1. GEARS: A General and Efficient Algorithm for Rendering Shadows
Lili Wang1, Shiheng Zhou1, Wei Ke2,Voicu Popescu3
1State Key Laboratory of Virtual Reality Technology and Systems, Beihang University
2Macao Polytechnic Institute, Macau
3Purdue University, West Lafayette

2. Contents Motivation Related work Algorithm Acceleration Results
Limitations
Conclusion 1

3. Motivation Shadow for Realistic rendering
Area lights give soft shadows
Point light
Area light 2

4. Motivation
Ray Tracing is a solution, multiple rays for each sample. But slow.
Challenge is to estimate what fraction of an area light source is visible from each of the surface pixel samples in real time.
A pixel
Lots of shadow rays per pixel
Area light 3

5. Related work shadow simulation methods shadow approximation methods 1
Screen-space percentage closer soft [MOHAMMADBAGHER M.et al.2010]
Percentage-closer soft shadows [FERNANDO R.2005]
Approximate soft shadows with an image-space flood-fill algorithm [ARVO J. et al.2004]
shadow approximation methods
Highquality adaptive soft shadow mapping. [GUENNEBAUD G.et al.2007]
Microquad soft shadow mapping revisiteds. [SCHWARZ M. et al. 2008]
Packet-based hierarchal soft shadow mapping.[YANG B. et al.2009]
[MOHAMMADBAGHER M.et al.2010]
[YANG B. et al.2009] 1 4

6. Related work Accurate soft shadow methods
Alias-free shadow maps [AILA T. et al 2004]
Samplebased visibility for soft shadows using alias-free shadowmaps [SINTORN E. et al.2008]
Efficient ray traced soft shadows using multi- frusta tracing[BENTHIN.C. et al. 2009]
Visibility sampling on gpu and applications [EISEMANN E. et al. 2007]
[AILA T. et al 2004]
[SINTORN E. et al.2008 1 5

7. Algorithm - Basic solution.
Replace the shadow rays
With rendering all triangles on visibility bit mask
Construct a camera frustum containing sample rays
Fractional visibility per pixel can be computed
A pixel
Area light
Area light 6
Lots of shadow rays per pixel
Resterize triangles on visibility bit mask

8. Algorithm - Basic solution
Replace the shadow rays
With rendering all triangles on visibility bit mask
Construct a camera with pixel sample as its eye and the light as its image plane
Fractional visibility per pixel is computed as the percentage of unoccluded light samples
visibility mask
(resolution)
Render triangles
Light source
Shadow caster triangles
Pixel sample as a camera 7

9. Algorithm GEARS algorithm 8
Compute the output image without the soft shadows
Unproject each pixel p in I0I1 to pixel sample P
Assign all tris. to pixel samples P.(acceleration is introduced later)
For each P, compute the frac. visibility vp of L0L1
Construct camera
Render with PL0L1 all blocking triangles
Compute vp as the percentage of unoccluded light samples
Add the contribution of light L0L1 to each pixel p of I0I1 using the computed fractional visibility vp. 8

10. Acceleration- Finding potential triangles
Assign all tris. to pixel samples is naïve
Time
Memory
A small portion of triangles are blocking
We want to find the potential blocking triangles per pixel
HOW? 9

11. Acceleration- Finding potential triangles
Generating grid cells in light space
Triangles and samples are transferred in a light space
Assign triangles to all the cells that are touched by the projection of area light
Projecting pixel samples with camera and assign them to the grid cells.
Pixels in a same cell shares the blocking triangles
Spatial coherence 10

12. Acceleration- Generating grid cells
Generating light camera :
View frustum of camera contains 3-D AABB of all sample pixels.
The far plane is given by the farthest corner of the 3-D AABB. 11

13. Acceleration- Triangle to cell assignment
A triangle is assigned to :
All grid cells for which the triangle might block the light.
Established a mapping from triangles to cells by pairs (triID; cellID) . 12

14. Acceleration- Invert the mapping
Creates an array of pairs (triID; cellID) by calculating all the cellsN touched by the triangle
Mapping needs to be inverted to be able to retrieve the triangles that potentially block a given cell.
computed in parallel by radiax sorting 13

15. Acceleration- Rasterize potential triangles
For a given pixel we find the potential triangles in the sorted array from the begin to the end of belonging cell.
Rasterize
Construct a camera with pixel sample as its eye and the light as its image plane
Scan line rasterization
Bit mask is computed
Fractional visibility is computed as the percentage of unoccluded light samples 14

16. Result - Quality
Soft shadows rendered with GEARS compared to that of ray tracing – No Difference !
Our Method
Ray-tracing 15

17. Result - Quality
Soft shadows rendered with GEARS compared to that of ray tracing, dynamic scene with tree moving in winds – No Difference ! 16
Our Method
Ray-tracing

18. Result - Quality
Video 17

19. Result - Quality
Quality dependence on resolution of visibility masks –Better results with higher resolution
4x4
8x8
16x16
32x32 18

20. Results - Performance
Our method is between 10 and 25 times higher than ray tracing.
Scene
GEARS
[fps]
(A)
RT static
(B)
Speedup
(A)/(B)
RT dyn.
(C)
Chess
29.8
2.91 10
2.52 12
Church
36.2
3.72
3.05
Dragon
38.7
4.01
2.96 13
Bird Nest
26.5
1.07 25
0.95 28
Spider
75.6
5.33 15
3.54 22
Garden
21.4
1.98 11
1.65 19

21. Results - Performance
Quality comparison between GEARS and ray tracing for equal time performance
GEARS
ray tracing
GEARS
ray tracing 20

22. Results - Performance
Average frame rate for our test scenes for various output image resolutions
Frames per second
Image resolution
512x512
1024x1024
1280x1280
Cow Matrix
30.5
20.2
13.4
Chess
29.8
19.9
11.8
Church
36.2
17.1
12.3
Dragon
38.7
15.6
11.9
Bird Nest
26.6
11.3
8.8
Spider
75.6
40.8
27.7
Garden
21.4
11.4
9.0 21

23. Results - Performance
Frame rate for various visibility mask resolutions.
Bit mask res
4x4
8x8
16x16
32x32
Chess
36.7
35.6
29.8
22.0
Church
51.0
44.3
36.2
21.7
Dragon
48.7
46.2
38.7
24.9
Bird Nest
57.7
55.3
26.5
16.0
Spider
11.6
110.0
75.6
48.4
Garden
34.2
30.1
21.4
15.2 22

24. Limitations
Blockers have to be large enough for their projection to be detected.
Regular grid is used which is an inefficient modeling of non-uniform sampling
Edges of blocking geometry exhibit some degree of temporal aliasing
Cannot efficiently support a set of disparate point light sources 23

25. Conclusion GEARS is an algorithm for computing Soft Shadows
General: support fully dynamic scenes, scenes without restrictions
Efficient: fast to compute and tight
Accurate: guaranteed to render all triangles that block the light 24

26. Thanks
Q&A