Bearbeiter: Ben Othman kabil

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Proseminar WEB 3D

• Bearbeiter Ben Othman kabil
• Betreuer Boulila
  Naoufel
• Aufgabensteller Prof. Boulila
  Naoufel
• Abgabedatum 13. Febraur
  2003

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Illumination and Shading

• Light Sources
• Empirical Illumination
• Shading
• Transforming Normals

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Illumination Models

• Illumination
• The transport luminous flux
• from light sources between
• points via direct and indirect paths
• Lighting
• The process of computing the luminous
• intensity reflected from a specified 3-D point
• Shading
• The process of assigning a colors to a pixels
• Illumination Models
• Simple approximations of light transport
• Physical models of light transport

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Two Components of Illumination

• Light Sources (Emitters)
• Emission Spectrum (colour)
• Geometry (positional and directional)
• Directional Attenuation
• Surface Properties
• Reflectance Spectrum (colour)
• Geometry (position, orientation, and
  micro-structure
• Absorption

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Light Sources

• Ambient Light Sources
• No spatial or directional characteristics
• Independent of object's position and orientation
• Directional Light Sources
• All rays have common direction
• Example sunlight
• Spot Light Sources
• Emits rays in a radial direction from its source
• Example light bulb
• Other Light Sources

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Diffuse Reflection and Lighting

• Ideal diffuse Reflection
• A very rough on microscopic level
• Examplechalk
• Computer Diffuse reflection
• Lamberts cosine Law
• How of the incoming light energy is
  reflected

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Diffuse reflection und lighting

• Ideal Diffuse Reflection
• A very rough surface on microscopic level
• Example chalk
• Lambert's Cosine Law
• How much of the incoming light energy is
  reflected
• Computing Diffuse Refelection

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Diffuse reflection und lighting
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Specular Reflection and Lighting

• Ideal Specular Reflection
• A very smooth surface on microscopic level
• Example mirror
• Snell's Law
• Non-ideal Reflectors
• Empirical model

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Specular Reflection and Lighting
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Phong Illumination

• The Phong Illumination Model
• Phong examples
• Variable Direktion of the light source

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Simple Shading

• Flat Shading
• A only one illumination calculation for each
  primitive
• Facet Shading
• Apply illumination equation at each point

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Gouraud Shading

• First applies the illumination model one subset
  of surface points
• Then interpolates the intensity of the
  remaining points on the surface
• But the facet artefacts are still visible

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Phong Schading

• Based on Goraud Shading
• First the surface normal is linearly
  interpolated across polygonal facets
• Then the illumination model is applied at every
  point

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Texture Mapping

• Texture mapping is a successful technique for
  high-quality image synthesis
• Results depend on key elements
• Mapping
• Filtering
• Sampling

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What will we cover?

• Image warping
• Affine, bilinear, projective mappings
• Scan algorithms
• Anti-aliasing
• EWA Filter (Elliptical Weighted Average)

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What is texture mapping?

• Purpose To create rich visual illusions
  inexpensively
• Definition Mapping a texture image onto a
  surface in a 3D scene

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Mapping spaces
(u, v)
(x, y)
Texture Space (2D)
Screen Space (2D)
Screen Space (3D) (z buffer)
Object Space (3D)
parameterization
projection
World Space (3D)
rendering
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Texture rendering

• Texture mapped to cube model
• Cube projected onto screen space

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Hidden Surface Removal

• Problem Given a set of 3D objects and a
  viewing specification, determine which lines or
  surfaces are invisible.
  
  
• Primitives outside of the field of view
• Back-facing primitives on a closed, convex object
• Primitives occluded by other objects closer to
  the camera

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General Approaches

• Image-Based Approach (Image Precision)
  
• for(each pixel in the image)
• determine the ray through the pixel
• determine the closest object along the
  ray
• draw the pixel in the color of the
  closest object
• 
  
• Object-Based Approach (Object Precision)
• for (each object in the world)
• determine unobstructed object parts
• draw the parts in the appropriate color

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Coherence

• Objects typically have properties which vary
  smoothly
• Face coherence
• Edge coherence
• Scan line coherence
• Area coherence
• Depth coherence
• Frame coherence
• Object coherence
• Helps to avoid making expensive calculations, or
  to turn them into simple differences (like line
  drawing)

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Perspective Transformations

• Depth comparisons are typically performed after
  the perspective transformation
• Since were looking down the z axis, two points
  are now on the same ray if x1x2 and y1y2
• The perspective transformation preserves
• Relative depth
• Straight lines
• Planes

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Normalizing the Viewing Frustum

• Transform frustum to a cube before clipping -
  that is convert perspective frustum to
  orthographic frustum

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Handling Occlusion

• Problem some polygons will overlap, therefore
  we must determine which portion of each polygon
  is visible to eye.

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Back Culling

• Reduces complexity only
• Convex self-occlusion
• Angle between the viewing direction an the
  normal vector of a polygon to determine
  visibility
• Criterion vector product positive

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Back To Front Sorting

• aka The Painter's method
• First Step The list of surfaces are sorted
  according to their distance from the viewpoint
• Second Step The objects are then painted from
  back-to-front.
• Criterion for polygon comparison simplest
  solution compare the maximum z coordinate

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Problems with Painters

• Choosing a suitable sorting algorithm
• Bubble sort complexity O(n2)
• Radix sort complexity O(n)
• Quick sort complexity O(n lg n)
• The criterion of comparing the maximum z
  coordinate is not correct in general
• Mutual overlapping and piercing

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Constructing a BSP Tree

• The Binary Space Partition (BSP) algorithm
• Select a partitioning plane/facet
• Partition the remaining planes/facets according
  to the side of the partitioning plane that they
  fall on ( or -)
• Repeat with each of the two new sets
• In the case of a crossing facet we clip it into
  two halves

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Computing Visibility with BSP Trees

• Starting from the root of the tree
• Classify viewpoint as being in the positive or
  negative half-space of our plane
• Call this routine with the opposite half-space
• Draw the current partitioning plane
• Call this routine with the same half-space

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Pixel-level Visibilty

• Thus far - visibilty at the level of primitives
• Now - visibilty at the level of each pixel

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Ray Casting

• A very general algorithm
• Works with any primitive we can write
  intersection tests for
• but it's hard to make it run fast
• And thats why
• can use it for much more than visibility testing
• shadows, refractive objects, reflections, motion
  blur,...

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Scanline Visibilty

• Looks a lot like polygon rasterization
• Maintains active object table
• Looks at one scanline at a time - no need to
  store entire image - nice if memory scarce