Granular Visibility Queries on the GPU

University of Stuttgart - Visualization Research Center (VISUS) 19 April 2023

Granular Visibility Queries on the GPU

Thomas Engelhardt & Carsten Dachsbacher

Visualization Research Center

University of Stuttgart


Motivation:Culling


Motivation: Culling

Remove rendering workload from the pipeline

Prevent draw calls from execution- Frustum Culling- Hardware Occlusion Queries (HOQ) - Occlusion Predicates

Prevent shaders from execution- Backface Culling- Early-Z


Motivation: Culling

Control of shader execution based on visibilityGeometry ShaderPixel Shader when early-z is disabled

Visibility not only per object / draw call but perPrimitive / primitive clusterScreen space region

Evaluate and use visibility on GPU, no application feedback


Image Space Visibility

How to determine image space visibility?

Take some objects

Rasterize

Count pixels that passed the depth test

8But how to count? 6 11


Contribution

Two output sensitive pixel counting methods for from point visibility

Pixel Counting Summed Area Tables (PiC-SAT)Hierarchical Item Buffer (HIB)

Can also be done with HOQs. Why not use them?Granularity limitation & synchronization

Application toCulling of individual instancesControl of GS and PS execution for per pixel displacement mapping


Pixel Counting using Summed Area Tables


Pixel Counting using SATs

SAT stores sum of pixel values

Pixel sum of any rectangular region with just 4 lookups- Screen space bounding box

𝑆𝐴𝑇ሺ𝑥,𝑦ሻ= 𝐶𝑂𝑏𝑗(𝑖,𝑗)𝑦−1𝑗=0

𝑥−1𝑖=0

S=1 + 17 – 1 – 6 = 11

1

1

1 1 0 1 1

1 1

1 1 1

1

1

1

1

1

1 1 1

1

1

1

1

1

1

0

1

1

3

1

4

1

4

1

4

1

5

1

6

4

9

7

12

9

14

1

1

4

4

6

6

6

6

7

8

9

11

11

14

14

17

17

20

19

22

1

1

4

4

6

6

6

6

9

9

13

13

17

17

20

20

23

23

25

25

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

2

2

4

3

6


Pixel Counting using SATs

Crucial: Query regions must not overlap!

Can‘t differentiate to which object the pixels in the overlap belong


Conflict Objects

Conflict ObjectsObjects whose bounding rectangles overlap

How to resolve conflict?Distribute objects among color channels without overlap- 4 parallel SATs per RGBA texture

What is the distribution strategy?


Graph Coloring Algorithms

Graph Coloring AlgorithmsAssign colors to vertices in a graph- Vertices connected by an edge must

not share the same color

Difficult problem- Requires heuristic approaches like

Chaitin‘s algorithm- What if more edges than colors

available?

correct coloring

false coloring


Object Distribution by Graph Coloring

Construct a conflict graphEach object‘s bounding rectangle one vertexEach overlap one edge

Graph Construction

How to color the graph?

OVERLAP


Chaitin‘s Algorithm

Heuristic algorithm desgined for register allocation

InputConflict GraphSet of colors

OutputColor coded graph

Some vertices may remain uncolored

Complexity: O(N²)

color 1 color 2


Chaitin‘s Algorithm: Deconstruction

3

1

2 5 4

stack

4

1

5

3

21. Find any vertex with least

number of incident edges

2. Remove vertex and put onto a stack

3. Repeat until graph deconstructed


Chaitin‘s Algorithm: Reconstruction

3

1

2 5 4

stack

4

1

5

3

2

color 2color 11. Reinsert top vertex on stack

into graph

2. Find a color not used by any reconsructed neighbor

3. Repeat until entire graph is reconstructed

No color available

What to do about uncolored objects?


