Volume xx (200y), Number z, pp. 1–26

State-of-the-Art in
GPU-Based Large-Scale Volume Visualization
Johanna Beyer1 , Markus Hadwiger2 , Hanspeter Pfister1
1 Harvard University, USA
2 King Abdullah University of Science and Technology, Saudi Arabia

Abstract
This survey gives an overview of the current state of the art in GPU techniques for interactive large-scale volume
visualization. Modern techniques in this field have brought about a sea change in how interactive visualization
and analysis of giga-, tera-, and petabytes of volume data can be enabled on GPUs. In addition to combining theparallel processing power of GPUs with out-of-core methods and data streaming, a major enabler for interactivity
is making both the computational and the visualization effort proportional to the amount and resolution of data thatis actually visible on screen, i.e., “output-sensitive” algorithms and system designs. This leads to recent outputsensitive approaches that are “ray-guided,” “visualization-driven,” or “display-aware.” In this survey, we focus on
these characteristics and propose a new categorization of GPU-based large-scale volume visualization techniques
based on the notions of actual output-resolution visibility and the current working set of volume bricks—thecurrent subset of data that is minimally required to produce an output image of the desired display resolution.
Furthermore, we discuss the differences and similarities of different rendering and data traversal strategies involume rendering by putting them into a common context—the notion of address translation. For our purposes
here, we view parallel (distributed) visualization using clusters as an orthogonal set of techniques that we do notdiscuss in detail but that can be used in conjunction with what we discuss in this survey.
Categories and Subject Descriptors (according to ACM CCS): I.3.6 [Computer Graphics]: Methodology andTechniques—I.3.3 [Computer Graphics]: Picture/Image Generation—Display algorithms

1. Introduction
Visualizing volumetric data plays a crucial role in scientific visualization and is an important tool in many domainsciences such as medicine, biology and the life sciences,
physics, and engineering. The developments in GPU technology over the last two decades, and the resulting vast parallel processing power, have enabled compute-intensive operations such as ray-casting of large volumes at interactive
rates. However, in order to deal with the ever-increasing resolution and size of today’s volume data, it is crucial to use
highly scalable visualization algorithms, data structures, andarchitectures in order to circumvent the restrictions imposedby the limited amount of on-board GPU memory.
Recent advances in high-resolution image and volume acquisition, as well as computational advances in simulation,
have led to an explosion of the amount of data that must be
visualized and analyzed. For example, high-throughput electron microscopy can produce volumes of scanned brain tissue at a rate above 10-40 megapixels per second [BLK∗ 11],
with a pixel resolution of 3-5 nm. Such an acquisition prosubmitted to COMPUTER GRAPHICS Forum (10/2014).

cess produces almost a terabyte of raw data per day. For
the next couple of years it is predicted that new multibeam
electron microscopes will further increase the data acquisition rate by two orders of magnitude [Hel13, ML13]. A
current prototype with 61 beams can capture 1.2 billion
pixels per second already [Mar13]. This trend of acquiringand computing more and more data at a rapidly increasing pace (“Big Data”) will continue in the future [BCH12].
This naturally poses significant challenges to interactive visualization and analysis. For example, many established algorithms and frameworks for volume visualization do notscale well beyond a few gigabytes, and this problem cannot
easily be solved by simply adding more computing power
or disk space. These challenges require research on novel
techniques for data visualization, processing, storage, andI/O that scale to extreme-scale data [MWY∗ 09, AAM∗ 11,
BCH12].
Today’s GPUs are very powerful parallel processors thatenable performing compute-intensive operations such as
ray-casting at interactive rates. However, the memory sizes

