Embree Ray Tracing Kernels

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Embree Ray Tracing Kernels
Sven Woop

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 Embree Overview
 Embree Performance
 Embree API
 Catmull Clark Subdivision Surfaces
Outline
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Embree Overview
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Copyright © 2015, Intel Corporation. All rights reserved. *Other names and brands may be claimed as the property of others.Copyright © 2015, Intel Corporation. All rights reserved. *Other names and brands may be claimed as the property of others.8/18/2015
• Movie industry transitioning to ray tracing
(better image quality, faster feedback)
• High quality rendering for commercials, prints, etc.
• Provides higher fidelity for virtual design
(automotive industry, architectural design, etc.)
• Various kind of simulations
(lighting, sound, particles, collision detection, etc.)
• Prebaked lighting in games
• etc.
Usage of Ray Tracing Today
7

 Need to multi-thread: easy for rendering but difficult for hierarchy
construction
 Need to vectorize: efficient use of SIMD units, different ISAs (SSE,
AVX, AVX2, AVX-512, KNCNI)
 Need deep domain knowledge: many different data structures
(kd-trees, octrees, grids, BVH2, BVH4, ..., hybrid structures) and
algorithms (single rays, packets, large packets, stream tracing, ...) to
choose
 Need to support different CPUs: Different ISAs/CPU types favor
different data structures, data layouts, and algorithms
Writing a Fast Ray Tracer is Difficult
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Observations
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 Ray tracers are often not sufficiently optimized
 Ray traversal consumes a lot of cycles of renderer
(often over 70%)
 Ray tracing can be expressed by small number of
commonly used operations (build and traversal)
 Ray tracing kernel library has potential to speed up
many rendering applications
9

 Provides highly optimized and scalable Ray Tracing
Kernels (data structure build and ray traversal)
 Highest ray tracing performance
(1.5x – 6x speedup reported by users)
 Support for latest CPUs (e.g. AVX512 support)
 Targets application developers in professional
rendering environment
 API for easy integration into applications
 Free and Open Source under Apache 2.0 license
(http://embree.github.com)
Embree Ray Tracing Kernels
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 Find closest and any hit kernel (rtcIntersect, rtcOccluded)
 Single Rays and Ray Packets (4, 8, 16)
 High quality and high performance hierarchy builders
 Intel® SPMD Program Compiler (ISPC) supported
 Triangles, Instances, Hair, Catmull Clark Subdivision
Surfaces, Displacement Mapping
 Extensible (User Defined Geometry, Intersection filter
functions, Open Source)
 SSE, AVX, AVX2, and AVX512 support
Embree Features
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 Catmull Clark Subdivision Surfaces
– Smooth surface primitive
 Vector Displacement Mapping
– Add geometric detail
 Initial AVX512 support
– 16 wide AVX512 traversal kernels
New Embree Features
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Embree System Overview
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Embree API (C++ and ISPC)
Ray Tracing Kernel Selection
Accel. structure
bvh4.triangle4,
bvh8.triangle8,
bvh4aos.triangle1,
bvh4.grid
…
Builders
SAH builder
Spatial split builder
Morton code
builder
BVH Refitter
Traversal
Single ray (SSE2,
AVX, AVX2),
packet (SSE2),
hybrid
(SSE4.2),
...
Common Vector and SIMD Library
(Vec3f, Vec3fa, float4, float8, float16, SSE2, SSE4.1, AVX, AVX2, AVX512)
Intersection
MöllerTrumbore,
Plücker Variant,
Bezier Curve,
Triangle Grids
Subdiv
Engine
B-Spline Patch
Gregory Patch
TessellationCache
Displ. Mapping

 High ray tracing performance for photorealistic rendering
 Large memory capacity to render really complex models
 Robust tools to develop and debug rendering application
 Complex shading and rendering applications are
executed efficiently (e.g. light cuts with large per pixel
state)
Why Ray Tracing on CPUs?
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Copyright © 2015, Intel Corporation. All rights reserved. *Other names and brands may be claimed as the property of others.Copyright © 2015, Intel Corporation. All rights reserved. *Other names and brands may be claimed as the property of others.8/18/2015
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 Hides complexity of writing high performance ray tracing kernels
 gives you more time developing your renderer
 High performance on latest Intel® Xeon® Processor family and
Intel® Xeon Phi™ coprocessor products
 Embree always up to date with latest ISA instruction sets
 High potential performance gain
(1.5x – 6x rendering speedup reported by Embree users)
Why should I use Embree?

