1. Michael I. Gold NVIDIA Corporation
Emboss Bump Mapping
Michael I. Gold
NVIDIA Corporation

2. 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),

3. Emboss Bump Mapping Emboss bump mapping is a hack
Diffuse lighting only, no specular component
Under-sampling artifacts
Possible on today’s consumer hardware
If it looks good, do it!

4. Diffuse Lighting Calculation
C = (L•N)  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 (L•N)

5. Approximate diffuse factor L•N
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
(Fd+m) approximates (L•N) per pixel

6. 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 m=H1-H0

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

8. Compute the Lighting Evaluate fragment color Cf Cf = (L•N)  Dl  Dm
(L•N)  (Fd + (H1-H0))
Dm  Dl encoded in surface texture color Ct
Could control Dl seperately if you’re clever
Cf = (Fd + (H1-H0))  Ct

9. Is that all? Its so easy! We’re 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

10. 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 !!

11. 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

12. Calc Texture Offsets (cont’d)
Use normal-space light vector for offsets
L’ = Mn  L
Use L’x, L’y for s, t
Use L’z for diffuse factor!
If light vector is near normal, L’x, L’y are small
If light vector is near tangent plane, L’x and L’y are large
What is L’z is less than zero?
Light is on opposite side from normal
Fade contribution toward zero

13. 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)

14. Implementation on TNT (cont’d)
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
(T0c*C0a + T0c*Fda - 0.5) * 2
scale by 2 brightens the image