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Shading

CMPT 361

Introduction to Computer GraphicsTorsten Mller

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Reading

Chapter 5.5 - Angel

Chapter 16 - Foley, van Dam, et al

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Shading

Illumination Model: determine the color of a

surface (data) point by simulating some light

attributes.

Local IM: deals only with isolated surface

(data) point and direct light sources.

Global IM: takes into account the

relationships between all surfaces (points) inthe environment.

Shading Model: applies the illumination models

at a set of points and colors the whole scene.

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Shading of surfaces

We can shade a point on a surface using a

local IM

How about surfaces? Flat (planar) polygons are computationally

attractive

Normals, intersections, visibility, projections, etc.,

are all easy to compute hardware acceleration available

Curved surfaces are often tessellated into many

small flat polygons in graphics polygonal

meshes

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Polygonal meshes

Composed of a set of polygons (often

quadrilaterals or triangles) pasted along

their edges

The dominant surface model for 3D shapes

(esp. free-form shapes), e.g., in games

Well, now there are points

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Shading of flat polygons

Flat (constant, faceted) shading

compute illumination once per polygon and

apply it to whole polygon:

glShadeModel(GL_FLAT)

Interpolated/smooth/Gouraud shading

compute illumination at borders (e.g. vertices)

and interpolate: glShadeModel(GL_SMOOTH) Accurate shading e.g., Phong shading

compute illumination at every point of the

polygon

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Flat shading

N1

N2

A single color across the polygon efficient

Intensity discontinuity across edges

Flat shading would be correct if

Light source is at infinity, i.e., light vector

l is constant, so nl is constant across polygon, and

viewer is at infinity, so rv is constant across polygon,

and

polygons represent actual surface, not an approximation

Also, if there are a very large number of very

small polygons, the faceting effects is less obvious

spatial integration of our eyes

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Results of flat shading

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Mach Banding

Creates discontinuities in colour

easily visible

hence we can see the distinct surface patcheseasily

Machbands!

caused by lateral

inhibition of thereceptors in the eye

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Mach Banding (2)

Problems - Machbands!

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Gouraud shading

This shading model is interpolative:

Given colors of the polygon vertices, interior

points are colored through bilinear interpolation

How to compute the normal at a mesh

vertex?

E.g., take the (normalized) average of the

normals of adjacent faces

Also, area weighted normals

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Flat vs. Gouraud Shading

Gouraud ShadingFlat Shading

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Phong shading

Gouraud shading does not properly handle

specular highlights because of color interpolation

Phong shading accurate shading

Interpolates normals at each point instead of colors

Apply LIM at each point according to approximated

normal

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Phong vs. Gouraud shading

Phong shading:

Handles specular highlights much better

Does a better job in handling Mach bands But much more expensive than Gouraud

shading

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Further problems with shading

models so far (1) Polygonal silhouette we are quite sensitive

in picking these up

Solution: subdivide further Exercise: How to determine

whether an edge in a mesh

is a silhouette edge?

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Orientation dependence

Note first that interpolation is done along

horizontal scanlines

When the orientation of the same polygon

changes, the same point p may be colored

differently

Solution: triangulate

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Further problems with shading

models so far (2)

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Further problems with shading

models so far (3) Unrepresentative normals

As shown, all vertex and interpolated normals

are the same

Solution: subdivide further