Emboss Bump Mapping

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Emboss Bump Mapping

• Michael I. Gold
• NVIDIA Corporation

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Bump Mapping

• Real bump mapping uses per-pixel lighting
• Lighting calculation at each pixel based on
  perturbed normal vectors
• Computationally expensive
• For more information see
• Blinn, J. Simulation of Wrinkled Surfaces.
  Computer Graphics. 12, 3 (August 1978), 286-292

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Emboss Bump Mapping

• Emboss bump mapping is a hack
• Diffuse lighting only, no specular component
• Under-sampling artifacts
• Possible on todays consumer hardware
• If it looks good, do it!

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Diffuse Lighting Calculation

• C (LN) ? Dl ? Dm
• L is light vector
• N is normal vector
• Dl is light diffuse color
• Dm is material diffuse color
• Bump mapping changes N per pixel
• Emboss bump mapping approximates (LN)

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Approximate diffuse factor LN

• Texture map represent height field
• 0,1 height represents range of bump function
• First derivative represents slope m
• m increases/decreases base diffuse factor Fd
• (Fdm) approximates (LN) per pixel

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Approximate derivative

• Embossing approximates derivative
• Lookup height H0 at point (s,t)
• Lookup height H1 at point slightly perturbed
  toward light source (s?s, t?t)
• subtract original height H0 from perturbed height
  H1
• difference represents instantaneous slope mH1-H0

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Compute the Bump
Original bump (H0) overlaid with second bump (H1)
perturbed toward light source
Original bump (H0)
brightens image
darkens image
Subtract original bump from second (H1-H0)
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Compute the Lighting

• Evaluate fragment color Cf
• Cf (LN) ? Dl ? Dm
• (LN) ? (Fd (H1-H0))
• Dm ? Dl encoded in surface texture color Ct
• Could control Dl seperately if youre clever
• Cf (Fd (H1-H0)) ? Ct

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Is that all? Its so easy!

• Were not quite done yet. We still must
• Build a texture
• Calculate texture coordinate offsets ?s, ?t
• Calculate diffuse factor Fd
• Both are derived from normal N and light vector L
• Now we have to do some math

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Building a Texture

• Conserve Textures!
• Current multitexture hardware only supports two
  textures
• Bump map in ALPHA channel
• Maximum bump 1.0
• Level ground 0.5
• Maximum depression 0.0
• Surface color in RGB channels
• Set internalformat to RGBA8 !!

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Calculate Texture Offsets

• Rotate light vector into normal space
• Need Normal coordinate system
• Derive coordinate system from normal and up
  vector
• Normal is z-axis
• Cross product is x-axis
• Throw away up vector, derive y-axis as cross
  product of x- and z-axes
• Build 3x3 matrix from axes
• Transform light vector into Normal space

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Calc Texture Offsets (contd)

• Use normal-space light vector for offsets
• L Mn ? L
• Use Lx, Ly for ?s, ?t
• Use Lz for diffuse factor!
• If light vector is near normal, Lx, Ly are
  small
• If light vector is near tangent plane, Lx and
  Ly are large
• What is Lz is less than zero?
• Light is on opposite side from normal
• Fade contribution toward zero

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Implementation on TNT

• Calculate vectors, texcoords on the host
• Pass diffuse factor as vertex alpha
• Could use vertex color for light diffuse color
• H0 and surface color from texture unit 0
• H1 from texture unit 1 (same texture, different
  coordinates)
• ARB_multitexture extension
• Combiners extension (TBD)

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Implementation on TNT (contd)

• Combiner 0 alpha setup
• (1-T0a) T1a - 0.5
• T1a-T0a maps to -1,1 but hardware clamps to
  0,1
• 0.5 bias balances the loss from clamping
• Could modulate light diffuse color with T0c
• Combiner 1 rgb setup
• (T0cC0a T0cFda - 0.5) 2
• 0.5 bias balances the loss from clamping
• scale by 2 brightens the image