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GR2 Advanced Computer Graphics AGR. Lecture 12 Solid Textures Bump Mapping Environment Mapping. Marble Texture. Y. object space. V. texture space. U. X. W. Z. Solid Texture. A difficulty with 2D textures is the mapping from the object surface to the texture image

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GR2 Advanced Computer Graphics AGR

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Gr2 advanced computer graphics agr

GR2Advanced Computer GraphicsAGR

Lecture 12

Solid Textures

Bump Mapping

Environment Mapping


Marble texture

Marble Texture


Solid texture

Y

object space

V

texture

space

U

X

W

Z

Solid Texture

  • A difficulty with 2D textures is the mapping from the object surface to the texture image

    • ie constructing fu(x,y,z) and fv(x,y,z)

  • This is avoided in 3D, or solid, texturing

    • texture now occupies a volume

    • can imagine object being carved out of the texture volume

Mapping functions trivial: u = x; v = y; w = z


Defining the texture

Defining the Texture

  • The texture volume itself is usually defined procedurally

    • ie as a function that can be evaluated, such as:

      texture (u, v, w) = sin (u) sin (v) sin (w)

    • this is because of the vast amount of storage required if it were defined by data values


Example wood texture

V

texture

space

U

W

Example: Wood Texture

  • Wood grain texture can be modelled by a set of concentric cylinders

    • cylinders coloured dark, gaps between adjacent cylinders coloured light

radius r = sqrt(u*u + w*w)

if radius r = r1, r2, r3,

then

texture (u,v,w) = dark

else

texture (u,v,w) = light

looking down:

cross section view


Example wood texture1

Example: Wood Texture

  • It is a bit more interesting to apply a sinusoidal perturbation

    • radius:= radius + 2 * sin( 20*) , with 0<<2

  • .. and a twist along the axis of the cylinder

    • radius:= radius + 2 * sin( 20* + v/150 )

  • This gives a realistic wood texture effect


Wood texture

Wood Texture


How to do marble

How to do Marble?

  • First create noise function (in 1D):

    • noise [i] = random numbers on lattice of points

  • Next create turbulence:

    • turbulence (x) = noise(x) + 0.5*noise(2x) + 0.25*noise(4x) + …

  • Marble created by:

    • basic pattern:

      • marble (x) = marble_colour (sin (x) )

  • with turbulence:

    • marble (x) = marble_colour (sin (x + turbulence (x) ) )


Marble texture1

Marble Texture


Bump mapping

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’


Bump mapping1

Bump Mapping


How does it work

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


How it works the maths

How It Works - The Maths!

  • Any 3D surface can be described in terms of 2 parameters

    • eg cylinder of fixed radius r is defined by parameters (s,t)

      x=rcos(s); y=rsin(s); z=t

  • Thus a point P on surface can be written P(s,t) where s,t are the parameters

  • The vectors:

    Ps = dP(s,t)/ds and Pt = dP(s,t)/dt

    are tangential to the surface at (s,t)


How it works the maths1

How it Works - The Maths

  • Thus the normal at (s,t) is:

    N = Ps x Pt

  • Now add a bump map to surface in direction of N:

    P’(s,t) = P(s,t) + b(s,t)N

  • To get the new normal we need to calculate P’s and P’t

    P’s = Ps + bsN + bNs

    approx P’s = Ps + bsN - because b small

  • P’t similar

    • P’t = Pt + btN


How it works the maths2

How it Works - The Maths

  • Thus the perturbed surface normal is:

    N’ = P’s x P’t

    or

    N’ = Ps x Pt + bt(Ps x N) + bs(N x Pt) + bsbt(N x N)

  • But since

    • Ps x Pt = N and N x N = 0, this simplifies to:

      N’ = N + D

    • where D = bt(Ps x N) + bs(N x Pt)

      = bs(N x Pt) - bt(N x Ps )

      = A - B


Worked example for a cylinder

Worked Example for a Cylinder

  • P has co-ordinates:

  • Thus:

  • and then

x (s,t) = r cos (s)

y (s,t) = r sin (s)

z (s,t) = t

Ps : xs (s,t) = -r sin (s)

ys (s,t) = r cos (s)

zs (s,t) = 0

Pt : xt (s,t) = 0

yt (s,t) = 0

zt (s,t) = 1

N = Ps x Pt :

Nx = r cos (s)

Ny = r sin (s)

Nz = 0


Worked example for a cylinder1

Worked Example for a Cylinder

  • Then: D = bt(Ps x N) + bs(N x Pt) becomes:

  • and perturbed normal N’ = N + D is:

D : bt *0 + bs*r sin (s)= bs*r sin (s)

bt *0 - bs*r cos (s)= - bs*r cos (s)

bt*(-r2) + bs*0= - bt*(r2)

N’ :r cos (s) + bs*r sin (s)

r sin (s) - bs*r cos (s)

-bt*r2


Bump mapping a bump map

Bump MappingA Bump Map


Bump mapping resulting image

Bump MappingResulting Image


Bump mapping another example

Bump Mapping - Another Example


Bump mapping another example1

Bump MappingAnother Example


Bump mapping procedurally defined bump map

Bump MappingProcedurally Defined Bump Map


Environment mapping

Environment Mapping

  • This is another famous piece of trickery in computer graphics

  • Look at a highly reflective surface

    • what do you see?

    • does the Phong reflection model predict this?

  • Phong reflection is a local illumination model

    • does not convey inter-object reflection

    • global illumination methods such as ray tracing and radiosity provide this

  • .. but can we cheat?


Environment mapping recipe

Environment Mapping - Recipe

  • Place a large cube around the scene with a camera at the centre

  • Project six camera views onto faces of cube - known as an environment map

projection of scene

on face of cube -

environment map

camera


Environment mapping rendering

Environment Mapping - Rendering

  • When rendering a shiny object, calculate the reflected viewing direction (called R earlier)

  • This points to a colour on the surrounding cube which we can use as a texture when rendering

environment

map

eye

point


Environment mapping limitations

Environment Mapping - Limitations

  • Obviously this gives far from perfect results - but it is much quicker than the true global illumination methods (ray tracing and radiosity)

  • It can be improved by multiple environment maps (why?) - one per key object

  • Also known as reflection mapping

  • Can use sphere rather than cube


Environment mapping1

Environment Mapping


Environment mapping2

Environment Mapping


Jim blinn

Jim Blinn

  • Both bump mapping and environment mapping concepts are due to Jim Blinn

  • Pioneer figure in computer graphics

www.research.microsoft.com/~blinn

www.siggraph.org/s98/conference/

keynote/slides.html


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