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Multi-Layered ImpostorsMulti-Layered Impostorsforfor
Accelerated RenderingAccelerated Rendering
Xavier Decoret, iMAGIS
This is joint work withGernot Schaufler and Julie Dorsey at MIT
and François Sillion at iMAGIS
Multi layered impostors for accelerated rendering
Complex EnvironmentsComplex Environments
Paris:
• 411537 vertices
• 137179 triangles
• 32 textures(most 256x256)
• 6.1 MB geometry
Multi layered impostors for accelerated rendering
IBR in RT-GraphicsIBR in RT-Graphics
• Image-based representations simplify the scene
• Images are unaffected by the depicted scene’s
complexity
• Images are fast to render as textured triangles
• Images themselves can be generated with hardware
• This works if a geometric model is available
Multi layered impostors for accelerated rendering
Previous WorkPrevious Work
• Pre-generated RepresentationsGrossman98, Dally96, Maciel95, Aliaga96, Chen95, McMillan95,
Xiong96, Sillion97, Darsa98, Pulli97, Laveau94, Max96,Rafferty98
• Dynamically Updated RepresentationsTorborg96, SGI97, Lengyel97, Regan94,Shade96, Schaufler96, Mark97, Mann97Aliaga99
Multi layered impostors for accelerated rendering
ImpostorsImpostors (pre-generated) (pre-generated)
SGI Performer(Billboards)
Id Software(3D Sprites)
Maciel95
Multi layered impostors for accelerated rendering
Impostors Impostors (dynamically generated)(dynamically generated)
Schaufler95
SchauflerStürzlinger‘96
Multi layered impostors for accelerated rendering
Meshed ImpostorsMeshed Impostors [Sillion97][Sillion97]
Distant Geometry Depth Map
Discontinuities Triangulation
Multi layered impostors for accelerated rendering
Artefacts in IBRArtefacts in IBR
• Deformation caused by mesh
• Resolution mismatch
Geometry
Multi layered impostors for accelerated rendering
Artefacts in IBRArtefacts in IBR
• Incomplete representation
• Rubber sheet effects
Geometry
?
Multi layered impostors for accelerated rendering
Artefacts in IBRArtefacts in IBR
• Image cracks
• and more …– Static Shading:
• no highlights
• no reflections
– No moving Objects
Geometry
Multi layered impostors for accelerated rendering
Our ContributionsOur Contributions
• Reducing Rubber Sheet Triangles
– Multi Mesh Impostors
• Reducing distortions to improve quality
– Dynamic Update
Reduce some of the identified artefacts
Multi layered impostors for accelerated rendering
The databaseThe database
• The geometry is organized into objects
• We have a set of viewcells
The street graphO
ne e
dge
Another edge
Multi layered impostors for accelerated rendering
Model segmentationModel segmentation
Complete Geometry PVS
Local model
Distant model
Multi layered impostors for accelerated rendering
Single mesh impostorSingle mesh impostor
Distant model......replaced by impostor
Combined withlocal model
Multi layered impostors for accelerated rendering
Rubber Sheet Triangles Rubber Sheet Triangles due to parallaxdue to parallax
impostor
viewpoint
View from
above
Multi layered impostors for accelerated rendering
Locating Rubber Sheet TrianglesLocating Rubber Sheet Triangles• Parallax creates rubber sheets between objects
when objects overlap in depth
View from
above
Frontview
• In urban walkthrough, parallax is mainly horizontal
Assume a 2D problem
Multi layered impostors for accelerated rendering
Multi Mesh ImpostorMulti Mesh Impostor
• The critical zone identifies overlaps between 2 objects
• When overlapping occurs in image space, one object can uncover the other one
Edge
O1
Critical zone
O2
Edge
O1
O2
• If uncovering is too much, objects must not be on the same mesh
Multi layered impostors for accelerated rendering
Multi Mesh ImpostorMulti Mesh Impostor
• Quantifying overlapping between 2 objects• Place those objects in different layers which are
too distant in depth:– construct a relation graph– partition the graph
Single mesh Several meshes
Multi layered impostors for accelerated rendering
Objects and LayersObjects and Layers
• Two objects must go into different layers if two points on their geometry can be seen under sufficiently different viewing angles min and max.
Q
P
A
B
max
edge
min
Object 1
Object 2
y
P Q
A
B
x
M1, M2 [PQ] such as
AM1B minimal
AM2B maximal
Multi layered impostors for accelerated rendering
Relation GraphRelation Graph
• Edges joining overlapping objects
• Coloriate the graph so that joined vertices have different colors
• Each color represent a layer• Non unique
• One node per object
Multi layered impostors for accelerated rendering
All Geometry
Results & ExampleResults & Example
Edge
PVS
SingleMesh
MultiMesh
Multi layered impostors for accelerated rendering
Results & ExamplesResults & Examples
Start pointFrom which the impostor
is computed
End pointNotice rubber sheet triangles
Multi layered impostors for accelerated rendering
Results & ExamplesResults & Examples
Start pointFrom which the impostor
is computed
End pointThe view is no longer blocked
Multi layered impostors for accelerated rendering
Results & ExamplesResults & ExamplesGeometry
Single Mesh MultiMesh
Multi layered impostors for accelerated rendering
Offline vs OnlineOffline vs Online
• If the user stops, the correct image should appear.
• This requires knowledge of the current viewpoint.
• Offline approaches do not have this information
• Motion hides small artifacts of stored impostors.
Multi layered impostors for accelerated rendering
Combining preprocessingCombining preprocessingwith dynamic updateswith dynamic updates
• Single mesh impostor replace too much geometry to be updated.• Layers and their contents are suitable for regeneration of part of
the distant model.• Layers are updated front to back
to improve image quality:
– silhouettes
– distortions
– resolution mismatches
Multi layered impostors for accelerated rendering
System ArchitectureSystem Architecture
Preprocessing:• Take geometry and view cells• Find visible geometry for cells• Split into near and far part• Create impostors for far part• Store as scene per view cell
Walkthrough:• Page in geometry and textures• Do dynamic updates if possible• LOD management
Minimal Scenegraph per viewcell
Visibility Extractor
Model Segmentation
MMI ExtractorOffl
ine
View Cells Scene
Dynamic Update
On
line
Rendering
Multi layered impostors for accelerated rendering
Results & Video (Paris)Results & Video (Paris)
Storage requirements :70 Mbs
Computation time : 100 edges per hour
Achieved frame rate: 50 Hz
Multi layered impostors for accelerated rendering
Future WorkFuture Work
• Smooth transitions between different representations
• Automatic generation of street graph from street mesh
• Extension of viewcells from edges to areas and volumes
• Reverse approach: construction of viewcells to optimize use of impostors.
Multi layered impostors for accelerated rendering
AcknowledgementsAcknowledgements
This is a joint project between iMAGIS, Grenoble and MIT, Cambridge.
It was supported in part by a joint collaborative research grant of NSF and INRIA (INT-9724005), an Alfred P. Sloan Foundation Research Fellowship (BR-3659), and by a grant from Intel Corporation.
The following people contributed to the presented results :
Julie Dorsey, François Sillion, Gernot Schaufler, Max Chen, Byong Oh Mok, Yann Argotti and Sami Shalabi.