Interactive View-Driven Evenly Spaced Streamline Placement Zhanping Liu Robert J. Moorhead II Visualization Analysis and Imaging Lab High Performance Computing.

Interactive View-Driven Evenly Spaced Streamline Placement

Zhanping Liu Robert J. Moorhead II

Visualization Analysis and Imaging LabHigh Performance Computing Collaboratory

Mississippi State University

IS & T / SPIE EI-VDA 2008

Outline

Results

Conclusions

Introduction

IVDESS

IVDESS Pipeline

— physical-space streamline integration — view-space streamline density control

Temporally-Coherent Seeding Strategy (TCSS)

vs. Temporally-Incoherent Seeding Strategy (TISS)

View-Sensitive Streamline Representation


Introduction


Texture-based (e.g., LIC) — powerful in visualizing 2D flows

Evenly Spaced Streamlines (ESS)

There have been many flow visualization methods Geometry-based (e.g., arrow plots)

Introduction



Texture-based techniques may be ineffective for 2.5D/3D flows due to view occlusion, depth ambiguity, direction vagueness, & aliasing artifacts Streamlines remain one of the most important 3D approaches for the straightforward direction cueing and the low computational expense

Introduction



Without an effective placement strategy, streamlines tend toresult in an incomplete coarse view or a global but cluttered image

A heavily cluttered image may still missan important flow feature (saddle here)

Introduction



A layout of evenly spaced streamlines may provide an aesthetic & informative pattern to facilitate mental reconstruction of the flow

here the saddleis clearly shown

Introduction


To apply ESS to 3D exploration of volume flows, surface flows, & planar flows in a perspective-view setting, we need to address

the foreshortening effect to obtain a visually uniform streamline placement — streamlines evenly spaced in 3D physical space (the flow field) may not visually retain the uniformity when projected to 2D view space (the output image)


the inter-frame transition

to enable a temporally coherent flow exploration

the practical applicability to provide an interactive grid-friendly solution

O. Mattausch, T. Theubl, H. Hauser, and E. Groller

Uniform in physical space but non-uniform in view space

Streamlines that are evenly spaced in a 2D flow field are visually non-uniform in a perspective-view setting

Introduction

Existing ESS Algorithms Image-guided methods

Sample-based methods

Take a streamline placement as a binary-valued image

Low-pass filter each intermediate placement and then compare it against a reference gray-scale image to guide iterative refinement toward an optimal

Use inter-sample distance control to approximate inter-line distance control

Distance checking is performed on each newly generated sample against other existing samples to determine if the distance is less than a threshold d


Introduction


ESS for Surface & Volume Flows

Physical-space ESS placement strategy

multi-density representation — Mattausch et al [03]

surface flows — Mao et al [98]

volume flows — Ye et al [05]

View-space ESS placement strategy

surface & volume flows — Li-Shen [07]

Streamlines are indeed not evenly spaced in the output image

Introduction


IVDESS (Interactive View-Driven ESS) built on ADVESS (ADVanced ESS, Liu & Moorhead[06])

a 2D engine for sample-based streamline placement

supports fast high-quality ESS placement with robust loop detection

for ESS-based 3D (through perspective projection) exploration of

a planar flow

a surface flow

essentially different from previous work in

placing streamlines that are indeed evenly spaced in the output image

providing a solution for coherent exploration of flows

delivering high performance on a low-end PC

IVDESS


Basic Idea

the non-uniform streamline placementof a planar flow in 3D physical space

the resulting visually uniform layoutin 2D view space (the output image)

surface rendering

depth acquisition

streamline integration in physical space streamline-density control in view space

whether a streamline is further advectedor immediately terminated in physical spaceis governed by the status (accepted/rejected)

of the newly generated point

the projection of each streamline pointand the associated view-space samplesundergo inter-sample distance controlto achieve inter-line distance control

accept orreject point

do pointprojection

IVDESS The Pipeline


Dividing ADVESS Components into Two Spacesphysical-space seeding is used toestablish inter-frame coherence

view-space seeding is used to create a separateframe of view-dependent evenly spaced streamlines

each line segment is uniformlysampled in view space by thres. d

inter-line distance control & intra-line distance controlare both achieved using inter-sample distance control

