1 Light Properties Property Exam ple Properties: Define - - PDF document

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CS 4 7 3 1 : Com put e r Gr a phics Le ct ur e 1 6 : I llum ina t ion M ode ls Pa r t 2 Em m anuel Agu

Adding Color

• Som etim es light or surfaces are colored
• Treat R,G and B com ponents separately
• I llum ination equation goes from :

I llu m = a m bie nt + diffuse + sp e cu la r = Ka x I + Kd x I x ( N .L) + Ks x I x ( R.V )

• To:

I llu m_ r = Ka _ r x I _ r + Kd_ r x I _ r x ( N .L) + Ks_ r x I _ r x ( R.V ) I llu m_ g = Ka _ g x I _ g + Kd _ g x I _ g x ( N .L) + Ks_ g x I _ g x ( R.V ) I llu m_ b = Ka _ b x I _ b + Kd _ b x I _ b x ( N .L) + Ks_ b x I _ b x ( R.V ) n

Adding Color

8 9 .6 0.773911 0.773911 0.773911 0.2775 0.2775 0.2775 0.23125 0.23125 0.23125 Polished Silver 27.8974 0.992157 0.941176 0.807843 0.780392 0.568627 0.113725 0.329412 0.223529 0.027451 Brass 3 2 0 .5 0 .5 0 .5 0 .0 1 0 .0 1 0 .0 1 0 .0 0 .0 0 .0 Black plastic Exponent, n Specular Ksr, Ksg, ksb Diffuse Kdr, Kdg,kdb Ambient Kar, Kag, kab Material

Figure 8.17, Hill, courtesy of McReynolds and Blythe

Lighting in OpenGL

• Adopt Phong light ing m odel

• Interpolate across surface (Gouraud/smooth shading)
• Use a constant illumination (get it from one of the vertices)
• Setting up OpenGL Lighting:

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Light Properties

• Properties:

glLight fv( light , pr ope r t y, va lue )

(1) constant: specify which light you want to set the property E.g: GL_LI GHT0, GL_LI GHT1, GL_LI GHT2 … you can create multiple lights (OpenGL allows at least 8 lights) (2) constant: specify which light property you want to set the value E.g: GL_AMBI ENT, GL_DI FFUSE, GL_SPECULAR, GL_POSI TI ON

( check t he r ed book for m or e)

(3) The value you want to set to the property

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Property Exam ple

• Define colors and position a light

GLfloat light_ambient[ ] = { 0.0, 0.0, 0.0, 1.0} ; GLfloat light_diffuse[ ] = { 1.0, 1.0, 1.0, 1.0} ; GLfloat light_specular[ ] = { 1.0, 1.0, 1.0, 1.0} ; GLfloat light_position[ ] = { 0.0, 0.0, 1.0, 1.0} ; glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient); glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse); glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular); glLightfv(GL_LIGHT0, GL_POSITION, light_position); colors Position What if I set Posit ion t o ( 0,0,1,0) ?

Types of lights

• OpenGL supports two types of lights

• Determ ined by the light positions you provide

GLfloat light_position[ ] = { x,y,z,w} ; glLightfv(GL_LIGHT0, GL_POSITION, light_position);

Turning on the lights

• Turn on the power (for all the lights)

• Flip each light’s switch

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Controlling light position

• Modelview m atrix affects a light’s position
• Two options:
• Option a:

glLight fv position, popm atrix

• Option b:

Material Properties

• The color and surface properties of a m aterial (dull,

shiny, etc)

• How m uch the surface reflects the incident lights

(am bient/ diffuse/ specular reflecetion coefficients) glMaterialfv(face, property, value)

Face: m at erial propert y for which face ( e.g. GL_FRONT, GL_BACK, GL_FRONT_AND_BACK) Propert y: what m at erial propert y you want t o set ( e.g. GL_AMBI ENT, GL_DI FFUSE,GL_SPECULAR, GL_SHI NI NESS, GL_EMI SSI ON, et c) Value: t he value you can t o assign t o t he pr oper t y

