Image-Based Techniques Hierarchical Image Caching Michael Chung.

Image-Based TechniquesHierarchical Image Caching

Michael Chung

Papers• Hierarchical Image Caching for

Accelerated Walkthroughs of Complex Environments– Jonathan Shade, Dani Lischinski, David H.

Salesin, Tony DeRose, John Snyder (SIGGRAPH, 1996)

• A Three Dimensional Image Cache for Virtual Reality– Gernot Schaufler, Wolfgang Sturzlinger

(EUROGRAPHICS, 1996)

Motivation• Unable to render large and

complex environments at a sufficiently high frame rate.

• Would be nice to be able to. (ex: crowded forests, cities…)

Insights• Dynamically Generated Impostors (Schaufler, 1995)

– Path coherence between frames– No need to render every frame from scratch

Impostor – What is it?• An impostor is a billboard that mimics and

replaces 3d geometry in a scene.

Impostor – Why use it?• Fast to render

• Reusable for objects that are coherent between frames– Especially distant objects

• Essentially, a high quality image at the cost of a very low LOD model (a quad)– But the impostor is short-lived.

Impostor Generation• Face viewpoint towards center of bounding box• Project bounding box into screen space• Generate tight rectangle• Reverse project rectangle into world space• Render impostor texture onto rectangle

Example: Virtual Dublin Project

• Geopostors: A Real-Time Geometry / Impostor Crowd Rendering System

Simon Dobbyn, John Hamill, Keith O’Conor, Carol O’Sullivan



• New insight:– Further optimize rendering using a

hierarchical cache of impostors• Creating impostors out of smaller impostors saves

time over rendering actual geometry• If possible, draw fewer impostors per frame

Proposed ContributionsShade, Lischinski, Salesin, DeRose, Snyder• “New method for accelerating

walkthroughs of geometrically complex and largely unoccluded static scenes by hierarchically caching images of scene portions.”

• “New simple error metric that provides automatic quality control.”

Proposed ContributionsSchaufler, Sturzlinger

• “New approach to software accelerated rendering which improves and generalizes the concept of impostors”

– Handles intersecting objects

– Handles indoor as well as outdoor scenes

– Deals with primitives, does not rely on organization of scene into object groupings


• Indoor intersecting objects problem:


• With the 3d image cache:

Method Overview• Partition scene into a tree

(BSP-tree or k-d-tree)

• For each frame:– First traversal of tree

• Cull nodes outside of the view frustum (optional)• Update the cached impostors of nodes

– Second traversal of tree• Render scene into framebuffer, back to front

Algorithm DetailsShade, Lischinski, Salesin, DeRose, Snyder

• Partition scene into BSP-tree– All splitting planes perpendicular to x and z

axes.– Place splitting planes in the “best” gap found.

• Compute a cost for each gap that is a function of: – the number of its active objects– and, the ratio of the number of primitives on either side of

the gap

– Choose best gap such that the longest side of the bounding box is split.

Algorithm DetailsSchaufler, Sturzlinger

• Partition scene into k-d-tree– Recursively subdivide along longest side.– If primitive intersects more than one bounding

box, store primitive in all intersected bounding boxes.

Updating Image CachesShade, Lischinski, Salesin, DeRose, Snyder

UpdateCaches(node, viewpoint)if node is outside the view frustum then

node.status CULL

else if node.cache is valid for viewpoint thennode.status DRAW CACHE

else if node is a leaf thenUpdateNode(node, viewpoint)

elseUpdateCaches(node.back, viewpoint)UpdateCaches(node.front, viewpoint)UpdateNode(node, viewpoint)

Updating Image CachesA Visual Example


UpdateCaches(node, viewpoint)

if node is outside the view frustum then

node.status CULL

else if node.cache is valid for viewpoint then DETERMINE VALIDITY?

node.status DRAW CACHE

else if node is a leaf then

UpdateNode(node, viewpoint)

else

UpdateCaches(node.back, viewpoint)

UpdateCaches(node.front, viewpoint)

UpdateNode(node, viewpoint)

Determining Impostor ValidityShade, Lischinski, Salesin, DeRose, Snyder

• Error metric

• In practice, only consider the eight corners of a node’s bounding box when computing error.

Determining Impostor Validity• Error threshold

• Impostor can be used if…– – AND, imposter is not too close to the viewpoint

• Too close if tex >= screen (Schaufler, Sturzlinger)• OR, too close if viewpoint is in the same node (Shade et al)


UpdateCaches(node, viewpoint)if node is outside the view frustum then

node.status CULL

else if node.cache is valid for viewpoint thennode.status DRAW CACHE

else if node is a leaf thenUpdateNode(node, viewpoint)

elseUpdateCaches(node.back, viewpoint)UpdateCaches(node.front, viewpoint)UpdateNode(node, viewpoint) CACHE NEW IMAGE OR NOT?

