3/23/2005 © Dr. Zachary Wartell 1 Illumination Models and Surface- Rendering Methods.

3/23/2005 © Dr. Zachary Wartell 1

Illumination Models and Surface-Rendering Methods


light models and surface rendering

• light model – model for calculating the light intensity at a single surface point

• surface rendering – procedure for applying a lighting model to obtain pixel colors for each pixel on a projected surface. Options:– compute light model at every pixel

• ray-tracing, scan-line graphics hardware circa 2000 (but more computation→lower framerate)

– compute light model at few surface locations and interpolate color of other pixels

• older scan-line graphics hardware, modern hardware must still balance realism vs framerate.


Light in real world

medium

emission spectrum

reflection spectrum

phototopic curve(eye sensitivity)


Light in graphics

medium

emission spectrum

reflection spectrum

Display

RGB

RGB

RGB

RGBRGB

RGBpixels

space “outside”display typically

not computationallymodeled


Light interactions

Light interacts with a surface in some combination of:

• emission• reflection

– on surface : mirror, specular or diffuse– suspended particles: random scattering

• transmission– transparent, translucent, refraction

• absorption


Distance Attenuationemission spectrum

Display

RGB

RGBpixel

At farther distances same amount of energy is

spread over a larger area (section of sphere). In

particular:

energy/area = 1 / distance2


Directional Light Sources

PointLight Source

Vlight

θl

Vobj

α

If α < θl then P is illuminatedby light; otherwise it is not.

Vobj Vlight = cos α, so test

Vobj Vlight ≥ cos θl

P

assumes vectorsare normalized and 0 ≤ θl ≤ 90◦


Surface Lighting Effects: Diffuse Reflection

• diffuse reflection – reflection intensity equal in all outgoing directions. Reflected light color is due to combination of incoming spectrum and surface absorption spectrum


Surface Lighting Effects: Specular Reflection

• Specular reflection – reflected light is concentration in a particular outgoing direction. It appears as a highlight or bright spot on surface. In physical world reflected light is primarily due to incoming light spectrum, but graphics models usually include surface reflection spectrum also.


Surface Lighting Effects: Ambient Light

• ambient light – approximates all the combined light that a surface receives from all other non-emitting surfaces. Light is not direct from light source but reflected off an intermediate surface.

This is a very coarse approximation to reality. Doesn’t model “color bleeding”.


Diffuse reflection of Point Light Source

CGwOG, Hearn&Baker


Diffuse Reflection of Ambient and Point Light

CGwOG, Hearn & Baker


Specular Reflection

• incident angle = reflection angle = θ• mirror reflection – all reflected light follows R• specular reflection – reflected light distributed in

“reflection” cone around R

NL

V

Rθ θ Φ

mirror shiny specular dull specular

NL

V

RΦ

NL

V

RΦ

NL

V

RΦ

NL

V

RΦ

NL

V

RΦ

NL

V

RΦ


Combined Illumination Model

, , , ,1

,

,

,

is radial/distance attenuation equation

is angular attenuation equation (spotlight)

0.0, if 0.0 (lig

n

surfemission ambdiff l radatten l angatten l diff l specl

l radatten

l angatten

l

l diff

I I I f f I I

f

f

I

N L

,

ht source behind object)

, otherwise

0.0, if 0.0 (light source behind object)

max{0, }, otherwises

d l l

l

l spec n

s l

k I

Ik I

l

N L

N L

N H

●to keep I in [0,1] range: clamp final result, normalizeindividual terms, or scale based on maximum of all pixels

3/23/2005 © Dr. Zachary Wartell 1 Illumination Models and Surface- Rendering Methods.

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Transcript of 3/23/2005 © Dr. Zachary Wartell 1 Illumination Models and Surface- Rendering Methods.