 As a benchmark to identify performance issues in
existing applications
 Adopt algorithms from Embree to your code
– However Embree internals change frequently!
 As a library through the Embree API (recommended)
– Benefit from future Embree improvements!
How can I use Embree?
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Embree v2.6.1 Performance
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 Models and illumination effects representative for
professional rendering environment
 Path tracer with different material types, different
light types, about 2000 lines of code
 Evaluation on typical Intel® Xeon® rendering
workstation* and Intel® Xeon Phi™ Coprocessor**
 Compare against state of the art GPU*** methods
(using OptiX™ 3.8.0 and CUDA® 7.0.28)
 Identical implementations in ISPC (Xeon®), ISPC
(Xeon Phi™), OptiX™ (GTX™ Titan X)
Performance Methology
20
Imperial Crown of Austria
4.3M triangles
Bentley 4.5l Blower (1927)
2.3M triangles
Asian Dragon
12.3M triangles
* Dual Socket Intel® Xeon® E5-2699 v3 2x18 cores @ 2.30GHz ** Intel® Xeon Phi™ 7120, 61 cores @ 1.238 GHz *** NVIDIA® GeForce® GTX™ Titan X

Build Performance for Static Scenes
40 41 45
32.3 31.7 35.1
0
50
100
150
Intel® Xeon® E5-2699 v3
Processor
2 x 18 cores, 2.3 GHz
Intel® Xeon Phi™ 7120
Coprocessor
61 cores, 1.28 GHz
21
MillionTriangles/Second
SAH Build (high quality)

Build Performance for Dynamic Scenes
112 108 105
160.1
140.4
162.1
0
50
100
150
Processor
Coprocessor
61 cores, 1.28 GHz
22
MillionTriangles/Second
Morton Build

Ray Tracing Performance (incl. Shading)
107.2
129.6 134.98
64.96
75.36
82.62
29.472 35.04 38.76
0
20
40
60
80
100
120
140
Processor
Coprocessor
61 cores, 1.28 GHz
NVIDIA® GeForce®
GTX™ Titan X
Coprocessor
12 GB RAM
23
MillionRays/Second

Embree API
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 Version 2 of the Embree API
 Compact and easy to use
 C++ and ISPC version
 Hides implementation details
(e.g. different spatial index structures)
Embree API Overview
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 Scene is container for set
of geometries
 Scene flags passed at
creation time
 Scene geometry changes
have to get commited
(rtcCommit) which
triggers BVH build
Scene Object
26
/* include embree headers */
#include <embree2/rtcore.h>
int main ()
{
/* initialize at application startup */
rtcInit ();
/* create scene */
RTCScene scene = rtcNewScene
(RTC_SCENE_STATIC,RTC_INTERSECT1);
/* add geometries */
... later slide ...
/* commit changes */
rtcCommit (scene);
/* trace rays */
... later slide ...
/* cleanup at application exit */
rtcExit ();
}

 Static Scenes
– Geometry cannot get changed
– High quality BVH build (SAH)  faster ray traversal
– For final frame rendering
 Dynamic Scenes
– Geometries can get added, modified, and removed
– Faster build (Morton)  slower ray traversal
– Preview mode during geometric modeling
Scene Types
27

Triangle Mesh
 Contains vertex and
index buffers
 Number of triangles
and vertices set at
creation time
 Linear motion blur
supported (2 vertex
buffers)
/* add mesh to scene */
unsigned int geomID = rtcNewTriangleMesh
(scene, numTriangles, numVertices, 1);
/* fill data buffers */
... later slide ...
/* add more geometries */
...
rtcCommit (scene);
29

 Recommended to use buffer sharing
 Reduces memory consumption
 Application manages buffers (buffer has to stay alive as
long as geometry is alive)
 Support for stride and offset allows application
flexibility in its data layout
Buffer Sharing
30

Buffer Sharing Example
/* application vertex and index layout */
struct Vertex { float x,y,z,s,t; };
struct Triangle { int materialID, v0, v1, v2; };
/* share buffers with application */
rtcSetBuffer(scene,geomID,RTC_VERTEX_BUFFER,vertexPtr,0,sizeof(Vertex));
rtcSetBuffer(scene,geomID,RTC_INDEX_BUFFER ,indexPtr ,4,sizeof(Triangle));
31