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available to GPUs are not increasing at the same rate as
the amount of raw data. In recent years, several GPU-based
methods have been developed that employ out-of-core methods and data streaming to enable the interactive visualization of giga-, tera-, and petabytes of volume data. The crucial property that enables these methods to scale to extremescale data is their output-sensitivity, i.e., that they make boththe computational and the visualization effort proportional
to the amount of data that is actually visible on screen (i.e.,the output), instead of being proportional to the full amountof input data. In graphics, the focus of most early work on
output-sensitive algorithms was visibility determination ofgeometry (e.g., [SO92, GKM93, ZMHH97]).
An early work in output-sensitive visualization on GPUs
was dealing with 3D line integral convolution (LIC) volumesof flow fields [FW08]. In the context of large-scale volume
visualization, output-sensitive approaches are often referred
to as being ray-guided (e.g., [CNLE09, Eng11, FSK13]) or
visualization-driven (e.g., [HBJP12, BHAA∗ 13]). These are
the two terms that we will use most in this survey.
We use the term visualization-driven in a more generaland inclusive way, i.e., these methods are not necessarily
bound to ray-casting (which is implied by “ray-guided”),
and they can encompass all computation and processing ofdata in addition to rendering. In principle, the visual output can “drive” the entire visualization pipeline—including
on-demand processing of data—all the way back to theraw data acquisition stage [HBJP12,BHAA∗ 13]. This would
then yield a fully visualization-driven pipeline. However, toa large extent these terms can be used interchangeably.
Another set of output-sensitive techniques are displayaware multi-resolution approaches (e.g., [JST∗ 10, JJY∗ 11,
HSB∗ 12]). The main focus of these techniques is usually
output-sensitive computation (such as image processing)rather than visualization, although they are also guided by
the actual display resolution and therefore the visual output.
Ray-guided and visualization-driven visualization techniques are clearly inspired by earlier approaches for occlusion culling (e.g., [ZMHH97, LMK03]) and level of detail (e.g., [LHJ99, WWH∗ 00]). However, they have a muchstronger emphasis on leveraging actual output-resolution
visibility for data management, caching, and streaming—inaddition to the traditional goals of faster rendering and antialiasing. Very importantly, actual visibility is determined onthe-fly during visualization, directly on the GPU.
1.1. Survey Scope
This survey focuses on major scalability properties of volume visualization techniques, reviews earlier GPU volume renderers, and then discusses modern ray-guided andvisualization-driven approaches and how they relate to andextend the standard visualization pipeline (see Figure 1).
Large-scale GPU volume rendering can be seen as being

in the intersection of volume visualization and high performance computing. General introductions to these two topics
are given in books on real-time volume graphics [EHK∗ 06]
and high performance visualization [BCH12], respectively.
We mostly focus on techniques for stand-alone workstations with standard graphics hardware. We see the othercore topics of high performance visualization (i.e., parallel rendering on CPU/GPU clusters, distributed visualization
frameworks, and remote rendering) as an orthogonal set oftechniques that can be used in combination with modern rayguided, visualization-driven, and display-aware techniques
as discussed here. Therefore, for more details on parallel visualization we refer the reader to previous surveys in thisarea [Wit98,BSS00,ZSJ∗ 05]. Nonetheless, where parallel or
distributed rendering methods do directly relate to our courseof discussion we have added them to our exposition.
We focus on volume rendering of regular grids and mostly
review methods for scalar data and a single time step. However, the principles of the discussed scalable methods are
general enough that they also apply to multi-variate, multimodal, or time series data. For a more in-depth discussionof the visualization and visual analysis of multi-faceted scientific data we refer the reader to a recent comprehensive
survey [KH13]. Other related recent surveys can be foundon the topics of compression for GPU-based volume rendering [RGG∗ 14], and massive model visualization [KMS∗ 06].
1.2. Survey StructureThis survey gives an overview of the current state of theart in large-scale GPU volume visualization. Starting fromthe standard visualization pipeline in Section 2, we discuss
required modifications and extensions to this pipeline toachieve scalability with respect to data size and define someunderlying concepts of this report.
We continue by examining scalable data structures thatare the basis for rendering large data (Section 3). Next, we
discuss general scalability strategies and how they relate toand are used in volume visualization (Section 4). In particular, we focus on how, when, and where data is processed andrendered to achieve scalable performance, including ways toreduce the computational load.
Section 5 discusses recent advances in large-scale volume rendering in depth. We propose a new categorization ofGPU-based large-scale volume visualization techniques (Table 2) based on the notion of the active working set—the current subset of data that is minimally required to produce an
output image of the desired display resolution. Based on thiscategorization, we review traditional GPU volume rendering
techniques, their advantages and limitations (Section 5.2).
In Section 6 we present a unified categorization of address translation methods, putting popular address translation methods like tree traversal and virtual texturing into
submitted to COMPUTER GRAPHICS Forum (10/2014).

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Data

Processing
Data (Pre-)
Processing

Acquisition &Simulation

Visualization

Filtering

Mapping

Image

Renderingon-demand

Processing Strategies
Section 4

where
when

Working Set
Representation

Address
Translation

Working Set
Determination

Section 3 & 6.3

Section 6

Section 7

Ray-Guided
Rendering
Section 8

Scalable Volume RenderinghowData Representation

How

When

Where

Section 3

Section 4.1

Section 4.2

Section 4.3• bricking• multi-res. data structures• data layout, compression• object-space decomp.
• image-space decomp.
• single-/multi-pass• pre-processing.
• on-demand processing• streaming• in-core/out-of-core• in-situ• parallel/distributed

