CS 184: COMPUTER GRAPHICS

Lecture #19 -- Mo: 4/5, 1999.

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Preparation:

Read:  Ch. 16.1 -- 16.3, "Illumination and Shading"

Lecture Topics

Review

  • Lighting models tell us what brightness and what color to expect on each surface.
  • Shading/rendering concerns techniques to produce the apparent brightness values on the display efficiently!
  • GLIDE Lighting Model
  • Various lights: ambient(sky), directed(sun), point(bulb), spot(focused).
  • Superposition Law: sum effects of individual lights

    • IDEAL AND REAL SURFACES

    Ideal Lambert Surface

    Perfect Mirrors

    Approximation of Real Surfaces

    Real surfaces are a mixture of chalky properties and of a dull, dusty mirror.
    ==> They have some diffuse as well as some specular reflection.
    Reflected beam is spread out in a small angle around the unit vector R (club shape).
    ==> Phong model: Models the reflective component on a real surface.
    Intensity falls of with a user definable power of the cosine of the deviation from ideal R dir.
     
  • Phong Lighting Model

    • Show effect of exponent on cos function.
     
  • Phong Highlight on glossy surface

    • Even uniform directional light, with flat surface can produce nonuniform brightntess, if surface is partially reflective and we use a Phong illumination model.
     

    Better Approximations of Real Surfaces:

    Real material are more complicated: Spreading of light (Phong exponent) may depend on:
    -- incident angles {Torrence Sparrow model },
    -- the material  in metal, reflected light penetrates deeper into substrate, picks up color of metal {metalness factor},
    -- color of light {spectral behavior}.

    Surface Characterization

  • Relationship between ka, kd, ks; and the C’s and the K’s
  • Shading Examples using spheres.

    • Brightness Normalization.

    Normally one tries to carefully choose lights so that the produced brightnesses fall into the range 0 to 1.0 which is then mapped onto the, say, 256 discrete beam intensities of CRT.
    In spite of best intentions the sum of all calculated brightnesses may exceed unity,
    this may produce funny effects if left unchecked !
    --> at the very least: clip to unity
    --> better: detect and scale downwards all brightness values on object or in scene.

    Now that we know what the brightness is that we would like to see,
    the question arises how can we efficiently generate all the pixels of the right brightness ?

    Shading and Rendering: Techniques to produce desired display brightness.

    Needs to be efficient ! We cannot afford to do full shading calculation for each pixel.

    Lambert lighting model + flat faces + uniform illumination -> easy:

    ONE brightness value per polygon.
    ==> just take a representative value at centroid of face, and apply brightness values to all pixels of the polygon.

    All other cases are harder: cause non-uniform apparent brightness.

  • Causes of non-uniform brightness

    • Point light or spotlight close to surface, or a curved surface will have varying brightness.
    Could sample more finely, and subdivide large polygons into many smaller coplanar ones. But this would still look patchy with sharp, contrasty brightness boundaries in between.
    Need to smoothly blend these patches together: -->  Sample at corners of patches and then interpolate between these sample points.
    We cast this procedure into the framework of our scan-line algorithm,  and do an interpolation similar to the interpolation of the z-values in the z-buffer.

    1. Gouraud Shading Technique: 
    Linear Interpolation of (Scalar) Brightness Values

    Good for: handling small nonuniformities in illumination intensity, or for
    Polyhedral approximations to round objects.
    Principle: Use barycentric intensity interpolation;
    ==>  Implementation:
    GOURAUD SHADING TECHNIQUE:

    1. compute shading at visual vertices, A...D
    2. compute shading along edges by linear interpolation.
    3. compute shading along spans by linear interpolation.

    This also works for round objects:
    Take an averaged brightness value at each corner, determined by averaged vertex normal
    == weighted face normals of all the adjacent polygons by angle subtended at vertex.
    But this is fake: edges remain straight! -- but it is efficient.

    Smooth shading can be applied whenever the apparent brightness changes across a surface,
    -- either because the illumination is non-uniform,
    -- the reflection has highlights,
    -- or the surface is curved.
     

  • Limitations of Gouraud Shading:

    •  Some flat spots; extrema may be "short-circuited";   discontinuities at concave corners;   missing highlights...

    2nd Technique: Phong Shading: 
    Interpolate normal vector directions

    We can fix some of the above problems with a more sophisticated interpolation scheme.
    PHONG SHADING
    Phong can interpolate over flat spots.

    1. compute normals at visual vertices, A...D
    2. compute normal along edges: (interpol)
    3. compute normal along spans: (interpol)
    4. for every pixel compute brightness from interpolated normal orientation.

    Problems with Phong shading:
    Not perfect either ! --> example of "corrugated surface"
    Limitations of Phong Shading
    “Corrugated” structures with too few vertices (just zig-zag) may have all parallel normal directions and cannot represent periodic shading variations.
    Geometry that is not seen at vertices cannot be inferred !!
    Illumination changes that are not seen at vertices cannot be inferred !!
     

    Procedural Shading

    Texture Mapping, Bump Mapping, ...

    Current Homework Assignment:

    ASG#8  "Lighting Calculations and Gouraud Shading"
    DUE: Saturday 4/10/99, 11:59pm.
    CAN BE DONE WITH YOUR PARTNER OF CHOICE !

    Midterm Exam will be held in class on April 14, 1999.

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