IVDESS The Pipeline


TISS (Temporally Incoherent Seeding Strategy) — for separate frames

a view-space seeding scheme

sort and insert

Candidates introduced by the seed sample of a streamline are

saved&sorted by the view-space streamline length in primary queue — a sorting queue Candidates introduced by each regular (non-seed) sample of a streamline are simply appended to the tail of secondary queue — a FIFO queue

append to tail

adopts a double-queue seed scheduler

primary queue head

secondary queue head

Primary queue takes priority over secondary queue in providing candidates

Only when primary queue is temporarily empty is secondary queue used to either init the layout process or guarantee view coverage

IVDESS Temporally Coherent Seeding Strategy


Building on top of TISS

IVDESS provides a multi-resolution (in physical space) flow representation and hence requires smooth inter-frame transition to achieve coherent flow exploration with visually uniform lines

TISS is an intra-frame view-space seeding mechanism without addressing explorative issues

IVDESS employs an inter-frame physical-space seeding scheme on top of TISS to constitute a

The inter-frame physical-space seeding scheme maintains temporal coherence by reusing and lengthening the streamlines of the previous frame under normal density control in the current frame

Temporally Coherent Seeding Strategy (TCSS) — physical-space seeding prior to view-space seeding

IVDESS TCSS

IVDESS TCSS


Efficient Greedy Non-split Streamline Reuse+Lengthening

Each streamline of the previous frame is accessed from physical- space storage and processed beginning with the seed in both directions — reprojection + resampling + possible lengthening

A streamline is potentially reused in either direction as long as the first in-view-segment sample passes inter-sample distance check

greediness: a streamline with the seed out of the view may be reused

otherwise: the disappearance of such streamlines brings big view change

A streamline is saved if it passes the view-space length check

the accepted in-view part + the rejected in-view part + the out-of-view part

IVDESS TCSS



Point projection and segment sampling continue until any sample (I1) fails to pass inter-sample distance check

The first in-view segment sample (I0) in either direction is a raw segment sample — the projection of an in-view seed (S) an intermediate segment sample from line-view clipping (seed S out of view)


Check view-space length to decide if the streamline needs saving

Lengthening+projection+sampling occurs if the line end is reachedI0 R0 R1 I1; I: Intermediate segment sample; R: Raw segment sample

prevents the number of streamlines from excessively increasing suppresses incoherence / artifacts over the view boundaries

Non-split streamline reuse+lengthening

IVDESS TCSS


Projection+sampling continues until any sample (I1) fails to pass inter-sample distance check

The first in-view segment sample (I0) in either direction is a raw segment sample — the projection of an in-view seed (S) an intermediate segment sample from line-view clipping (seed S out of view)

Lengthening+projection+sampling occurs if the line end is reached

Efficient (projection + sample-in-view check distance check in comp. cost)

allows closed streamlines to form

Otherwise discontinuitieswould occur

IVDESS View-Sensitive Streamline Representation


Complete Storage & Visibility Description

A streamline successfully reused in an IVDESS frame may include an out-of-view part and / or an in-view but rejected part while neither should be rendered to the output image

Physical-space raw points of a streamline are sequentially stored in the main body of a buffer from the negative end to the positive end

number of raw points, seed’s buffer-index, view-space streamline length

Header of the streamline buffer

2 VSDs (View-Sensitive Descriptors, one per direction) after the header

the first accepted in-view segment sample I0 — 3D coordinate the first accepted in-view raw point R0 — buffer index the last accepted in-view raw point R1 — buffer index the last accepted in-view segment sample I1 — 3D coordinate instantaneous adaptive step size closing point of a closed streamline

Lengthening+projection+sampling occurs if the line end is reachedI0 R0 R1 I1; I: segment-view clip sample; R: Raw segment sample

The unprojection point of a line-view clip sample is temporarily stored

in a VSD to render the current frame properly

Otherwise jaggy lines might emerge as unintended unprojection points

are stored in the main body and then used in the subsequent frames to

lengthen streamlines

VSDs avoid jaggy lines resulting from unprojection errors

Thorough Reuse & Proper Rendering

VSDs provide a general description of the accepted viewable parts of a streamline to allow for greedy reuse+lengthening

Redundancy may occur between fields and padding may be needed

Fields need to be dynamically updated to keep track of the change

IVDESS View-Sensitive Streamline Representation


Jaggy lines emerge when unintended unprojection points (due to numerical error) of the current frame are reused in the subsequent frames to lengthen the streamlines

