SI23 Introduction to Computer Graphics

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SI23Introduction to Computer Graphics

• Lecture 16 Some Special Rendering Effects

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Where were we?

• Phong reflection model tells us how light
  reflects from surfaces

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Phong Reflection Model
light source
N
R
L
eye
?
V
?
surface
dist distance attenuation factor
In practice, we evaluate IRED, IGREEN, IBLUE for
red, green, blue intensities IRED KaREDIaRED
( KdRED( L . N ) Ks( R . V )n ) IRED/dist
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Interpolated Shading

• Polygons can be efficiently shaded using
• Flat shading
• Gouraud shading
• Phong shading
• Visible surfaces are determined using the
  z-buffer algorithm

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Flat, Gouraud and Phong Shading
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Phong versus Gouraud Shading

• A major advantage of Phong shading over Gouraud
  is that specular highlights tend to be much more
  accurate
• vertex highlight is much sharper
• a highlight can occur within a polygon
• Also Mach banding greatly reduced
• The cost is a substantial increase in processing
  time because reflection model applied per pixel
• OpenGL only supports Gouraud shading

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• Adding Realism Through Texture Effects

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Adding Realism

• Objects rendered using Phong reflection model and
  Gouraud or Phong interpolated shading often
  appear rather plastic and floating in air
• Texture effects can be added to give more
  realistic looking surface appearance
• Simple texture
• Bumps
• Light maps

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Adding Surface Detail

• The most obvious solution is not the best
• breaking the scene into smaller and smaller
  polygonal objects increases the detail
• ..BUT it is very hard to model and very
  time-consuming to render
• Preferred solution is texture mapping
• typically a 2D image painted onto objects

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A Simple Example

• Suppose we have a 2D image...
• .. and a 3D box
• .. we can paint the image on a face of the box

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or a teapot
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Basic Concept

• Replace the shading calculation with a look-up
  into a texture map (ie 2D image) to get the
  colour of a pixel
• May replace shaded value - or modulate it in some
  way

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Question

• We could apply the texture in screen space (ie
  after projection)
• ... or we could apply it in object space (ie
  before projection)
• Which is more sensible?

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Texture Mapping - Overview
texture space
screen space
object space
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Mapping Texture to Polygons

• For polygon texture mapping, we explicitly define
  the (u,v) co-ordinates of the polygon vertices
• That is, we pin the texture at the vertices
• We interpolate within the triangle at the time of
  scan converting into screen space

texture space
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Texture Mapping Triangles
Interpolation is done during scan
conversion, similar as is done for Gouraud
interpolated shading
But rather than interpolate to get RGB values,
we get (u,v) values which point to elements of
texture map.
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Interpolation in Texture Space

• The interpolation in texture space has to be done
  carefully
• Equal steps in screen space do not correspond to
  equal steps in object space (and hence texture
  space)
• Why?

V
texture
U
Y
object
X
Z
J
screen
I
A line is a line in all 3 spaces
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Interpolation in Texture Space

• The rate of change in texture space will depend
  on the depth of the points from the viewer
• Correct approach is to scale by the distance (zP,
  zQ) of the points from the viewer

P
Q
P
Q
If Q further away than P, then as we take equal
steps from P towards Q, we want to take
increasingly large steps in (U,V) space from P
to Q.
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Interpolation in Texture Space

• Suppose (uP, vP) and (uQ,vQ) are texture co-ords
  at end-points P, Q
• Linear interpolation would be
• u auQ (1-a)uP
• with a increasing from 0 to 1 (similarly for v)
• Correct texture interpolation is
• u auQ / zQ (1-a)uP / zP / D
• where D a/ zQ (1-a)/ zP

P
Q
P
Q
Note this is equivalent to a linear
interpolation in projective space
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Check for Yourself

• Suppose P is one unit from viewer, and Q is two
  units from viewer

• Show that the mid-point in screen space is
  equivalent to one-third of the distance along the
  line in texture space

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Summing Up

• We have seen how a 2D texture image can be mapped
  to an object, at the rendering stage
• for a polygon, we pin texture to vertices and
  interpolate (correctly!) at scan conversion time
• The texture value is used to modify the colour
  that would otherwise be drawn
• options include replacing completely, or
  modulating (eg by multiplying shaded value with
  texture value)

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• Bump Mapping

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Bump Mapping

• This is another texturing technique
• Aims to simulate a dimpled or wrinkled surface
• for example, surface of an orange
• Like Gouraud and Phong shading, it is a trick
• surface stays the same
• but the true normal is perturbed, or jittered, to
  give the illusion of surface bumps

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Bump Mapping
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How Does It Work?

• Looking at it in 1D

original surface P(u)
bump map b(u)
add b(u) to P(u) in surface normal direction,
N(u)
new surface normal N(u) for reflection model
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Bump MappingA Bump Map
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Bump MappingResulting Image
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Bump Mapping - Another Example
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Bump MappingAnother Example
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Bump MappingProcedurally Defined Bump Map
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Another pioneer

• Jim Blinn
• Creator of bump mapping and many other graphics
  effects

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• Light Maps

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The Problem with Gouraud.

• Gouraud shading is established technique for
  rendering but has well known limitations
• Vertex lighting only works well for small
  polygons
• but we dont want lots of polygons!

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Pre-Compute the Lighting

• Solution is to pre-compute some canonical light
  effects as texture maps
• For example

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Rendering using Light Maps

• Suppose we want to show effect of a wall light
• Create wall as a single polygon
• Apply vertex lighting
• Apply texture map
• In a second rendering pass, apply light map to
  the wall

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Light Maps

• Widely used in games industry
• Latest graphics cards will allow multiple texture
  maps per pixel