Tracing Rays
 rtcIntersect (scene, ray) reports first intersection
 rtcOccluded (scene, ray) reports any intersection
 Packet versions for ray packets of size 4,8, and 16
32

rtcIntersect: Ray Structure Inputs
 Ray origin and direction (org, dir)
 Ray interval (tnear, tfar)
 Time used for motion blur [0,1]
struct RTCRay
{
Vec3f org;
Vec3f dir;
float tnear;
float tfar;
float time;
Vec3f Ng;
float u;
float v;
int geomID;
int primID;
int instID;
}
33

rtcIntersect: Ray structure Outputs
 Hit distance (tfar)
 Unnormalized geometry normal (Ng)
 Local hit coordinates (u,v)
 Geometry identifier of hit geometry
(geomID)
 Index of hit primitive of geometry
(primID)
 Geometry identifier of hit instance
(instID)
 No shading normals, texture
coordinates, etc.
struct RTCRay
{
Vec3f org;
Vec3f dir;
float tnear;
float tfar;
float time;
Vec3f Ng;
float u;
float v;
int geomID;
int primID;
int instID;
}
34

35
 Simplifies writing vectorized renderer
 C-based language plus vector extensions
 Scalar looking code that gets vectorized automatically
 Guaranteed vectorization
 Compilation to different vector ISAs (SSE, AVX, AVX2,
AVX512, Xeon Phi™)
 Available as Open Source from http://ispc.github.com
Intel® SPMD Program Compiler (ISPC)

/* loop over all screen pixels */
foreach (y=0 ... screenHeight-1, x=0 ... screenWidth-1)
{
/* create and trace primary ray */
RTCRay ray = make_Ray(p,normalize(x*vx + y*vy + vz),eps,inf);
rtcIntersect(scene,ray);
/* environment shading */
if (ray.geomID == RTC_INVALID_GEOMETRY_ID) {
pixels[y*screenWidth+x] = make_Vec3f(0.0f); continue;
}
/* calculate hard shadows */
RTCRay shadow = make_Ray(ray.org+ray.tfar*ray.dir,neg(lightDir),eps,inf);
rtcOccluded(scene,shadow);
if (shadow.geomID == RTC_INVALID_GEOMETRY_ID)
pixels[y*width+x] = colors[ray.primID]*(0.5f + clamp(-dot(lightDir,normalize(ray.Ng)),0.0f,1.0f));
else
pixels[y*width+x] = colors[ray.primID]*0.5f;
}
Embree Rendering: ISPC Example
36

Dynamic Scenes
 Create scene with
RTC_SCENE_DYNAMIC flag
 Report modified meshes
with rtcUpdate call
 Possibly
enable (rtcEnable),
disable (rtcDisable),
add (rtcNewXX), and
delete (rtcDeleteGeometry)
geometries
for each frame
{
for each dynamic mesh
{
/* modify shared buffers */
modify mesh->indices
modify mesh->vertices
/* signal mesh update */
rtcUpdate(scene,mesh);
}
rtcCommit (scene);
/* trace rays */
...
}
37

 Per geometry callback that is called
during traversal for each primitive
intersection
 Callback can accept or reject hit
 Can be used for:
– Trimming curves (e.g. modeling tree leaves)
– Transparent shadows (reject and
accumulate)
– Find all hits (reject and collect)
Intersection Filter Functions
42

/* procedural intersection filter function */
void intersectionFilter(void* userPtr, RTCRay& ray)
{
Vec3fa h = ray.org + ray.dir*ray.tfar;
float v = abs(sin(4.0f*h.x)*cos(4.0f*h.y)*sin(4.0f*h.z));
float T = clamp((v-0.1f)*3.0f,0.0f,1.0f);
if (T > 1.0f) return; // accept hit
ray.geomID = RTC_INVALID_GEOMETRY_ID; // reject hit
}
/* set intersection filter for the cube */
rtcSetIntersectionFilterFunction(scene, geomID, (RTCFilterFunc)&intersectionFilter);
rtcSetOcclusionFilterFunction (scene, geomID, (RTCFilterFunc)&intersectionFilter);
rtcSetUserData (scene, geomID, NULL);
Filter Function Example
43