Results


Implementation & Test

Current implementation (using VC++ and OpenGL) — IVDESS for 3D exploration of planar flows in a perspective-view setting

Notebook PC (Celeron M 1.60GHz/512MB RAM/Window XP/no GPU)

Test platform — a nowadays low-end facility

Test aspects — placement speed / placement quality / temporal coherence

Test dataset — a 468337 2D flow field of the Northeast Pacific ocean

Perspective projection near clipping plane = 1.0 field-of-view angle = 90°

far clipping plane = 10000.0 aspect ratio = 1.0

view size = 990700

Initial step size (0.0625) & the adaptive range [10-5, 10-4] in cells Threshold distance (10) & min streamline length (30) in pixels 100 IVDESS-TCSS (IVDESS) frames & 100 IVDESS-TISS frames were generated based on exactly the same exploration of the flow

over one hundred critical points making a very complex flow pattern

Streamlines are evenly spaced in an IVDESS-TCSS frame without cluttering or distractingdiscontinuities. In particular, there are 3 closed streamlines successfully detected and formed.

The IVDESS-TCSS layout demonstrates the capability of our seeding strategy, even withouttopology-based seed distribution, in placing evenly spaced streamlines around critical points.

Results


Play the IVDESS-TISS movie!

Play the IVDESS-TCSS movie!

number of streamlines per frame

Results


This demonstrates the effectiveness of the streamline reuse+lengthening scheme of TCSS

For more than half of the TCSS frames, there are far more reused streamlines than advected ones per frame. Even for the other frames, the number of reused streamlines is only a little bit less than that of advected ones per frame.

TCSS-reused

TCSS-advected

TCSS total

TISS total

The total number of streamlines in a TCSS frame is very similar to that in a TISS frame.

This indicates the high-performance of TCSS in preventingthe number of streamlines from excessively increasing.

Results


streamline reuse percentage for each TCSS frame

The high percentages demonstrate the effectiveness of the greedy non-split streamline reuse+lengthening scheme adopted in TCSS.

streamlines obtained by reuseall streamlines in the previous framepercentage =

streamlines obtained by reuseall streamlines in the current framepercentage =

time required per frame

Results


The variation in frame generation time for TCSS is much less than that for TISSand this is also the case with frame generation+rendering time.

For nearly every frame and for either case (generation time / generation+rendering time),less time was consumed by TCSS than by TISS.

TCSS generation

TCSS generation+renderingTISS generation

TISS generation+rendering

frames per second

Results


The interactive and nearly constant frame rates of TCSS indicate that IVDESS-TCSS (IVDESS) is well suited for coherent flow exploration.

TCSS generation

TCSS generation+renderingTISS generation

TISS generation+rendering

Conclusions

IVDESS is a physically non-uniform but visually uniform representation of planar or curved surface flows in a perspective-view setting

IVDESS divides the view-dependent uniform placement process into physical-space flow integration & view-space streamline density control

A projection-unprojection pair is used via off-screening surface rendering to link the two spaces

Greedy but efficient non-split streamline reuse+lengthening is an inter-frame physical-space seeding scheme that is adopted on top of an intra-frame view-space seeding method to constitute a hybrid-space multi-level seeding mechanism — Temporally Coherent Seeding Strategy

A view-sensitive streamline representation is used to support thorough reuse+lengthening while guaranteeing proper rendering

IVDESS is well suited for coherent level-of-detail 3D exploration of large complex flows at interactive frame rates without either pre-processing or GPU support on a nowadays low-end PC


Conclusions


DoD HPCVI Program

Dr. David Kao

Anonymous reviewers

Acknowledgments

to enhance the current version of IVDESS in support of flows on curvilinear grids and unstructured grids

to investigate adaptive depth selection issues in an effort to extend IVDESS for explorative visualization of volume flows

Future Work


Thank you!

Any questions?

Interactive View-Driven Evenly Spaced Streamline Placement Zhanping Liu Robert J. Moorhead II Visualization Analysis and Imaging Lab High Performance Computing.

Documents

spaced streamlines ess

view space streamlines

t spie eivda

d view space

d flows

d physical space

interactive view

perspectiveview setting