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Restart Trail for Stackless BVH Traversal Samuli Laine NVIDIA Research

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Background Ray casting in usual acceleration hierarchies is tree traversal kd-trees Bounding volume hierarchies Typically, traversal requires a stack Traversal stack is expensive Storage Bandwidth

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Closer Look at Traversal Basic non-recursive tree traversal while (not terminated) if (not a leaf node) fetch bounds of children of current node check which children are intersected by the ray if (intersections) sort intersected children according to proximity make closest intersected child the current node push other children into stack else pop stack, terminate if empty end if else process primitives in leaf node pop stack, terminate if empty end if end while

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Closer Look at Traversal Basic non-recursive tree traversal while (not terminated) if (not a leaf node) fetch bounds of children of current node check which children are intersected by the ray if (intersections) sort intersected children according to proximity make closest intersected child the current node push other children into stack else pop stack, terminate if empty end if else process primitives in leaf node pop stack, terminate if empty end if end while

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Stackless kd-Tree Traversal [Foley and Sugerman 2005] while (not terminated) (ray not shrunk to a point) if (not a leaf node) fetch bounds of children of current node check which children are intersected by the ray if (intersections) sort intersected children according to proximity make closest intersected child the current node push other children into stack do nothing else pop stack, terminate if empty shorten ray, restart end if else process primitives in leaf node pop stack, terminate if empty shorten ray, restart end if end while

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Why Does this Work? Near childFar child Original ray before restart Split plane Node Intersect! Go here Forget

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Why Does this Work? Near childFar child Shortened ray after restart Split plane Node Intersect! Go here (not intersected)

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kd-Trees and Other BSP Trees Sorted according to traversal order

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kd-Trees and Other BSP Trees Sorted according to traversal order Ray part ARay part B Processed nodes Unprocessed nodes

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Short-Stack kd-tree Traversal [Horn et al. 2007] Store only n topmost stack entries Shorten ray and restart when stack is exhausted With n=3, only 3% too many nodes processed Stackless (n=0) roughly doubles the amount of work

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What About BVH? In BVH, nodes may overlap Unprocessed part of BVH tree does not correspond to a shortened ray Restarting traversal by shortening the ray is not possible Stackless and short-stack traversal cannot be used

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Example of Impossible Restart A A1 A2 B1 B2 B A B A1 A2 B1 B2

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Example of Impossible Restart A A1 A2 B1 B2 B A B A1 A2 B1 B2 Processed nodes Run out of stack here, need to restart after processing A2

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Example of Impossible Restart A A1 A2 B1 B2 B A B A1 A2 B1 B2 Processed nodes Does not work, misses B entirely

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Example of Impossible Restart A A1 A2 B1 B2 B A B A1 A2 B1 B2 Processed nodes Does not work, revisits A and A1

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Example of Impossible Restart A A1 A2 B1 B2 B A B A1 A2 B1 B2 Processed nodes Problem: Any ray that covers B1 will revisit A and A2

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Solution: Restart Trail If we sort a BVH according to ray traversal order, we can partition it into processed and unprocessed parts Unlike in kd-trees, these parts do not correspond to ray segments, but there’s no need to worry about that Therefore, we can cheaply store which part of the hierarchy has been processed Upon restart, consult the information to steer the traversal to the correct part of the tree

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Processed vs Unprocessed Nodes Sorted according to traversal order Processed nodes Unprocessed nodes

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Finding the Next Unprocessed Node Sorted according to traversal order This is the node we need to find 0 1 1 Restart trail

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Restart Trail Encoding, Issues One bit per level is enough This leaves many options for the actual encoding Must remain consistent during traversal What if only one child node is intersected? What if the ray is shortened from the end during traversal? Needs to be updated efficiently After processing a node, we want to update the trail without remembering anything about our ancestors Should be transparent during traversal No splitting into before-restart and after-restart codepaths

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Our Encoding Scheme 0 = Node not visited yet OR node has two children to be traversed, subtree under near child not fully traversed 1 = Node has one child to be traversed OR node has two children to be traversed, subtree under near child has been fully traversed One sentinel bit on top of actual trail Allows easy detection of when to terminate the traversal

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Use of Trail During Traversal If node has two children 0 = Go to near child 1 = Go to far child When initialized to all zeros, does not affect usual traversal order Always set bit when going through one-child node Does not affect traversal, but enables efficient updates!

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Efficient Updates Other view of encoding: 0 = I have a far child that is not processed yet 1 = otherwise Trail update must make trail point to next unprocessed node Find last zero bit, toggle it to one, clear the rest Sounds suspiciously like binary addition

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Traversal Example 0 0 0 0 … 0

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0 0 0 0 … 0

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0 0 0 0 … 0

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0 1 0 0 … 0

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0 0 1 0 … 0

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0 1 1 0 … 0

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0 1 1 0 … 0

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0 0 0 1 … 0

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0 0 1 1 … 0

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0 0 1 1 … 0

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0 0 0 0 … 1

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0 0 0 0 … 1 I’m outta here

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Gotcha: Rays Shortened at the End During traversal, primitive intersection may shorten the ray at the end Makes it possible to terminate traversal before ray exits scene Can change two-child node into one-child node Node has two children during descent, but only one during restart If not handled properly, this can cause multiple traversal

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Trouble with Shortened Ray 0 0 … …

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0 0 … … While traversing this subtree This branch gets culled

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Trouble with Shortened Ray 0 1 … … Trail is updated after this subtree is processed

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Trouble with Shortened Ray 0 1 … … Stack is exhausted, and a restart is initiated

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Trouble with Shortened Ray 0 1 … … The same subtree is processed again This node does not have a far child anymore, so we cannot go there

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Solving the Shortened Ray Problem Notice which bit was toggled to one when updating trail When traversing, detect if there is a one-child node on this level If so, the only explanation is that the far child was culled Proper action is to update trail and restart again

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Practical Implementation Keep trail and current level bit in registers Set and query bits trivially using Boolean operations Trail update Find last zero above current level, toggle to one, clear the rest TrailReg &= -LevelReg; TrailReg += LevelReg; Determining level pointer after trail update Find last one temp = TrailReg >> 1; LevelReg = ((temp – 1) ^ temp) + 1;

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Results Overvisit factor of 2.2–2.4 for stackless traversal Similar to results of Foley and Sugerman [2005] Three-entry short stack causes 5–8% overvisit Compared to 3% reported by Horn et al. [2007] Possibly because two-child intersections are more frequent in BVHs than kd-trees, yielding more restarts Fairy Forest, 174K trisConference, 282K tris

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Results on Hardware Restart trails were originally implemented in BVH ray cast kernels Used to improve performance for a long while, but did not pay off in the most optimized variants Not tested on Fermi hardware yet Cache trashing caused by stack traffic could make stackless (or register-only) traversal attractive again

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Possible Use Cases Situations where Not enough local storage for storing full traversal stack Not enough memory bandwidth for stack traffic Custom traversal hardware? Cleaner memory interface Read-only memory client Pays off whenever cost of extra node traversal is less than cost of stack pushes and pops

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Extensions, Future Work Storing two bits per level to avoid doing node intersection tests again Avoiding intersection tests might not pay off on wide SIMD Extension to higher branching factors Could a similar approach be applied in other kinds of traversals? e.g. closest-point and kNN searches for density estimation

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Thank You Questions

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