1. Shuen-Huei Guan Seminar in CMLab, NTU
Paper Report: Shadow
Shuen-Huei Guan
Seminar in CMLab, NTU

2. Shadow Silhouette Maps
Siggraph 2003
Pradeep Sen, Mike Cammarano, Pat Hanrahan
Standford University

3. Overview Introduction Shadow Algorithms Shadow Silhouette Maps
Shadow Volume [Crow 1977]
Shadow Maps [Williams 1978]
Shadow Silhouette Maps

4. Introduction What is Shadow? Umbra vs. Penumbra
Hard Shadow, Soft Shadow

5. What is Shadow?
An area that is only partially irradiated or illuminated due to blockage of light by an opaque object.
A point is in shadow relative to a given light source if rays from that light source cannot directly reach the point.
Light + Caster (Obstacle) + Scene (Receiver) + Camera (Eye).

6. Shadow Visually effect Algorithms Self shadow Real-time, off-line...
Hard shadow
Soft shadow
Algorithms
Shadow volume
Shadow maps
Self shadow
Real-time, off-line...

7. Umbra vs. Penumbra Umbra: Penumbra: Hard Shadow Soft Shadow:
Area in full shadow
Penumbra:
Boundry Area; Lesser shadowed
Hard Shadow
Umbra
Soft Shadow:
Umbra + Penumbra

8. Shadow Volume Shadow planes Volume formed from shadow planes
Point inside is shadowed
Point outside is lit
Stencil Buffer

9. Shadow Volume cont.

10. Shadow Maps
Step1
For each light, make a depth buffer from the point of view of light (Shadow Maps)
Step2
For each rendered point in camera space, transform it to light coordinate. Then, compare the distance of the transformed point with the Shadow Maps to get shadowed or lit.

11. Shadow Maps cont.
Eye
Light

12. Shadow Maps cont.
Eye
Light

13. Aliasing in Shadow Maps

14. Aliasing in Shadow Maps

15. Aliasing in Shadow Maps cont.
Insufficient resolution of shadow maps
Dist (Eye, Obj) <-> Dist (Obj, Light)
Casting area from light <-> casting area from eye
Sampling & Quantization

16. After Shadow Maps Blur – 4-corners comparison
Percentage Closer Filtering [Reeves 1987]
Adaptive Shadow Maps [Fernando 2001]
Perspective Shadow Maps [Stamminger 2002]

17. Adaptive Shadow Maps Observation:
Only regions that contain shadow boundaries need to be high-quality.
ASM hierarchically subdivides the shadow map in visually important regions.
N’ = N * (Projected Area in Eye) / (Projected Area in Shadow Map)

18. Adaptive Shadow Maps cont.
Eye
Light

19. Perspective Shadow Maps
Observation:
Aliasing shadow boundaries are near the camera.
Apply a transformation to get Perspective Shadow Map, providing high resolution for near objects.

20. Perspective Shadow Maps cont.

21. Perspective Shadow Maps cont.
eye
obstacle
light
aliasing shadow
smooth shadow

22. Shadow Silhouette Maps
Observation:
Aliasing only occurs in the shadow boundaries.
Solution:
Using silhouettes of the shadow to recognize the boundaries linearly.

23. Recall: Shadow Maps Step1 Step2
For each light, make a depth buffer from the point of view of light (Shadow Maps)
Step2
For each rendered point in camera space, transform it to light coordinate. Then, compare the distance of the transformed point with the Shadow Maps to get shadowed or lit.

24. Shadow Silhouette Maps Algo
Step1
Shadow Maps + Silhouette Maps (both from the point of camera)
Step2
for each point p
If (ShadowMaps(p))
p is in shadow
else if (SilhouetteMaps(p))
else
p is lit

25. Silhouette Maps Definition:
From the point of view of light, silhouette map is used to store the shadow edges (silhouette).

26. Silhouette Maps cont.
Silhouette maps can be thought of as warping a regular grid so that grid lines align with boundaries in the geometry.

27. Results

28. Conclusion Silhouette is introduced to shadowing.
Shadow maps: constant (discrete) approximation of shadow.
Silhouette maps: linear approximation.
Better representation of silhouettes -> better performance.

29. The End
Thanks for listening!