Material Exam ple

• Define am bient/ diffuse/ specular reflection and shininess

GLfloat m at_amb_diff[ ] = { 1.0, 0.5, 0.8, 1.0} ; GLfloat m at_specular[ ] = { 1.0, 1.0, 1.0, 1.0} ; GLfloat shininess[ ] = { 5.0} ; (range: dull 0 – very shiny 128) glMaterialfv(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE, m at_am b_diff); glMaterialfv(GL_FRONT, GL_SPECULAR, mat_speacular); glMaterialfv(GL_FRONT, GL_SHININESS, shininess);

• refl. coeff.

Global light properties

• glLightModelfv(property, value)
• Enable two sided lighting

lighting)

• Global am bient color

• Check the red book for others

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Surface Norm als

• Correct norm als are essential for correct lighting
• Associate a norm al to each vertex

glBegin(… ) glNorm al3f( x,y,z) glVertex3f(x,y,z) … glEnd()

• The norm als you provide need to have a unit length

OpenGL norm alize all t he norm als

Lighting revisit

• Where is lighting performed in the graphics pipeline?

m odeling and viewing

v1, m 1 v2, m 2 v3, m 3

per vert ex lighting proj ect ion clipping interpolate vert ex colors viewport m apping Rast erizat ion texturing shading Display

Polygon shading m odel

• Flat shading - com pute lighting once and assign the

color to the whole (m esh) polygon

Flat shading

• Only use one vertex norm aland m aterial property to

com pute the color for the polygon

• Benefit: fast to com pute
• Used when:

• OpenGL com m and: glShadeModel(GL_FLAT)

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Mach Band Effect

• Flat shading suffers from “m ach band effect”
• Mach band effect – hum an eyes accentuate the

discontinuity at the boundary

Side view of a polygonal surface perceived intensity

Sm ooth shading

• Fix the m ach band effect – rem ove edge discontinuity
• Com pute lighting for m ore points on each face

Fla t sha ding Sm oot h sha ding

Sm ooth shading

• Two popular m ethods:

Gouraud Shading

• The sm ooth shading algorithm used in OpenGL

g lSh a d e M od e l( GL_ SM OOTH )

• Lighting is calculated for each of the polygon vertices
• Colors are interpolated for interior pixels

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Gouraud Shading

• Per- vertex lighting calculation
• Norm al is needed for each vertex
• Per- vertex norm al can be com puted by averaging the

adjust face norm als

n n 1 n 2 n 3 n 4 n = ( n1 + n2 + n3 + n4) / 4.0

Gouraud Shading

• Com pute vertex illum ination (color) before the

projection transform ation

• Shade interior pixels: color interpolation (norm als are

not needed)

C1 C2 C3 Ca = lerp(C1, C2) Cb = lerp(C1, C3) Lerp( Ca, Cb) for all scanlines * lerp: linear int erpolat ion

Gouraud Shading

• Linear interpolation
• I nterpolate triangle color: use y distance to interpolate

the two end points in the scanline, and use x distance to interpolate interior pixel colors

a b v1 v2 x x = a / (a+ b) * v1 + b/ (a+ b) * v2

Gouraud Shading Problem

• Lighting in the polygon interior can be inaccurate

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Phong Shading

• I nstead of interpolation, we calculate lighting for each

pixel inside the polygon (per pixel lighting)

• Need norm als for all the pixels – not provided by user
• Phong shading algorithm interpolates the norm als and

com pute lighting during rasterization ( need t o m ap t he

norm al back t o w orld or eye space t hough)

Phong Shading

• Norm al interpolation
• Slow – not supported by OpenGL and m ost graphics

hardware

n1 n2 n3 nb = lerp(n1, n3) na = lerp(n1, n2) lerp(na, nb)

References

• Hill, chapter 8