Scalable
Processing
Strategies

Figure 1: The visualization pipeline for large-scale visualization. Top: Data are generated on the left (either through acquisition/measurement or through computation/simulation) and then pass through a sequence of stages that culminate in the desired
output image. Bottom: Extensions to the visualization pipeline to achieve scalability: A ray-guided or visualization-driven approach can drive earlier pipeline stages so that only what is required by (visible in) the output image is actually loaded orcomputed. In a fully visualization-driven pipeline, this approach can be carried through from rendering (determining visibility)
on the right all the way back to data acquisition/simulation on the left (stippled line). The bottom row shows different scalable
processing strategies (related to where, when and how data is being processed and rendered), and where they are discussed inthis report.
the same reference framework (Section 6.1). Section 6.2
presents their individual advantages and shortcomings inmore detail. We also address how GPU memory managment
is handled by different address translation approaches (Section 6.3).
We discuss methods for determining the working set (i.e.,culling) in Section 7, moving from basic view frusum andglobal attribute-based culling to occlusion culling and finally
ray-guided culling.
In Section 8 we reflect on and emphasize the recent advances of ray-guided algorithms, and sum up the most important works in that area.
Finally, we review the major challenges and current limitations and give an outlook on future trends and open problems in large-scale GPU volume visualization (Section 9).
2. Fundamentals
Before delving into the depths of scalable GPU volume rendering algorithms and data structures we first give a conceptual overview of the visualization pipeline with focus onlarge-scale volume visualization and define and clarify somebasic concepts that are used throughout this survey.
submitted to COMPUTER GRAPHICS Forum (10/2014).

2.1. Large-Scale Visualization Pipeline
A common abstraction used by visualization frameworks isthe visualization pipeline [Mor13]. In essence, the visualization pipeline is a data flow network where nodes or modules are connected in a directed graph that depicts the data
flow throughout the system (see Figure 1). After data acquisition or generation through computation or simulation, thefirst stage usually consists of some kind of data processing,which can include many sub-tasks from data pre-processing(e.g., computing a multi-resolution representation) to filtering. The second half of the pipeline comprises the actual visualization, including visualization mapping and rendering.
For large-scale rendering, all the stages in this pipeline haveto be scalable (i.e., in our context: output-sensitive), or they
will become the bottleneck for the entire application.
Ray-guided or visualization-driven approaches can drive
earlier stages in the visualization pipeline so that only the required data (i.e., visible data in the output image) is loaded,
processed and rendered (stippled line in Figure 1). Actual
scalability in volume rendering also depends on how dynamically and accurately the working set is determined, how
volumes are represented, and how ray traversal and addresstranslation is performed.

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The bottom part of Figure 1 shows the main processing
strategies employed by state-of-the-art visualization-driven
pipelines to achieve this scalability and lists where they are
discussed in this report. Efficient data structures such as
multi-resolution and compact data representations are necessary as well as deciding on how, when and where data isprocessed and rendered.

