refresher on simple lighting models– Plus some new stuff

FogGamma correctionTransparency and alpha


Compute lighting at vertices,Compute lighting at vertices,then interpolate over trianglethen interpolate over triangle

How compute lighting? We could set colors per vertex manually For a little more realism, compute lighting from

– Light sources– Material properties– Geometrical relationships

light

Geometry

blue

red green

Rasterizer


Refresher on lightingRefresher on lightingDiffuse component : Diffuse component : iidiffdiff i=iamb+idiff+ispec

Diffuse is Lambert’s law: cos lndiffi

Photons are scattered equally in all directions

diffdiffdiff smlni )(


LightingLightingSpecular component : Specular component : iispecspec

Diffuse is dull (left)Specular: simulates a highlight


Specular component: PhongSpecular component: PhongPhong specular highlight modelReflect l around n:

l)n2(nlr

n

lr

-lnln )(

lnshishi mm

speci )(cos)( vr

specspecm

specshi smvri ))(,0max(

Read about Blinns highlight formula: (n.h)m


Ambient component: Ambient component: iiambamb

Ad-hoc – tries to account for light coming from other surfaces

Just add a constant color:

ambambamb smi


LightingLightingi=ii=iambamb+i+idiffdiff+i+ispecspec

This is just a hack!Has little to do with how reality works!

++

=

DEMO


Additions to the lighting equationAdditions to the lighting equationDepends on distance: 1/(a+bt+ct )Can have more lights: just sum their

respective contributionsDifferent light types:

2


What’s lighting andWhat’s lighting andwhat’s shading?what’s shading?Lighting: the interaction between light

and matterShading: do lighting (at vertices) and

determine pixel’s colors from theseThree types of shading:

– Flat, Goraud, and Phong


FogFog

Simple atmospheric effect– A little better realism – Help in determining distances

Color of fog: color of surface: fc sc

]1,0[ )1( fff ssp ccc How to compute f ? 3 ways: linear, exponential, exponential-squared Linear:

startend

pend

zz

zzf


Fog exampleFog example

Often just a matter of – Choosing fog color– Choosing fog model– Turning it on


Gamma correctionGamma correction

If input to gun is 0.5, then you don’t get 0.5 as output in intensity

Instead, gamma correct that signal: gives linear relationship


Gamma correctionGamma correction

I=intensity on screenV=input voltage (electron gun)a,and are constants for each systemCommon gamma values: 2.3-2.6Assuming =0, gamma correction is:

)( VaI

)/1( icc


Why is it important to care about Why is it important to care about gamma correction?gamma correction?

Portability across platforms Image quality

– Texturing– Interpolation

One solution is to put gamma correction in hardware…


Transparency and alphaTransparency and alpha

Transparency– Very simple in real-time contexts

The tool: alpha blending (mix two colors)Alpha () is another component in the

frame buffer, or on triangle– Represents the opacity – 1.0 is totally opaque– 0.0 is totally transparent

The over operator: dso ccc )1( Rendered object


TransparencyTransparency

Need to sort the transparent objects– Render back to front

Lots of different other blending modes

Can store RGB in textures as wellTwo ways:

– Unmultiplied– Premultiplied

visual appearance chapter 4 tomas akenine-möller department of computer engineering chalmers...

Documents