 Hair curves represented as
cubic bezier curves with
varying radius
 High performance through
use of oriented bounding
boxes
 Low memory consumption
through direct ray/curve
intersection
Hair Geometry
44
p0/r0 p1/r1
p2/r2
p3/r3

Catmull Clark Subdivision Surfaces
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 Converts coarse mesh into smooth
surface by subdivision
 Generalization of bi-cubic B-Spline
surfaces to arbitrary topology
 Embree is compatible with
OpenSubdiv 3.0
Catmull Clark Subdivision Surfaces
46 46

Catmull Clark Subdivision
47

 Low resolution base mesh controls high
resolution surface
 Smoothness always guaranteed
(C2 continous almost everywhere)
 Support for arbitrary topology
(no trimming required as with NURBS)
 Creases allow introducing sharp features
 Support in most modeling tools
 Established as standard in movie production
CC Subdivision Surface Advantages
Inside-Out (2015)
Pixar, Walt Disney Pictures
48

 Semi-sharp edge creases
 Semi-sharp vertex creases
 Vertex attribute interpolation
 Tessellation level per edge
 Non-manifolds and Holes
 Boundary modes
 Triangles, Quads, Pentagons, ...
 Displacement mapping
Embree Subdivision Features
49
49

Embree Subdivision Example
50
unsigned geomID = rtcNewSubdivisionMesh (scene, RTC_GEOMETRY_STATIC,
numFaces, numIndices, numVertices,
numEdgeCreases, numVertexCreases, numHoles);
rtcSetBuffer (scene,geomID,RTC_VERTEX_BUFFER, vertices, 0, sizeof(float3));
rtcSetBuffer (scene,geomID,RTC_INDEX_BUFFER , indices, 0, sizeof(int));
rtcSetBuffer (scene,geomID,RTC_FACE_BUFFER , faces, 0, sizeof(int));
rtcSetBuffer (scene,geomID,RTC_LEVEL_BUFFER , levels, 0, sizeof(float));
rtcSetBuffer (scene,geomID,RTC_EDGE_CREASE_INDEX_BUFFER,...);
rtcSetBuffer (scene,geomID,RTC_EDGE_CREASE_WEIGHT_BUFFER,...);
rtcSetBuffer (scene,geomID,RTC_VERTEX_CREASE_INDEX_BUFFER,...);
rtcSetBuffer (scene,geomID,RTC_VERTEX_CREASE_WEIGHT_BUFFER,...);
rtcSetBuffer (scene,geomID,RTC_HOLE_BUFFER,holes,0,sizeof(char));

 Feature adaptive subdivision to
evaluate patch (subdivides only at
irregular vertices and crease
features)
 Fast path for B-Spline patches and
Gregory patches
 Tessellation Cache limits memory
consumption (trade memory for
performance)
Embree Subdivision Implemention
51
Feature adaptive subdivision
into B-Spline patches (green) and
Gregory Patches (blue)

Embree Subdivision Performance
Patches 16 52k 53k
Edge Creases 0 0 30k
Micro Quads 1048k 831k 837k
Walkthrough 32 fps 36 fps 23 fps
Same View 66 fps 51 fps 56 fps
Intel® Xeon® E5-2690
2.9 GHz
2x 8 cores
1024 x 1024 pixels
52

 Interpolates arbitrary user data over
geometries (non-trivial for subdivision
geometries)
 Interpolated data P as well as dPdu and
dPdv can be calculated at arbitrary location
 Enables smooth normals and anisotropic
texture lookups
 Different rules for interpolation of texture
coordinates supported (by evaluation of
second subdiv mesh)
Vertex Data Interpolation
53

Vertex Data Interpolation Example
rtcNewScene (RTC_STATIC, RTC_INTERSECT1 | RTC_INTERPOLATE);
...
unsigned geomID = rtcNewSubdivisionMesh (...);
rtcSetBuffer (scene,geomID,RTC_INDEX_BUFFER, indices, 0, sizeof(int));
rtcSetBuffer (scene,geomID,RTC_VERTEX_BUFFER, vertices, 0, sizeof(float3));
rtcSetBuffer (scene,geomID,RTC_USER_VERTEX_BUFFER, vertex_colors, 0, sizeof(float3));
...
rtcCommit (scene);
...
rtcIntersect (scene, ray);
...
float3 P, dPdu, dPdv;
rtcInterpolate (scene, geomID, primID, ray.u,ray.v, RTC_VERTEX_BUFFER, &P, &dPdu, &dPdv, 3);
float3 color;
rtcInterpolate (scene, geomID, primID, ray.u,ray.v, RTC_USER_VERTEX_BUFFER, &color, 0,0, 3);
54