2.2. Basic Concepts
Large-scale visualization. In the context of this survey,
large-scale visualization deals with volume data that do notcompletely fit into memory. In our case, the most important
memory type is GPU on-board memory, but scalability mustbe achieved throughout the entire memory hierarchy. Most
importantly, large-scale volume data cannot be handled directly by volume visualization techniques that assume thatthe entire volume is resident in memory in one piece.
Bethel et al. [BCH12] (Chapter 2) define large data basedon three criteria: They are too big to be processed: (1) intheir entirety, (2) all at one time, and (3) exceed the available memory. Scalable visualization methods and architectures tackle either one or a combination of these criteria.
Scalability. In contrast to parallel/distributed visualization,
where a major focus is on strong vs. weak scaling [CPA∗ 10],
we define scalability in terms of output-sensitivity [SO92].
Our focus are algorithms, approaches, and architectures thatscale to large data by making the computation and visualization effort proportional to both the visible data on screen
and the actual screen resolution. If the required size of theworking set of data is independent of the original data size,
we say that an approach is scalable in this sense.
Scalability issues. Based on the notion of large data, themain scalability issues for volume rendering deal with questions on how to represent data, and how, when and where
the data is processed and rendered. This includes strategies
to split up the work and/or data to make it more tractable,
and to reduce the amount of work and/or data that has to behandled. The bottom of Figure 1 lists these main processing
strategies to achieve scalability and mentions where they aredescribed in our report.
Acceleration techniques vs. data size. A common source ofconfusion when discussing techniques for scalable volume
rendering is the real goal of a specific optimization technique. While many of the techniques discussed in this survey were originally proposed as performance optimizations,they can also be adapted to handle large data sizes. A wellknown example of this are octrees. While octrees are often
used in geometry rendering to speed up view frustum culling(via hierarchical/recursive culling), an important goal of using octrees in volume rendering is to enable adaptive levelof detail [WWH∗ 00] (thereby limiting the amount of datathat has to be handled), in addition to enabling empty spaceskipping. This “dual” purpose of many scalable data structures and algorithms is an important issue to keep in mind.
Output-sensitive algorithms. The original focus of outputsensitive algorithms [SO92] was making their running timedependent on the size of the output instead of the size ofthe input. While this scalability in terms of running time isof course also important in our context, for the work that
we discuss here, it is even more important to consider the
dependence on “output data size” vs. input data size, using
the concept of the working set as described above.
Ray-guided and visualization-driven architectures. Inline with the concepts outlined above, scalable volume
rendering architectures rely heavily on data management(e.g., processing, streaming, caching) and how to reduce theamount of data that has to be processed, rather than just rendering. While ray-casting intrinsically could be called “rayguided”, this by itself is not very meaningful. The difference
to standard ray-casting first arises from how and which data
are streamed into GPU memory, i.e., ray-guided streamingof volume data [CNLE09]. Again considering the working
set, a ray-guided approach determines the working set ofvolume bricks via ray-casting. That is, the working set comprises the bricks that are intersected during ray traversal. It is
common to determine the desired level of detail, i.e., the locally required volume resolution, during ray-casting as well.
In this way, data streaming is guided by the actual visibility of data in the output image. This is in contrast to
the approximate/conservative visibility obtained by all common occlusion culling approaches. As described in the introduction, visualization-driven architectures generalize theseconcepts further to ultimately drive the entire visualization
pipeline by actual on-screen visibility [HBJP12,BHAA∗ 13].
3. Data Representation and Storage
Efficient data representation is a key requirement for scalable volume rendering. Scalable data structures should becompact in memory (and disk storage), while still being efficient to use and modify. Figure 2 shows common related
data structures, and Table 1 lists their scalability aspects.
Additional GPU representations and management of thesedata structures, as they are used for rendering, is discussed
in Section 6.3.
3.1. Bricking
Bricking is an object space decomposition method that subdivides the volume into smaller, box-shaped sub-volumes,
the so-called bricks. All bricks usually have the same size invoxels (e.g., 323 or 2563 voxels per brick). Volumes that are
not a multiple of the basic brick size are padded accordingly.
Bricking facilitates out-of-core approaches because individual bricks can be loaded and rendered as required, without
ever having to load/stream the volume in its entirety.
submitted to COMPUTER GRAPHICS Forum (10/2014).

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Data Structure
mipmapsoctrees / kd-trees
uniform grids (bricking)hierarchical grids (bricking)Accelerationno [except level of detail]hierarchical traversal/culling(linear) culling of bricks(hierarchical) culling of bricks

Out-of-Coreclipmaps [TMJ98]
working set (subtree)
working set (bricks from grid)working set (bricks from hierarchy)

Multi-Resolution
yes
yes
no
yes

Table 1: Scalable data structures for volume visualization. Data structures have a huge impact in how acceleration during
ray-casting (skipping, culling), out-of-core processing/rendering, and multi-resolution rendering (i.e., adaptive level of detail)is supported.

(a)

(c)

(d)

resolution

resolution(b)

Figure 2: Common spatial data structures in volume rendering. (a) Original volume; (b) bricked volume; (c) octreewith fixed subdivision (only four of eight pointers to children
shown); (d) multi-resolution bricking with arbitrary downsampling ratios, which is beneficial for anisotropic volumes.
Bricked data often require special handling of brickboundaries. Operations where neighboring voxels are required (e.g., GPU texture filtering, gradient computation)
usually return incorrect results at brick boundaries, because
the neighboring voxels are not readily available. A typicalexample of this is tri-linear interpolation of samples during
ray-casting. Although boundary voxels can be fetched individually from the neighboring bricks [Lju06a], this is in
general very costly. More commonly, so-called ghost voxels [ILC10] are employed, which are duplicated voxels at
the brick boundaries that enable straightforward, fully correct hardware texture filtering. The use of ghost voxels is thestandard approach in most bricked ray-casters [BHWB07,
FK10]. Ghost voxels are generally stored together with eachbrick on disk—which increases disk storage—, but they canalso be computed on-the-fly in a streaming fashion [ILC10].

overhead grows for smaller bricks, and the total number ofbricks increases as well. The latter makes multi-pass rendering approaches that render bricks individually infeasible.
For these reasons, traditional multi-pass out-of-core volume renderers typically use relatively large bricks (e.g., 1283
or 2563 voxels) to reduce the number of required rendering passes. In contrast, modern single-pass ray-casters use
smaller bricks (e.g., 323 voxels), or a hybrid approach where
small bricks are used for rendering, and larger bricks areused for storage on disk [HBJP12, FSK13]. For 2D dataacquisition modalities such as microscopy, hybrid 2D/3D
tiling/bricking strategies have also been employed successfully. One example is the on-demand computation of 3D
bricks from pre-computed 2D mipmap tiles of electron microscopy images during visualization [HBJP12,BHAA∗ 13].
3.2. Multi-Resolution Hierarchies
One of the main benefits of multi-resolution hierarchies for
rendering large data is that they allow sampling the data froma resolution level that is adapted to the current screen resolution or desired level of detail, instead of only from theoriginal (full) resolution. This can significantly reduce theamount of data that needs to be accessed, and is also important for avoiding aliasing artifacts due to undersampling.

