Shading and Lighting (04) RNDr. Martin Madaras, PhD. - - PowerPoint PPT Presentation

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Shading and Lighting (04) RNDr. Martin Madaras, PhD. - - PowerPoint PPT Presentation

Mar 13, 2024 446 likes •1.28k views

Fundamentals of Computer Graphics and Image Processing Shading and Lighting (04) RNDr. Martin Madaras, PhD. madaras@skeletex.xyz Overview Direct Illumination Emission at light sources Scattering at surfaces Gouraud shading

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Fundamentals of Computer Graphics and Image Processing

Shading and Lighting (04)

  • RNDr. Martin Madaras, PhD.

madaras@skeletex.xyz

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Overview

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 Direct Illumination

 Emission at light sources  Scattering at surfaces  Gouraud shading

 Global Illumination

 Shadows  Refractions  Inter-object reflections

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SLIDE 3

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  • Ask questions, please!!!
  • Be communicative
  • www.slido.com #ZPGSO04
  • More active you are, the better for you!

How the lectures should look like #1

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SLIDE 4

Overview

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 𝐽𝑀(𝑦, 𝑧, 𝑨, 𝜄, 𝜒, 𝜇)

 describes intensity of energy,  leaving a light source, ...  arriving at location (x,y,z), ...  from direction (θ,φ), ...  with wavelength λ

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Light Source types

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 Omnidirectional  Spotlight  Area  Directional  Object

  • what are the differences?
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Light Source Models

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 Simple mathematical models  OpenGL support

 Point light  Directional light  Spot light

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Point Light Source

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 Models omni-directional source

 intensity (𝐽0),  position (𝑦, 𝑧, 𝑨),  factors (𝑙𝑑, 𝑙𝑚, 𝑙𝑟) for attenuation with distance (𝑒)

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Directional Light Source

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 Models point light source at infinity

 intensity (𝐽0),  direction (𝑒𝑦, 𝑒𝑧, 𝑒𝑨),  no attenuation with distance

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Spot Light Source

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 Models point light source with direction

 intensity (𝐽0),  position (𝑦, 𝑧, 𝑨),  direction (𝑒),  attenuation 𝑙𝑑, 𝑙𝑚, 𝑙𝑟 .

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Overview

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 Direct Illumination

 Emission at light sources  Scattering at surfaces  Gouraud shading

 Global Illumination

 Shadows  Refractions  Inter-object reflections

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Elementary theory

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 Light-surface interaction  Reflection  Refraction

 Snell’s law

 Surface normal

vector

 Real world is a bit different

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Surface types

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 Reflective  Diffuse – Lambertian  Both

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Surface types

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Mirror Matte directional indirectional component component

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Light models

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 Empiric – e.g. Phong lighting model

 cheap computation  physically incorrect  visually plausible

 Physically-based

 energy transfer, light propagation  closer to real-world physics  expensive

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Local illumination models

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 Fast but inaccurate  Empirical (no physical background)  Many physical effects are impossible to achieve  Computer games, real-time rendering

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Modeling Surface Reflectance

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 Rs(θ,φ,γ,ψ,λ) ...

 describes the amount of incident energy, ...  arriving from direction (θ,φ), ...  leaving in direction (γ,ψ), ...  with wavelength λ

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”  Bui Tuong Phong, 1973

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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Diffuse Reflection

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 Assume surface reflects equally in all directions

 Examples: chalk, clay

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Diffuse Reflection

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 How is light reflected ?

 Depends on an angle of incident light

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Diffuse Reflection

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 How is light reflected ?

 Depends on an angle of incident light

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Diffuse Reflection

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 Lambertian model

 cosine law (dot product)

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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Specular Reflection

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 Reflection is strongest near mirror angle

 Examples: Mirrors, Metals

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Specular Reflection

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 How much light is seen ?  Depends on:

 angle of incident light  angle of viewer

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Specular Reflection

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 Phong Model

 cos(α)n This is a physically motivated hack!

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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Emission

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 Represents light emitted directly from surface

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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Ambient term

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 Represents reflection of all indirect illumination  This is a total hack

 (avoids complexity of global illumination)!

