Interactive View-Driven Evenly Spaced Streamline Placement

1. IS & T / SPIE EI-VDA 2008 IVDESS Interactive View-Driven Evenly Spaced Streamline Placement Zhanping Liu Robert J. Moorhead II Visualization Analysis and Imaging Lab High Performance Computing Collaboratory Mississippi State University

2. 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 • Results • Conclusions IS & T / SPIE EI-VDA 2008 IVDESS Outline

3. Evenly Spaced Streamlines (ESS) • There have been many flow visualization methods • Geometry-based (e.g., arrow plots) • Texture-based (e.g., LIC) — powerful in visualizing 2D flows IS & T / SPIE EI-VDA 2008 IVDESS Introduction

4. 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 IS & T / SPIE EI-VDA 2008 IVDESS Introduction • Evenly Spaced Streamlines (ESS)

5. Without an effective placement strategy, streamlines tend to result in an incomplete coarse view or a global but cluttered image A heavily cluttered image may still miss an important flow feature (saddle here) IS & T / SPIE EI-VDA 2008 IVDESS Introduction • Evenly Spaced Streamlines (ESS)

6. A layout of evenly spaced streamlines may provide an aesthetic & informative pattern to facilitate mental reconstruction of the flow here the saddle is clearly shown IS & T / SPIE EI-VDA 2008 IVDESS Introduction • Evenly Spaced Streamlines (ESS)

7. Uniform in physical space but non-uniform in view space • 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 visuallyuniform 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 Streamlines that are evenly spaced in a 2D flow field are visually non-uniform in a perspective-view setting • the practical applicability to provide an interactive grid-friendly solution O. Mattausch, T. Theubl, H. Hauser, and E. Groller IS & T / SPIE EI-VDA 2008 IVDESS Introduction • Evenly Spaced Streamlines (ESS)

8. Existing ESS Algorithms • Image-guided 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 • Sample-based methods • Use inter-sample distance control to approximate inter-line distance control • Distance checking is performed oneach newly generated sample against other existing samples to determine if the distance is less than a threshold d IS & T / SPIE EI-VDA 2008 IVDESS Introduction

9. ESS forSurface & 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 IS & T / SPIE EI-VDA 2008 IVDESS Introduction

10. 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 IS & T / SPIE EI-VDA 2008 IVDESS Introduction

11. Basic Idea streamline integration in physical space streamline-density control in view space accept or reject point do point projection depth acquisition the non-uniform streamline placement of a planar flow in 3D physical space the resulting visually uniform layout in 2D view space (the output image) surface rendering IS & T / SPIE EI-VDA 2008 IVDESS IVDESS whether a streamline is further advected or immediately terminated in physical space is governed by the status (accepted/rejected) of the newly generated point the projection of each streamline point and the associated view-space samples undergo inter-sample distance control to achieve inter-line distance control

12. view-space seeding is used to create a separate frame of view-dependent evenly spaced streamlines physical-space seeding is used to establish inter-frame coherence • Dividing ADVESS Components into Two Spaces inter-line distance control & intra-line distance control are both achieved using inter-sample distance control each line segment is uniformly sampled in view space by thres. d IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  The Pipeline

13. TISS (Temporally Incoherent Seeding Strategy) —for separate frames • a view-space seeding scheme • adopts a double-queue seed scheduler • 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 • Candidates introduced by the seed sampleof a streamlinearesaved&sorted by the view-space streamline length in primary queue — a sorting queue • Candidates introduced by eachregular (non-seed) sampleof a streamline are simply appended to the tail of secondary queue — a FIFO queue sort and insert primary queue head append to tail secondary queue head IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  The Pipeline

14. IVDESS  TCSS • 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 explorationwith visually uniform lines • TISSisan intra-frame view-space seedingmechanismwithout addressing explorative issues IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  Temporally Coherent Seeding Strategy • IVDESSemploys an inter-frame physical-space seeding scheme on top of TISS to constitutea Temporally CoherentSeeding Strategy (TCSS) — physical-space seeding prior to view-space seeding • The inter-frame physical-space seeding scheme maintains temporal coherence by reusing and lengthening thestreamlinesof the previous frame under normal density control in the current frame

15. Efficient Greedy Non-split Streamline Reuse+Lengthening • Each streamline of the previous frame is accessed fromphysical- space storage and processed beginningwith 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 bereused • otherwise: the disappearance of such streamlines brings big view change • A streamline is saved if it passes the view-spacelength check • the accepted in-view part + the rejected in-view part + the out-of-view part IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  TCSS

16. The first in-view segment sample (I0) in either directionis • a raw segment sample — the projection of an in-view seed (S) IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  TCSS • Efficient Greedy Non-splitStreamline Reuse+Lengthening • an intermediatesegment sample from line-view clipping (seed S out of view) • Point projection and segment sampling continue untilany sample (I1) fails to pass inter-sample distance check