The recent OpenGL extension for virtual texturing(GL_ARB_sparse_texture) includes hardware support
for texture filtering across brick boundaries, and thus alleviates the need for ghost voxels to some extent.

Trees (octrees, kd-trees). Octrees [WWH∗ 00, Kno06] andkd-trees [FCS∗ 10] are very common 3D multi-resolution
data structures for direct volume rendering. They allow
efficient traversal and directly support hierarchical empty
space skipping. Traditional tree-based volume renderers employ a multi-pass rendering approach where one brick (onetree node) is rendered per rendering pass. Despite the hierarchical nature of these data structures, many early approaches assume that the entire volume fits into memory [LHJ99, WWH∗ 00, BNS01]. Modern GPU approaches
support traversing octrees directly on the GPU [GMG08,
CNLE09,CN09,RTW13], which is usually accomplished via
standard traversal algorithms adapted from the ray-tracing
literature [AW87, FS05, HSHH07, PGS∗ 07, HL09].

Choosing the optimal brick size depends on several criteria and has been studied in the literature [HBJP12, FSK13].
Small bricks support fine-grained culling, which facilitatessmaller working sets because less unnecessary data needs
to be fetched. On the other hand, however, the ghost voxel

In recent years, sparse voxel octrees (SVOs) have gained alot of attention in the graphics and gaming industry [LK10a,
LK10b]. Several methods for rendering large and complex
voxelized 3D models use SVO data structures for efficient
rendering [GM05, R0̈9, HN12, Mus13].

submitted to COMPUTER GRAPHICS Forum (10/2014).

