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Dynamically Generated Imposter

Weidong Shi

Georgia Institute of Technology, 2001

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Dynamically Generated Imposter

Exploit coherence in consecutive frames to speed uprendering

Reduce scene complexity by converting polygonmodels into imposters

Generate imposters on the fly to achieve flexibility

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Frame to Frame Coherence

Frame sequence generated by interactive graphicssystem contains significant amount of coherence.

Relative motion of distant objects on screen is

small.

Close objects often expose the same side to the

viewer when they are moving.

Re-use of rasterized object image to speed uprendering

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Real Time Rendering Techniques

LOD & Progressive Mesh

Static Imposter

object pre-generated and stored as texture, then

mapped to a transparent polygon in real time

huge texture storage space required

limited view direction and navigation space

Hierarchical Z- buffer

works best for highly occluded scene

Dynamic Imposter (Gernot Schaufler)

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Dynamic Imposter

Generate an object image for a certain view

Convert the image into texture

Map the texture to a transparent polygon.

Advantages

fast

more flexible than static imposter

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Dynamic Imposter -cont

Adjust imposter when viewer moves. Rotate aimposter so that its normal always points to theviewer.

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Validity of Old Imposter

Error Angle due to translation

In view V1, B1 = B2

In view V2, B1 and B2 has an angletrans.

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Validity of Old Imposter

Error Angle due to move-in

In view V1, B = B.

In view V2, B and B has an anglesize.

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Algorithm

For each object i

check validity of its old imposterif valid

render object i with imposter

else

if successfully generate a new imposter for object i

render object i with new imposter

else

render object i with its original mesh

endif

endif

endfor

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Real Time Performance of Dynamic

Imposter

View real time rendering not only as graphicsproblem but also a resource management problem.

Real time performance depends on both thecomputational requirement and the availableresources.

Dynamic imposter is beneficial only if leads to betteruse of rendering resources.

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Implementation

Hardware

Dell Inspiron 8000. ATI mobile graphics card withhardware 3D rendering support.

API

Direct X 8.

Imposter Validity Checking

Bounding sphere instead of bounding box used for

computingsize andtrans

CPU converts rendered image into texture with alphachannel.

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Result - cont

Opaque imposters

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Result - cont

Imposters with transparent texture

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Result - cont

Performance becomes worse with dynamic imposter

because of the slow video-system memory bus.

Video-System speed is about 50 times slower thanSystem-Video based on DirectDraw surface copytest.

Possible Solution (RGBA+Stencil buffer)

Render in RGBA format instead of RGB format (16bits). This only worksin full screen mode.

1. Clear rendering target (in Video Memory) with transparent alpha.2. Render and use stencil buffer to record all drawn pixels.3. Write opaque alpha value to all drawn pixels.4. Copy result into a texture buffer (in Video Memory).

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Result

0

50

100

150

200

250

300

1 3 5 7 9 11 13

ReusedImposter

NewImposter& Mesh

13 tigers, viewer uses one as center and

moves along a circle.

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Future Work

Develop adaptive algorithm that chooses how objectsare rendered based on real time performance andavailable resources.

Divide distant objects into groups and generate onlyone imposter for each group.

Combine with LOD. Map imposter texture tosimplified mesh.

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Thanks

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