Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area

Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area
Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area
Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area
Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area
Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area
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Transcript of Computer Graphics Shading - Arizona Computer .Computer Graphics Copyright Gotsman, Elber, ... Area

  • Shading

    Page 1

    Computer Graphics

    Copyright Gotsman, Elber, Barequet, Karni, Sheffer

    Computer Science, Technion

    The Physics

    2

    3

    Local vs. Global Illumination Models

    Local model direct and local interaction of each object with the light.

    Global model: interactions and exchange of light energy between different objects.

    Example

    Ambient Diffuse

    Specular Final Image

    5

    Light Sources Point source (A): All light originates at a point

    " Rays hit planar surface at different incidence angles Parallel source (B): All light rays are parallel

    " Rays hit a planar surface at identical incidence angles " May be modeled as point source at infinity " Also called directional source

    Area source (C): Light originates at finite area in space. " Inbetween the point and parallel sources " Also called distributed source A B

    C

    6

    Assume non-directional light in the environment Object illuminated with same light everywhere

    " Looks like silhouette

    The Illumination equation I = Iaka

    Ambient Light

    " Ia - ambient light intensity

    " ka - fraction of ambient light reflected from surface.

    Also defines object color

  • Shading

    Page 2

    Computer Graphics

    Copyright Gotsman, Elber, Barequet, Karni, Sheffer

    Computer Science, Technion

    Diffuse Light Dull surfaces such as solid matte plastic reflects incoming light

    uniformly in all directions. Called diffuse or Lambertian reflection Understand intensity as the number of photons per inch2. If a flow

    of m photos passing each second through an inch2 window orthogonal to the flow, is hitting the red surface, how many photons hit an inch2 of the surface ?

    Let is the angle between the direction of incoming light and normal to surface, and let L, N be corresponding unit vectors.

    L N

    Diffused Ball

    Moon Paradox

    10

    Diffuse Reflection

    11

    Assume object is a perfect mirror.

    Lights emits the object in direction V only if V=R

    Reflection from a perfect mirror N L R

    V

    =0

    12

    Shiny objects (e.g. metallic) reflect light in preferred direction R determined by surface normal N.

    Most objects are not ideal mirrors also reflect in the immediate vicinity of R

    Phong Model approximate attenuation by the form of cosn (no real physical basis)

    Specular Reflection

    N L R V

  • Shading

    Page 3

    Computer Graphics

    Copyright Gotsman, Elber, Barequet, Karni, Sheffer

    Computer Science, Technion

    13

    Specular Reflection (Phong Model) Illumination equation:

    ks - Specular reflection coefficient n - Specularity exponent

    14

    Specular Reflection (contd) Exponent n of cosine controls decay factor of attenuation function:

    No physical basis but looks good:

    Retroreflector " sends light back where it came from regardless of the angle

    of insidence

    18

    For multiple light sources:

    " Ip of all light sources are added together " Precautions should be taken from overflows

    More on Illumination Equation shadingmodel

  • Shading

    Page 4

    Computer Graphics

    Copyright Gotsman, Elber, Barequet, Karni, Sheffer

    Computer Science, Technion

    19

    For distance/atmospheric attenuation sources:

    dp - distance between surface and light source and/or distance between surface and viewer (heuristic atmospheric attenuation)

    Even More on Illumination Equation

    20

    Flat Shading

    Applied to piecewise linear polygonal models

    Simple surface lighting approximated over polygons

    Illumination value depends only on polygon normal each polygon is colored with a uniform intensity

    Looks non-smooth (worsened by Mach band effect)

    Normal per Vertex

    21 22

    Gouraud Shading

    Compute illumination intensity at vertices using those normals

    Interpolate intensity over polygon interior

    23

    Gouraud Shading

    24

    Phong Shading Interpolate (at the vertices in image space) normal vectors

    instead of illumination intensities Apply the illumination equation for each interior pixel with

    its own (interpolated) normal

  • Shading

    Page 5

    Computer Graphics

    Copyright Gotsman, Elber, Barequet, Karni, Sheffer

    Computer Science, Technion

    25

    Comments on Shading Phong shading is more expensive (why?) but well

    worth the effort

    Can achieve good looking specular highlight effects

    Both the Gouraud and Phong shading schemes are performed in the image plane and fit well into a polygonal scan-conversion fill scheme

    Both the Gouraud and Phong are view dependent

    Can cause artifacts during animation as they are transformation dependent

    26

    Comparison