About Uncolored Objects

Uncolored objects need additional treatment1. Split bounding rectangle of uncolored

object

2. Attempt to color sub rectangles

3. Assign any color if no unique color can be found

Visibility overestimation

4. Attempt to merge sub regions


The Pixel Counting SAT Pipeline

CPU (Application)Construct Conflict

Graph Graph Coloring

Treat Uncolored Objects

Calculate Look Up Coordinates

GPU

Render to texture and compute SAT [Hensley05]

Count Pixels by SAT Look Up

Objects

Shader Constantscolor information

look up information

[Hensley05: Fast Summed Area Table Generation and its Applications]


Pixel Counting using the Hierarchical Item Buffer


The Hierarchical Item Buffer (HIB)

Exploits histogram computation algorithmGPU implementation demonstrated by Scheuermann [Scheuermann07: Efficient histogram generation using scattering on GPUs]

Render unique IDs to texture

𝐼𝐷= 𝐼𝐷𝑂𝑏𝑗𝑒𝑐𝑡 ∗𝑅𝑒𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛+𝐼𝐷𝑆𝑢𝑏 21

30 31 32

41 42

115 116

124 125

134

126

135 136

144 145 146

167 168 169

177 178 179

187 188

8

10

Object ID: 0 1 2

Sub ID: Enumerate pixels

Resolution: 8x10 = 80


GPU Item Buffer

Reinterpret ID texture as point listVertex or Geometry Shader for scatteringBlending operations for counting

VS/GS Maps to histogram

bin

RasterizerRenders point

primitive

BlendingIncrements bin

21 30 … 31 32 … 124 115 125 188 179 … 167

0 0 0 0 0 … 0 0 0 0 0 0

histogram/item buffer

1 1 1 1 1 11


Hierarchical Queries

Intelligently distributing IDs enables hierarchical queries by mip mapping

Mip

Map

1 10 … 0 1 10 … 1 1 10 … 0

1 13 1 2 43 2 4 12 1

6 11 8


Applications and Results


Culling of Instances

Shadow volumes with instanced renderingVolumes entirely contained in others have no effect [Lloyd04: CC Shadow Volumes]- Test caster visibility from light- HOQ / Occl. Predicates cannot be applied directly

- Granularity: a single draw call, not individual instances

Instances of the same object

Shadow VolumesCull volume with no contribution


Culling of Instances

Granularity: Per Instance (Sub-ID: Instance ID)500 shadow casters (606 triangles each)ID texture/SAT resolution: 512x512 pixels

ATI (

HD

4780

X2)

0 5 10 15 20 25 30 35 40 45 50

24

11

29

26

43

31

12

27

Predicates

HIB

PiC-SAT

No Cull

Thomsa Engelhardt

Hier noch erwähnen, wie groß die Buffer sind (HIB/SAT) und bei welche Auflösung wir rendern


Culling of Individual Primitives

Displacement MappingSetup costs in GS (mesh extrusion, tetrahedra, texture gradients)

Ray-Casting in PS - Cannot exploit early-z due to depth write

in PS

Don‘t output triangles if extruded prism is not visible- Exact visibility requires ray-casting in

HIB/SAT pass- Conservative visibility estimation by mesh

extrusion


Culling of Individual Primitives

Granularity: Per prismLizard: 7132 trianglesID texture / SAT resolution: 512x512

Ful

ly V

isib

leP

artia

lly V

isib

leO

cclu

ded

0 20 40 60 80 100 120 140 160 180 200

22

42

45

71

90

90

22

32

42

20

30

110

24

87

158

66

131

186

19

50

67

57

89

112

PiC-SAT (NVIDIA)

PiC-SAT (ATI)

HIB (NVIDIA)

HIB (ATI)

Predicates (NVIDIA)

Predicates (ATI)

No Cull (NVIDIA)

No Cull (ATI)

NVIDIA: GTX280 ATI: HD3780


Discussion

Pixel Counting SAT

Not enough colors, if many objectsVisibility overestimation

Difficult implementationNot entirely transparent to application (overlap, coloring, …)

PerformanceDominated by treatment of uncolored objects (rectangle split/merge, texture access)

Can handle arbitrary screen regions for query

Hierarchical Item Buffer

No penalty for many objects

Easy ImplementationTransparent to application, GPU handles everything

PerformanceDominated by overdraw in item buffer caused by choice of IDs.Usage of many IDs better exploits parallelism. Mip map does the rest (memory consumption)

Query regions defined by ID assignment


Questions?

Granular Visibility Queries on the GPU

Documents

Transcript of Granular Visibility Queries on the GPU