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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SLIDE 33

OpenGL Reflectance Model

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 Simple empirical model:

 diffuse reflection +  specular reflection +  emission +  “ambient”

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Diffuse light

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Ambient light

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Diffuse + Ambient light

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Specular + Diffuse + Ambient light

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Phong lighting model

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 Ambient + Diffuse + Specular components  Simulates global light scattered in the scene and reflected

from other objects

without ambient with ambient

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Other lighting models

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 Blinn-Phong

 generalization of Phong’s model

 Cook-Torrance

 microfacets

 Oren-Nayar

 rough surfaces

 Anisotropic microfacet distribution

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Direct Illumination

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 Single light source example

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Direct Illumination

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 Multiple light source example

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3D rendering pipeline

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3D polygons Modeling Transformation Lighting Viewing Transformation Projection Transformation Clipping Scan Conversion 2D Image

1

Draw pixels

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Ray Casting

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 Independent lighting calculation for each pixel

 Computationally expensive

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Polygon Shading

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 Can take advantage of spatial coherence

 Illumination calculations of pixels of a triangle are related  Scanline rasterization

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Surface normal vector

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 Perpendicular to the surface at the point  Computation:

 Usually from tangent vectors  Vector cross product  Depends on the

  • bject representation

 Vector

normalization

v u n     = n n n   = ˆ u  v  n 

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Tangent vectors

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 Parametric representation

 X = x(u,v)  Y = y(u,v)  Z = z(u,v)

 Partial derivation by u,v → vectors tu, tv

 Polygonal representation

 Tangent vectors are edge vectors  Mind the orientation!

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SLIDE 47

Overview

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 Global Illumination

 Shadows  Refractions  Inter-object reflections

 Shading

 Flat  Gouraud  Phong

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Flat Shading

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 Fill triangles using single calculated color

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Flat Shading

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 One Illumination calculation per polygon  Assign all pixels of each polygon the same color

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Flat Shading

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 Objects look like they are composed of polygons  Ok for polyhedral objects  Not so good for smooth surfaces

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Gouraud Shading

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 One lighting calculation per vertex  Assign pixels inside polygon by interpolating colors

computed at vertices

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Gouraud Shading

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 Bilinearly interpolate colors of triangle across scan line

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Gouraud Shading

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 Smooth shading over adjacent polygons

 Curved surfaces  Illumination highlights

 Produces smoothly shaded polygonal mesh

 Fast linear approximation  Needs fine mesh to capture subtle lighting effects

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Gouraud Shading

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Phong Shading

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 NOT Phong lighting model  One lighting calculation per pixel

 Approximate surface normals inside polygon using bilinear

interpolation of normals from vertices

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Phong Shading

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 Bilinearly interpolate surface normals at vertices down

and across scan lines

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Polygon Shading Algorithms

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Example: Wireframe scene

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Example: Ambient only

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Example: Flat shading with Diffuse

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Example: Gouraud shading with Diffuse

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Example: Gouraud shading with Specular

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Example: Phong shading with Specular

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Shading Issues

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 Problems with interpolated shading

 Problems computing shared vertex normals  Perspective distortion  Problems at T

  • vertices
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Shading Benefits

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 Good performance and quality of output

 Excellent for hardware  Works well with subdivision surfaces

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  • Ask questions, please!!!
  • Be communicative
  • www.slido.com #ZPGSO04
  • More active you are, the better for you!

How the lectures should look like #2

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SLIDE 67

Overview

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 Advanced Shading and Mapping

 Deferred Shading  Shadow Mapping  Normal Mapping  Displacement Mapping  Vector Displacement Mapping

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Deferred Shading

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 Compute Lighting in Screen-Space  Two pass approach  Decoupling of geometry and lighting  G-Buffer stores positions, normals, materials …  Lighting is a per-pixel operation  Problems with transparency and G-buffer size  O(objects+lights)

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Deferred Shading

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Diffuse Color Z Buffer Surface Normals Final Composition

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Shadow Mapping

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 Two pass technique  Obtain Light view depth buffer  Compare each pixel rendered to with light depth  Pixels further away are in shadow  Needs margin of error for lit pixels  Implementation usually has artifacts

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Shadow Mapping

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Normal Mapping

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 Fake lighting of bumps and dents  “Dot3 bump mapping”  Add lighting details without additional geometry  Store normals from high-polygon object in texture  Encode X,Y,Z as R,G,B color information

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Normal Mapping

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Normal Mapping

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a)

Normal map

(encoded in object space)

b)

Original high-res model

c)

Rendered low-res model

d)

Applied normal map

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Displacement Mapping

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 Move geometry as specified in texture  Displacement in direction of surface normal  Can add additional detail to a subdivided model  Relies on dense geometry  Usually used with adaptive tessellation techniques

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Displacement Mapping

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Vector Displacement Mapping

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 Displace geometry in any direction  Generalization of displacement mapping  Possible to store detailed geometry in textures  Excellent for sculpting purposes (Z-Brush)

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Vector Displacement Mapping

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Visibility, Culling

Next Week

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Acknowledgements

 Thanks to all the people, whose work is shown here and whose

slides were used as a material for creation of these slides:

Matej Novotný, GSVM lectures at FMFI UK Peter Drahoš, PPGSO lectures at FIIT STU Output of all the publications and great team work Very best data from 3D cameras

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www.skeletex.xyz madaras@skeletex.xyz

Questions ?!