Displaced Subdivision Surface
55
 Support for vector displacement
 Callback function displaces vertex
positions
 Bounded displacement allows for lazy
evaluation
 Smooth normals possible through
approximation
Q = P + D*Ng
dQdu ≈ dPdu + dDdu*Ng
dQdv ≈ dPdv + dDdv*Ng

void displacementFunction(
void* ptr, int geomID, int primID,
const float* u, const float* v,
const float* nx, const float* ny, const float* nz,
float* px, float* py, float* pz,
size_t N)
{
for (size_t i = 0; i<N; i++) {
float D = displacement(...);
px[i] += D*nx[i];
py[i] += D*ny[i];
pz[i] += D*nz[i];
}
}
BBox3fa bounds(...);
rtcSetDisplacementFunction (scene,geomID,displacementFunction,&bounds);
Displaced Subdivision Surface Example
56

57
 Embree delivers highest ray tracing performance on CPUs
 Embree can speed up many ray tracing applications
 Embree is easy to use through its API
 Subdivision surface support compatible to OpenSubdiv 3.0
 Free and Open Source (https://embree.github.com)
Summary

Questions?
https://embree.github.io
embree@googlegroups.com
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C o p y r i g h t © 2 0 1 5 , I n t e l C o r p o r a t i o n . A l l r i g h t s r e s e r v e d . *O t h e r n a me s a n d b r a n d s ma y b e c l a i me d a s t h e p r o p e r t y o f o t h e r s .

60
Technical Overview

 Stochastic integration
of all potential light
paths between source
and pixel
 Follows light
backwards from pixel
to light source
 Produces incoherent
ray distributions
Monte Carlo Ray Tracing
Light
Pixel

Two Kinds of Ray Distributions
Incoherent Rays
(typical for Monte Carlo)
Coherent Rays

BVH Acceleration Structure

Solution Space for Vectorized Ray Tracing
Single Ray
SIMD
Traversal
Scalar
Traversal
Packet
Traversal
Independent
Ray TraversalMulti Ray
Single Ray
Single Box Multi Box

BVH4 Spatial Index Structure
struct Node4 {
ssef minx, miny, minz;
ssef maxx, maxy, maxz;
Node4* child[4];
}
struct Triangle4 {
ssef v0x,v0y,v0z;
ssef e1x,e1y,e1z;
ssef e2x,e2y,e2z;
ssef Nx,Ny,Nz;
ssei id0,id1;
}

For each dimension:
 Intersect ray with near plane of each box in SIMD
 Intersect ray with far plane of each box in SIMD
 Clip the near and the far parameters (box hit if near <= far)
BVH4 Traversal
near4
near1
near2
near3
far1
far1
far3 far4

• Reduce number of executed instructions
• Reduce data dependencies of critical paths
• Take advantage of special instructions (e.g. SSE, bitscan,
etc.)
• Optimize most frequently executed code paths
Optimizing BVH4 Traversal for CPUs

• Load front/back plane based on
direction sign of the ray.
• Balanced min/max trees
• Bitscans to iterate through hit children
• Early exit for 0 children hit (20%)
• 1 child hit (50%): keep next node in
register (instead of push/pop
sequence)
• 2 children hit (20%): keep next node
in register, sort using a branch
Optimizing BVH4 Traversal for CPUs (SSE)
while (true) {
if (isLeaf(node)) goto leaf;
ssef nearX = (norg.x + node[nearX]) * rdir.x;
ssef nearY = (norg.y + node[nearY]) * rdir.y;
ssef nearZ = (norg.z + node[nearZ]) * rdir.z;
ssef farX = (norg.x + node[farX ]) * rdir.x;
ssef farY = (norg.y + node[farY ]) * rdir.y;
ssef farZ = (norg.z + node[farZ ]) * rdir.z;
ssef near = max(max(nearX,nearY),
max(nearZ,ray.near));
ssef far = min(min(farX,farY),
min(farZ,ray.far));
int hitmask = movemask(near <= far);
if (hitmask == 0) goto pop;
int c = bitscan(hitmask);
hitmask = clearbit(hitmask,c);
if (hitmask == 0) {
node = node.child[c]; continue;
} …

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