世情Ambient Occlusion Overview
O
v e r a l l A l g o r i t h m f o r a g i v e n p o i n t The u of varied ambient box sizes rais discontinuity problem for neighborhood points located in different boxes.
To solve this problem, computations are performed in t wo pass . Firstly, an ambient box is affected to each point. Next, by analyzing the image plane , neighborhood ambient boxes are gathered in ts .Then, for each point, ambient occlusion is computed with its t of ambient boxes.
Ambient boxes directly result from octree leaves. The octree is refined until reaching one of the criteria :●The leaf size is lower or equal than a maximal size ;●The leaf contains no more than one surface .
In all our experiments, the maximal size is clamped between 4 and 32 times the projected pixel size, depend of speed or accuracy requirements.
The quick box visibility test is ud to count the leaf surface number.
调研函As a result, the overall refinement strategy gives us a well adapted ambient box size.
Check Uful Occlusion Buffers
References
An original query is ud to clip out all surfaces located outside an arbitrary box and a half-space .
Occlusion buffers are built with 6 square grids mapped on a cube . Grid resolution is 8x8 in all our experiments.
Each cell of the grid is computed on demand . When computation is queried, a ray is launched from the center of the cube towards a random point located in the cell .
Cells are gathered by packet of 2x2 in order to launch more efficient SIMD ray packets .
Find the Best Ambient Box
Box Visibility Test
The ambient occlusion is approximate for the whole space inside an ambient box . Each corner of this box contains an occlusion buffer . Accurate ambient occlusion is computed and stored in the occlusion buffers.
So, each point to be shaded inside the ambient box just have to interpolate already computed ambient occlusions.
Contrarily to classical approaches, we do not u a fixed size for the ambient box. Each box size is maximized to reduce :
●the number of occlusion buffers ;●the memory consumption .思想路线
The adaptive size strategy becomes possible thanks to the integration of the box visibility test .
Fast Approximate Ambient Occlusion (sap_0512)
Gilles Cadet 1
, Bernard Lécussan
2
1
Computer Graphics Department, Engineering School Supaéro, France
2
HPC-SA, , FRANCE
外婆味道
Occlusion Buffer can NOT be ud
Ambient occlusion measures the attenuation of light due to blocker objects. It evaluates the amount of light received by a point centered in a uniform lighting hemisphere.
Widely ud by the production rendering community [1,3], ambient occlusion is able to approximate realistic global effects without requiring global illumination , more complex and slower.
The ambient occlusion value is computed by an approximate version of the integral propod by [6]. There is two ways to resolve it, off-line and real-time approaches.
Off line approach us Monte Carlo integration . Ray tracing is ud to sample the coverage of the hemisphere above each point. This method is accurate but slow .
First frame: 320 ms, Walkthrough: 6 fps
First frame: 190 ms, Walkthrough: 8 fps
First frame: 145 s
First frame: 140 ms, Walkthrough: 8 fps
This test ensures there is no surface inside a box susceptible to occlude a given oriented point. This assumption allows to skip costly intervisibility tests between this point and occlusion buffers.
This query is computed very quickly . It is implemented like a ray tracing function. Instead of propagating a ray, the box is propagated through our acceleration
structure (which is a BVH tree).
[1] CHRISTENSEN P. H., FONG J., LAUR D. M. and BATALI D.: Ray Tracing for the Movie 'Cars'. Proceedings of the IEEE Symposium on Interactive Ray Tracing 2006, pages 1-6. IEEE, September 2006.
[2] KONTKANEN J., LAINE S.: Ambient occlusion fields. In Proc. I3D (2005), ACM Press, pp. 41–48.
[3] LANDIS H.: Production-ready global illumination. In SIGGRAPH 2002 cour note #16, pages 87–102, 2002.
[4] MALMER M., MALMER F., ASSARSSON U., HOLZSCHUCH N.: Fast Precomputed Ambient Occlusion for Proximity Shadows . Tech. Rep. RR-5779, INRIA, 2005.
[5] PHARR M.: Ambient occlusion. In GPU Gems (2004), Fernando R., (Ed.), Addison Wesley, pp. 667–692.
[6] ZHUKOV S., IONES A., KRONIN G.: An ambient light illumination model. In Rendering Techniques ’98 (Proc. EGWR) (1998), pp. 45–55.
Diffu shading
Occlusion Buffer Repartition
Fast Approximate Ambient Occlusion Fast Approximate Ambient Occlusion
Monte Carlo (1024 rays / pixel)
Ambient Occlusion
Repartition of Occlusion Buffers
Fixed Size Repartition of Occlusion Buffers
Adaptive Size
Discontinuity artifacts appear when using only one ambient box
Interpolation with
neighborhood ambient boxes
Point to be shaded
Point normal
Occlusion Buffer
Occlusion Buffer can be ud Ambient Box
旅游服务质量杨氏28式太极拳A cell stores : - visibility result
- ambient occlusion
Occlusion Buffer
Visibility Rays
Fast Approximate Ambient Occlusion
清蒸黑鱼Real time approaches u more approximate methods and are performed on GPU . [5] replaces the exact ray tracing visibility function by an occluder visibility heuristic. [2,4] firstly precompute ambient occlusion nearby moving objects, stored it into low-resolution grids, and then interpolate the ambient occlusion value in real-time.
Conquently, this means real-time approaches are limited either by a high number of primitives or by a long precomputing time .
Therefore, we suggest a new approach to approximate efficiently ambient occlusion. Bad on ray-tracing, the method can be ud in two ways :
●Off-line : a quite accurate ambient occlusion can be computed very quickly ;
●Interactive : Thanks to a high optimized ray tracing and a lazy ambient occlusion computing, interactive walkthrough in a complex model is achievable.
1 – F i n d t h e b e s t a m b i e n t b o x ;
2 – C h e c k u s e f u l o c c l u s i o n b u f f e r s ;
3 – C o m p u t e n o t a l r e a d y d o n e o c c l u s i o n b u f f e r s ;
4 –
I n t e r p o l a t e e a c h o c c l u s i o n b u f f e r c o n t r i b u t i o n .Fast Approximate
Ambient Occlusion
Visible surface
Rejected surfaces
Oriented point Clipping box
手机初始密码Clipping plane Occlusion Buffer Computing