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Mipmaps are a standard multi-resolution pyramid representation that is very common in texture mapping [Wil83].
Mipmaps are supported by virtually all GPU texture units.
Clipmaps [TMJ98] are virtualized mipmaps of arbitrary size.
They assume a moving window (like in terrain rendering)that looks at a small sub-rectangle of the data and use atoroidal updating scheme for texels in the current view. However, for general volume rendering this inherent inflexibilityof clipmaps would be very restrictive.
Hierarchical grids (mipmaps) with bricking. Anothertype of multi-resolution pyramids are hierarchical grids (ormipmaps) where each resolution level of the data is brickedindividually. These grids have become a powerful alternative to octrees in recent ray-guided volume visualization approaches [HBJP12, FSK13]. The basic approach canbe viewed as bricking each level of a mipmap individually. Moreover, more flexible systems do not use hardware mipmaps and therefore even allow varying downsampling ratios between resolution levels [HBJP12]—e.g.,
for anisotropic data—which is not possible with standard
mipmaps.
Since there is no tree structure in such a grid type, no tree
traversal is necessary during rendering. Rather, the entire
grid hierarchy can be viewed as a huge virtual address space(a virtual texture), where any voxel corresponding to data of
any resolution can be accessed directly via address translation from virtual to physical addresses [vW09, BHL∗ 11,
OVS12, HBJP12]. On GPUs, this address translation can be
performed via GPU textures that act as “page tables,” which
can be further extended to a multi-level page table hierarchy
for extremely large data [HBJP12] (see Section 6).
As in all approaches that employ some form of bricking,interpolation between bricks has to be handled carefully. Especially the transitions between different resolution levels
can introduce visual artifacts, and several methods have been
introduced that deal with correct interpolation between different resolution levels [Lju06a, Lju06b, BHMF08].
Wavelet representations. The earliest works using wavelet
transforms for volume rendering were Muraki [Mur93] andWestermann [Wes94]. Subsequent methods such as Guthe etal. [GGSe∗ 02, GS04] compute a hierarchical wavelet representation in a pre-process, and decompress the bricks thatare required for rendering on-the-fly.
Other representations. Younesy et al. [YMC06] have proposed improving the visual quality of multi-resolution volume rendering by approximating the voxel data distribution by its mean and variance at each level of detail.
The recently introduced sparse pdf volumes [SKMH14]
and sparse pdf maps [HSB∗ 12], respectively, represent thedata distributions more accurately. For sparse pdf volumes,
this allows for consistent multi-resolution volume rendering [SKMH14], i.e., the consistent application of transfer
functions independent of the actual resolution level used.
For sparse pdf maps (images), this allows for the accurate,anti-aliased evaluation of non-linear image operators on gigapixel images [HSB∗ 12]. The corresponding data structures are very similar to standard mipmaps (2D or 3D) interms of storage and access [SKMH14, HSB∗ 12].
3.3. Data Layout and Compression
Data layout. To efficiently access data on disk, data layout and access are often optimized. In general, reading
small bits of data at randomly scattered positions is a lot
more inefficient than reading larger chunks in a continuous layout. Therefore, locality-preserving data access patterns such as space filling curves, e.g., Morton (z-) order [Mor66] are often used in time-critical visualization
frameworks [SSJ∗ 11]. A nice feature of the Morton/z-order
curve is that by adjusting the sampling stride along the curve,
samples can be restricted to certain resolution levels. Pascucci and Frank [PF02] describe a system for progressive
data access that streams in missing data points for higher
resolutions. With the most recent solid state drives (SSDs),
however, trade-offs might be different in practice [FSK13].
Data compression. Another major related field is data compression, for disk storage as well as for the later stages ofthe visualization pipeline. We refer to the recent comprehensive survey by Rodriguez et al. [RGG∗ 14] for an indepth discussion of the literature on volume compression
and compression-domain volume rendering.
4. Scalable Processing Strategies
This section introduces the main considerations and techniques for designing scalable volume visualization architectures in general terms. We will focus on the questions ofwhen (Section 4.2) and where (Section 4.3) data processing
and rendering takes place. But first, we introduce different
strategies of how data handling and processing can be madescalable by different decomposition strategies (Section 4.1).
Reducing the amount of data that has to be processedor rendered is a major strategy for dealing with large data.
Techniques for data reduction cover a broad scope, ranging
from multi-resolution data representations and sub-sampling
to more advanced filtering and abstraction techniques. A distinction has to be made between data reduction for storage(e.g., compression) that tries to reduce disk or in-memory
size, and data reduction for rendering. The latter encompasses visualization-driven and display-aware rendering approaches as well as more general methods such as ondemand processing and query-based visualization.
In real-world applications, these strategies for handlingand rendering large data often have to be combined toachieve interactive performance and high-quality images.
Furthermore, for future ultra-scale visualization and exascale computing [ALN∗ 08, SBH∗ 08, MWY∗ 09, AAM∗ 11,
Mor12] it is essential that each step of the visualization
pipeline is fully scalable.
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4.1. Decompositions Strategies
A crucial technique for handling large data is to partition or
decompose data into smaller parts (e.g., sub-volumes). Thisis essentially a divide and conquer strategy, i.e., breaking
down the problem into several sub-problems until they become easier to solve. Partitioning the data and/or work can
alleviate memory constraints, complexity, and allow parallelization of the computational task. In the context of visualization, this includes ideas like domain decomposition (i.e.,
object-space and image-space decompositions), but also entails single-pass vs. multi-pass rendering approaches.
Object-space (data domain) decomposition. This type ofdecomposition is usually done by using bricking with orwithout a multi-resolution representation, as described in
Sections 3.1 and 3.2, respectively. Object-space decompositions are view-independent and facilitate scalability withrespect to data size by storing and handling data subsets separately.
Image-space (image domain) decomposition. Image domain subdivision splits the output image plane (the viewport) and renders the resulting image tiles independently.
A basic example of this approach is ray-casting (which isan image-order approach), where conceptually each pixel
is processed independently. In practice, several rays (e.g., arectangular image tile) are processed together in some sense.
For example, rendering each image tile in a single rendering pass, or assigning each tile to a different rendering node.
Another example is rendering on a large display wall, where
each individual screen is assigned to a different rendering
node.
Single-pass vs. multi-pass rendering. In single-pass approaches the volume is traversed in front-to-back order in asingle rendering pass as compared to multi-pass approaches
that require multiple rendering passes. The first GPU volume rendering approaches [CN93,CCF94,WE98,RSEB∗ 00,
HBH03], including the first octree-based renderers [LHJ99,
WWH∗ 00, GGSe∗ 02, GS04, HFK05], were all based on
multi-pass rendering. With the introduction of dynamic
branching and looping on GPUs, single-pass approaches
have been introduced to volume ray-casting [HSSB05,
SSKE05].
Multi-pass approaches offer a higher flexibility, however,they also have a significant management overhead comparedto single-pass rendering (i.e., context switching, final compositing) and usually result in lower performance. Furthermore, optimization techniques like early ray termination are
not trivial in multi-pass rendering and create an additional
overhead. Therefore, most state-of-the art ray-guided volume renderers use single-pass rendering [CNLE09, Eng11,
HBJP12]. A limitation of single-pass approaches, however,is the requirement for the entire working set to fit into thecache. One way to circumvent this requirement is to use
single-pass rendering as long as the working sets fits into
submitted to COMPUTER GRAPHICS Forum (10/2014).

