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
IS & T / SPIE EI-VDA 2008
Introduction
IS & T / SPIE EI-VDA 2008
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
Evenly Spaced Streamlines (ESS)
IS & T / SPIE EI-VDA 2008
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
Evenly Spaced Streamlines (ESS)
IS & T / SPIE EI-VDA 2008
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
Evenly Spaced Streamlines (ESS)
IS & T / SPIE EI-VDA 2008
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
Evenly Spaced Streamlines (ESS)
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)
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
Introduction
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
Efficient Greedy Non-split Streamline Reuse+Lengthening
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)
IS & T / SPIE EI-VDA 2008
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
Efficient Greedy Non-split Streamline Reuse+Lengthening
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
Play the IVDESS-TISS movie!
Play the IVDESS-TCSS movie!
number of streamlines per frame
Results
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
Conclusions
IS & T / SPIE EI-VDA 2008
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
IS & T / SPIE EI-VDA 2008
Thank you!
Any questions?