From Graphics to Visualization

From Graphics to Visualization

Introduction Light Sources

Surface Lighting Effects

Basic (Local ) Illumination Models Polygon-Rendering Methods

Texture Mapping

Transparency and Blending Visualization Pipeline

outline

Introduction

• Illumination Model (Lighting/Shading Model)

Calculation of color on an illuminated position on the surface of an object

• Surface Rendering

A procedure for applying a lighting model to

obtain pixels colors for all projected surface

positions

• Point Source

Diverging ray paths from a point light source

Light Sources

Light Sources

• Distributed Light Source

Light rays from an infinitely distant light source illuminate an object along nearly parallel light paths

Surface Lighting Effects

Diffusereflections from

a surface (dull/roughsurface) (shinysurface) Specular reflection superimposed on diffuse reflection vectors

(Global Illumination) Surface lighting effects are produced by a combination of illumination

from light sources and reflection from other surfaces.

Rendering methods differ in approximating lighting effects

• Global illumination: ray tracing, accurate by computationally expensive

• Local illumination: relate the illumination of a given scene point directly to the light set, not to any other scene points If we denote the angle of incidence between the incoming Light direction and the surface

Illumination Models

1) Ambient Light

2) Diffuse Reflection

If we denote the angle of incidence between the incoming Light direction and the surface normal as θ

a a ambdif f k I

I 

Angle of incidence θ between the unit light- source direction vector L and the unit normal vector N at a surface position.

Basic (Local )Illumination

Models

Basic (Local )Illumination Models

2) Diffuse Reflection

I

=K

I

cos θ I

=K

I

(N‧L)

k_d:

diffuse-reflection coefficient, or diffuse reflectivity.

◆ Lambertian reflectors

◆ Lambert’s cosine law

– Total Diffuse-reflection of a single point-source illumination















 

0 L

N if

,

0 L

N if ), (

a a

l d a

a

diff

k I

L N I k I

I k

Basic (Local )Illumination Models

3) Specular Reflection and Phong Model

– Phong Specular-Reflection Model (Phong Model)





spec

l

W I

I

(  ) cos

Specular reflection angle equals angle of incidence θ

 



















 

0 L N or 0 R V if

, 0 . 0

0 L N and 0 R V if , ) (

,

ns

l s spec

l

R V I I k

(2-4) (2-5)

Basic (Local )Illumination Models

3) Specular Reflection and Phong Model

Modeling specular reflections (shaded area) with parameter n_s

The projection of either

L or R onto the direction of the normal vector N has a magnitude equal to N˙L.

L N

L N

R  ( 2  ) 

3) Specular Reflection and Phong Model

Specular reflections from a spherical surface for varying specular parameter values and a single light source

Basic (Local )Illumination

Models

3) Specular Reflection and Phong Model

Halfway vector H along the bisector of the angle between L and V.

|

| L V V H L



 

Basic (Local )Illumination

Models

Color Intensity Calculations

• Phong Algorithm:

I = I

＋ I

＋ I

I

ambient reflection I

: diffuse reflection I

: specular reflection I

＝k

I

I

＝k

I

cosα here cosα ＝(L‧N) I

＝k

I

cos

β here cosβ＝(R‧V)

• Multiple light sources:

P

L

R N

α α β V





 



i i

s d

a

r C

I I I

I

1

ri i

Here r_iis the distance to the light i and C is a constant

I_l

L, N, R, V are vectors

Shading

• Technique To Render Solid Surfaces

• Determines How Surfaces Will Be Filled

• Process for Computing the Color Intensity Value for Each Pixel Contained in a Polygon

• The Most Common Shading Techniques Are:

– Flat Shading glShadeModel (GL_FLAT);

– Gouraud Shading glShadeModel (GL_SMOOTH);

– Phong Shading (OpenGL by default doesn't do phong shading )

Shading Techniques

Flat Shading

Gouraud Shading Phong Shading

No Shading

Flat Shading

Flat Shading

• Constant Shading Or Flat Shading

• The Simplest and Cheapest and Therefore Fastest Shading Method

• Filling An Entire Polygon with One Color Intensity

• This Model is Only Valid (Realistic) If:

– The light source is imagined to be at infinity – The viewer is at infinity

– The polygon is not an approximation to a curved surface

Gouraud Shading

Gouraud Shading

• Also called Smooth shading

• Color Interpolation Algorithm

– Interpolation along polygon edges – Interpolation across polygon surfaces

Color Values Given On A Per Vertex Basis

Interpolation Along The Edges

Interpolation Across The Surface

V1 V2

V3

Gouraud Shading Illustration

I

( x

, y

)

I

( , x y

)

I

( , x y

)

I

_I

b

I

 

I

 y y I y

y I y y

 1   

1 2

1

(

)

(

)

 

I

 y y I y

y I y y

 1   

1 3

1

(

)

(

)

 

I

x x I x x I x x

b a

a b s b s a

  1   

( ) ( )

y

I

 I

  I

I

 I

  I

I

 I

  I

I

 y y I I

 1 

1 2

1 2

( ) ^I

y y I I

b



 1 

1 3

1 3

( ) I

x x I I

s

b a

b a

  1 

( )

 1

 j

y

Phong Shading

Phong Shading

• An Interpolation Process Similar To Gouraud Shading

• Interpolation Over Normal Vector Instead of Vertex Color

– Normal vectors tell about an objects orientation

– Surface orientation is important in respect to the position of

• The observer/viewer of a scene

• The source of lighting

• Creates greater realism than Gouraud shading

– Specially when combined with an illumination model

– Usually implemented through application software

– Very computing intense

Phong Shading Illustration

( , x y

)

( , x y

)

y

N

N

N

N

N

N

¹ 

⁽

⁾

⁽

⁾ 

2 1

s s

a

N y y N y y

y

N y _{  }

   



 ^{( ) ( )} 

1

1 3 3

1 3 1

s s

b

N y y N y y

y

N y _{  }

   



 ^{( ) ( )} 

1

a s

b s

b a a

b

s

N x x N x x

x

N x _{  }

   

 )

,

( x

y

Fill-Area Primitives

• Fill (Filled) Area

-- An area filled with some solid color or a pattern

• Surface Tessellation

-- Approximating a curved surface with polygon facets (a

polygon mesh)

Texture Mapping

Texture Mapping

(a) Texture Image; (b) Texture-mapped object

Texture Mapping

Texture Image

Texture Mapping

Texture Mapping: Coordinates Transformations

Texture Mapping

Texture Mapping

Transparency and Blending

(a) as Background + (b) as Foreground

=> blended as (c) ^{The Grid}

The Gaussian Surface

Height plot of e

drawn

on top of the domain grid

Visualization Pipeline

float data[N_x,N_y]

Class Quad

) ( ^{2 2}

) ,

f( x y _ e

^x

^y Continuous data

Discrete dataset

Geometric object

Displayed image

Data Acquisition

Data Mapping

Rendering