7

the cache, and to switch to multi-pass rendering when theworking set gets too large [Eng11, FSK13].
4.2. Time-Based and Scheduling Strategies
A careful selection of the point in time when data is being
processed or rendered can have a tremendous influence of
the amount of data that needs to be handled. In this section
we focus on time-based processing strategies such as preprocessing, on-demand processing and streaming.
Pre-Processing. Running computationally expensive ortime-consuming computations as a pre-process to computeacceleration metadata or pre-cache data can often dramatically reduce the computation costs during rendering. Typicalexamples include pre-computing a multi-resolution hierarchy of the data that is used to reduce the amount of dataneeded for rendering. On the other hand, processing datainteractively during rendering can reduce the required disk
space [BCH12] (Chapter 9), and enables on-demand processing, which in turn can reduce the amount of data thatneeds processing.
On-Demand Processing. On-demand strategies determine
at run time which parts of the data need to be processed,
thereby eliminating pre-processing times and limiting theamount of data that needs to be handled. For example, rayguided and visualization-driven volume rendering systems
only request volume bricks to be loaded that are necessary
for rendering the current view [CNLE09, HBJP12, FSK13].
Data that is not visible is never rendered, processed, or evenloaded from disk.
Other examples for on-the-fly processing for volume visualization target interactive filtering and segmentation. For
example, Jeong et al. [JBH∗ 09] have presented a system
where they perform on-the-fly noise removal and edge enhancement during volume rendering only for the currently
visible part of the volume. Additionally, they perform aninteractive active-ribbon segmentation on a dynamicallyselected subset of the data. More recently, Solteszova et
al. [SBVB14] have presented a visibility-driven method foron-the-fly filtering (i.e., noise removal and feature detection)of 4D ultrasound data.
Query-driven Visualization. These approaches can be considered a special kind of on-demand processing, where selection is used as the main means to reduce the amountof data that needs to be processed [BCH12] (Chapter 7).
Prominent techniques are dynamic queries [AWS92], highdimensional brushing and linking [MW95], and interactive
visual queries [DKR97]. Shneiderman [Shn94] gives an introduction to dynamic queries for visual analysis and information seeking.
The DEX framework [SSWB05] focuses on query-driven
scientific visualization of large data sets using bitmap indexing to quickly query data. Recently, approaches for query-8

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

based volume visualization have been introduced in the context of neuroscience [BvG∗ 09, BAaK∗ 13], with the goal toanalyze the connectivity between individual neurons in electron microscopy volumes. The ConnectomeExplorer framework [BAaK∗ 13] implements visual queries on top of alarge-scale, visualization-driven system.
Streaming. In streaming approaches, data are processedas they become available (i.e., are streamed in). Streaming techniques are closely related to on-demand processing.
However, where the latter usually consists of a pull model(i.e., data is requested by a process), streaming can be a pullor a push model (i.e., new data is pushed to the next processing step).
Streaming also circumvents the need for the entire data
set to be available before the visualization starts and allows rendering of incomplete data [SCC∗ 02]. Hadwiger
et al. [HBJP12] have described a system for streaming
extreme-scale electron microscopy data for interactive visualization. This system has later been extended to include onthe-fly registration and multi-volume visualization of segmented data [BHAA∗ 13]. Further streaming-based visualization frameworks include the dataflow visualization system presented by Vo et al. [VOS∗ 10], which is built on top
of VTK and implements a push and pull model.

4.3. Location-Based Strategies
In this section, we focus on processing strategies that are categorized by where computations are performed in the visualization pipeline, or more precisely, where the data is locatedand stored when it is being processed. These techniques generally lower the in-core memory requirements by using outof-core, in-situ or distributed processing strategies.
Out-Of-Core Techniques. Unless when dealing with datathat is small enough to fit into memory (“in core”) in its
entirety, one always has to partition the data and/or computation in a way that makes it possible to process subsets of the data independently. This enables out-of-core processing and can be applied at all stages of the visualization
pipeline [SCC∗ 02, KMS∗ 06]. Different levels of out-of-coreprocessing exist, depending on where the computation is
performed and where the data is residing (either on the GPU,
CPU, hard-disk, or network storage).
Out-of-core methods include algorithms that focus
on accessing [PF02] and prefetching [CKS03] data,
creating on-the-fly ghost data for bricked representations [ILC10], and methods for computing multi-resolution
hierarchies [HBJP12] or other processing tasks such as
segmentation [FK05], PDE solvers [SSJ∗ 11], image registration and alignment [JST∗ 10], or level set computation [LKHW04]. Silva et al. [SCC∗ 02] give a comprehensive overview of out-of-core methods for visualization andgraphics.