Updating NodesShade, Lischinski, Salesin, DeRose, Snyder

UpdateNode(node, viewpoint)if viewpoint is inside node then

if node is a leaf thennode.status DRAW GEOMETRY

elsenode.status RECURSE

returnk EstimateCacheLifeSpan(node, viewpoint)amortizedCost (cost to create cache) / k + (cost to draw cache)if (amortizedCost < (cost to draw contents) then

CreateCache(node, viewpoint)node.status DRAW CACHEnode.drawingCost (cost to draw cache)

elseif node is a leaf then

node.status DRAW GEOMETRYnode.drawingCost (cost to draw geometry)

elsenode.status RECURSEnode.drawingCost node.back.drawingCost + node.front.drawingCost

Updating NodesShade, Lischinski, Salesin, DeRose, Snyder

UpdateNode(node, viewpoint)if viewpoint is inside node then

if node is a leaf thennode.status DRAW GEOMETRY

elsenode.status RECURSE

returnk EstimateCacheLifeSpan(node, viewpoint) HOW TO ESTIMATE LIFE

SPAN?amortizedCost (cost to create cache) / k + (cost to draw cache)if (amortizedCost < (cost to draw contents) then

CreateCache(node, viewpoint)node.status DRAW CACHEnode.drawingCost (cost to draw cache)

elseif node is a leaf then

node.status DRAW GEOMETRYnode.drawingCost (cost to draw geometry)

elsenode.status RECURSEnode.drawingCost node.back.drawingCost + node.front.drawingCost

Estimating Impostor Life SpanShade, Lischinski, Salesin, DeRose, Snyder

• Approximate the safety zone around current viewpoint.


• Approximating safety zone for a leaf node– Evaluate d for each corner of node’s bounding

volume.– Let r be the smallest such d.– Set the safety zone to be the axis-aligned

cube inscribed inside a sphere of radius r around current viewpoint.


• Approximating safety zone for an interior node.– Compute safety zone Z using the bounding

box of the node.– Intersect Z with the safety zones of the node’s

children

Main Algorithm Differences

• Shade et al.– View frustum culling– Cost-benefit analysis for cache update

• Schaufler, Sturzlinger– No view frustum culling– No cost-benefit analysis for cache update

• Replace invalid impostors with new ones no matter what

Questions so far?Method Overview

• Partition scene into a tree(BSP-tree or k-d-tree)

• For each frame:– First traversal of tree

• Cull nodes outside of the view frustum (optional)• Update the cached impostors of visible nodes

– Second traversal of tree• Render scene into framebuffer, back to front

Tests and Analysis of ResultsShade, Lischinski, Salesin, DeRose, Snyder

• Silicon Graphics Indigo2 workstation– 250 MHz R4400 processor– 320 MB RAM– Maximum Impact graphics board with 4 MB

texture memory

• Outdoor Scene (island forest)– 1117 willow trees– 40,599,982 triangles


• BSP-tree Preprocessing– 46 seconds construction time– No splitting of trees, only terrain is split– 13 levels, 1072 leaf nodes– 150 MB storage space

• Walkthroughs– 640 x 480 frame resolution– 2 pixel error threshold


Actual geometry Image cache 2 pixel error threshold

Tests and Analysis of ResultsSchaufler, Sturzlinger

• INDIGO R3000– 39k triangles per second– No hardware support for texture mapping

• One cache update visualization

• Two performance tests


Cache update visualization• Diagonal walkthrough of some complex scene

• k-d-tree preprocessing– 341 bounding boxes

• 256 x 256 maximum texture resolution

• Impostor updates for every 6th frame of 120 frames shown…


Cache update visualization


• Two performance tests– Make up for hardware insufficiencies

• 1st test: Render at a small screen resolution• 2nd test: Replace each textured polygon with two

gouraud shaded polygons

– Sideward translation in front of forest (100 frames)

– Zoom from one corner of forest towards its center (100 frames)

– 1 pixel threshold


• Procedurally generated forest scene– 100 trees– Approximately 105000 polygons

• k-d-tree preprocessing– Max depth of 4– 585 bounding boxes

Comments on Papers• Shade et al.

– Relatively thorough explanation of proposed method

– No justification of formulas for calculating cached image life expectancy.

• Schaufler, Sturzlinger– Insufficient explanation of testing procedure– Confusing graphs

Summary

Summary• Hierarchical image caching

– Impostors are cheap mimics of complex 3d geometry.• Take advantage of path coherence

– Caching impostors hierarchically further saves rendering time• Allows generation of impostors from children impostors

– Scene partitions are disjoint and does not rely on object groupings• Able to handle intersecting objects

• Automatic cache update control using error metric based on angular discrepancy

• Improved frame rates for large, complex static scenes

• Following work impacted by the insight that large and complex geometry can be efficiently rendered by hierarchically caching cheap representations

Questions?

Image-Based Techniques Hierarchical Image Caching Michael Chung.

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