17. IVDESS  TCSS • Efficient Greedy Non-splitStreamline Reuse+Lengthening Otherwise discontinuities would occur • The first in-view segment sample (I0) in either directionis • 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) • Projection+sampling continues untilany sample (I1) fails to pass inter-sample distance check • Lengthening+projection+sampling occurs if the line end is reached • Lengthening+projection+sampling occurs if the line end is reached I0 R0  R1  I1; I: Intermediate segment sample; R: Raw segment sample • Non-split streamline reuse+lengthening • prevents the number of streamlines from excessively increasing • suppresses incoherence / artifacts over the view boundaries • allows closed streamlines to form • Efficient (projection + sample-in-view check  distance check in comp. cost) IS & T / SPIE EI-VDA 2008 IVDESS • Check view-space length to decideif the streamline needs saving

18. 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 bodyof a bufferfrom the negative end to the positive end • Header of the streamline buffer • Lengthening+projection+sampling occurs if the line end is reached I0 R0  R1  I1; I: segment-view clip sample; R: Raw segment sample • number of raw points, seed’s buffer-index, view-space streamline length • 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 IS & T / SPIE EI-VDA 2008 IVDESS IVDESS  View-Sensitive Streamline Representation

19. Thorough Reuse & Proper Rendering • VSDs provide a general description of the accepted viewable parts of a streamline toallow for greedy reuse+lengthening • Redundancy may occur between fields andpadding may be needed • Fields need to be dynamically updated to keep trackof the change • VSDs avoid jaggy lines resulting from unprojection errors • 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 IS & T / SPIE EI-VDA 2008 IVDESS 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

20. Implementation & Test • Current implementation(using VC++ and OpenGL)—IVDESS for 3D exploration of planar flows in a perspective-view setting over one hundred critical points making a very complex flow pattern • Test platform— a nowadays low-end facility • Notebook PC(Celeron M 1.60GHz/512MB RAM/Window XP/no GPU) • Test aspects— placement speed / placement quality / temporal coherence • Perspective projection • view size = 990700 • far clipping plane = 10000.0 • aspect ratio = 1.0 • near clipping plane = 1.0 • field-of-view angle = 90° • Initialstep size(0.0625)& the adaptive range[10-5, 10-4] in cells • Threshold distance(10)&min streamline length(30)in pixels • 100IVDESS-TCSS (IVDESS) frames & 100 IVDESS-TISS frames were generated based on exactly the same exploration of the flow IS & T / SPIE EI-VDA 2008 IVDESS Results • Test dataset—a 468337 2D flow field of the Northeast Pacific ocean

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

22. The IVDESS-TCSS layout demonstrates the capability of our seeding strategy, even without topology-based seed distribution, in placing evenly spaced streamlines around critical points.

23. IS & T / SPIE EI-VDA 2008 IVDESS Results Play the IVDESS-TISS movie! Play the IVDESS-TCSS movie!

24. number of streamlines per frame TISS total TCSS total TCSS-reused TCSS-advected For more than half of the TCSS frames, there are farmore reused streamlines thanadvected 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. The total number ofstreamlines in aTCSS frame is very similar to that in a TISSframe. This indicates the high-performance of TCSS in preventing the number of streamlines from excessively increasing. This demonstrates the effectiveness of the streamline reuse+lengthening scheme of TCSS IS & T / SPIE EI-VDA 2008 IVDESS Results

25. streamlines obtained by reuse all streamlines in the current frame streamlines obtained by reuse all streamlines in the previous frame percentage = percentage = IS & T / SPIE EI-VDA 2008 IVDESS 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.

26. time required per frame TISS generation+rendering TCSS generation+rendering TISS generation TCSS generation For nearly every frame and for either case (generation time / generation+rendering time), less time was consumed by TCSS than by TISS. The variation in frame generation time for TCSS is much less than that for TISS and this is also the case with frame generation+rendering time. IS & T / SPIE EI-VDA 2008 IVDESS Results

27. frames per second TCSS generation TISS generation TCSS generation+rendering TISS generation+rendering IS & T / SPIE EI-VDA 2008 IVDESS Results The interactive and nearly constant frame rates of TCSS indicate that IVDESS-TCSS (IVDESS) is well suited for coherent flow exploration.

28. IS & T / SPIE EI-VDA 2008 IVDESS Conclusions • IVDESSisa physically non-uniform but visually uniform representation of planar or curved surface flows in a perspective-view setting • IVDESSdivides the view-dependent uniform placement process into physical-space flow integration&view-space streamline density control • A projection-unprojection pair isused viaoff-screening surface rendering to link the two spaces • Greedy but efficient non-splitstreamline reuse+lengtheningisan inter-frame physical-space seedingscheme that is adopted on top ofan intra-frame view-space seedingmethod to constitute ahybrid-space multi-level seeding mechanism —Temporally Coherent Seeding Strategy • A view-sensitive streamline representationis used to support thorough reuse+lengthening while guaranteeing proper rendering • IVDESS is well suited forcoherent level-of-detail 3D explorationof large complex flows at interactive frame rates without either pre-processingor GPU support on a nowadays low-end PC

29. IS & T / SPIE EI-VDA 2008 IS & T / SPIE EI-VDA 2008 IVDESS Conclusions • Future Work • 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 IVDESS Thank you! • Acknowledgments Any questions? • DoD HPCVI Program • Dr. David Kao • Anonymous reviewers