Figure 3: Parallel volume rendering using sort-first andsort-last strategies as proposed in [MCE∗ 94]. In sort-lastrendering (top) each process is responsible for rendering
one or several parts of the volume and requires a final compositing pass to generate the final image. Compositing is often the bottleneck in parallel volume rendering, even thoughparallel compositing algorithms exist [MPHK94, YWM08,
PGR∗ 09]. In sort-first approaches (bottom), each process is
responsible for rendering a part of the viewport. This does
not require compositing, but often exhibits load-balancing
issues when moving the viewport, as data needs to be moved
between different processes.

In-Situ Visualization. Traditionally, visualization is performed after all data have been generated—either by measurement or simulation—and have been written to disk. Insitu visualization, on the other hand, runs simultaneously tothe on-going simulation (e.g., on the same supercomputeror cluster: in situ—in place), with the aim of reducing theamount of data that needs to be transferred and stored on
disk [BCH12] (Chapter 9).
To avoid slowing down the primary simulation, in-transit
visualization accesses only “staging” nodes of a simulation
cluster. The goal of these nodes is to hide the latency of disk
storage from the main simulation by handling data buffering
and I/O [MOM∗ 11].
In-situ and in-transit visualization have been identified as being crucial for future extreme-scale computing [MWY∗ 09, AAM∗ 11, KAL∗ 11, Mor12]. Furthermore,when the visualization process is tightly coupled or integrated into the simulation, these approaches can be leveraged for computational steering, where simulation parameters are changed based on the visualization [PJ95,
TTRU∗ 06]. Yu et al. [YWG∗ 10] present a complete casestudy of in-situ visualization for a petascale combustion simulation. Tikhonova et al. [TYC∗ 11] take a different approachby generating a compact intermediate representation of large
submitted to COMPUTER GRAPHICS Forum (10/2014).

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

9

volume data that enables fast approximate rendering for preview and in-situ setups.
Parallel and Distributed Rendering. High-performance visualization often depends on distributed approaches that split
the rendering of a data set between several nodes of a cluster.
The difference can be defined such that parallel visualization
approaches run on a single large parallel platform, whereas
distributed approaches run on a heterogeneous network ofcomputers. Molnar et al. [MCE∗ 94] propose a classificationof parallel renderers into sort-first, sort-middle, and sortlast. In the context of large data volume rendering, sort-lastapproaches are very popular and refer to bricking the data
and making each node responsible for rendering one or several bricks before final image compositing. In contrast, sortfirst approaches subdivide the viewport and assign rendernodes to individual image tiles. Neumann [Neu94] examines the communication costs for different parallel volume
rendering algorithms.
Conceptually, all or any parts of the visualization pipeline
can be run as a distributed or parallel system. Recent developments in this field are promising trends towards exascale visualization. However, covering the plethora of distributed and parallel volume visualization approaches is outof scope of this survey. The interested reader is referred
to [Wit98, BSS00, ZSJ∗ 05] and [BCH12] (Chapter 3) for indepth surveys on this topic.
5. GPU-Based Volume Rendering
In this section we categorize and discuss the individual literature in GPU-based large-scale volume rendering. We startby introducing a categorization and then give an overview ofthe evolution of GPU-based volume rendering techniques.
5.1. Categorization
We categorize GPU-based volume rendering approacheswith respect to their scalability properties by using the central notion of the working set—the subset of volume bricks
that is required for rendering a given view. Using the conceptof this working set, our categorization distinguishes different
approaches according to:
1. How the working set is determined (“culling”).
2. How the working set is stored (represented) on the GPU.
3. How the working set is used (accessed) during rendering.
Earlier approaches for volume rendering large data havemainly focused on (2) and (3), but only recent developments
in ray-guided working set determination (1) achieve truly
scalable performance. We will discuss these respective issues bottom-up, and first describe how to access and traverse the working set during rendering (3) in Section 6.2,
and elaborate on working set storage and GPU memory management (2) in Section 6.3. Techniques for determining thecurrent working set via different methods for culling (1) aresubmitted to COMPUTER GRAPHICS Forum (10/2014).

Figure 4: Rendering a multi-gigabyte CT data set (as usedin [Eng11]) at different resolution levels using a ray-guided
rendering approach. Data courtesy of Siemens Healthcare,Components and Vacuum Technology, Imaging Solutions.
Data was reconstructed by the Siemens OEM reconstruction
API CERA TXR (Theoretically Exact Reconstruction).
discussed in Section 7. General volume data structures havealready been discussed in Section 3.
We also categorize the resulting scalability (low, medium,high), where only “high” scalability means full outputsensitivity and thus independence of the input volume size.
The properties of different volume rendering approaches—
and the resulting scalability—vary greatly between what werefer to as “traditional” approaches (corresponding to “low”
and “medium” scalability in Table 2), and “modern” rayguided approaches (corresponding to “high” scalability in
Table 2).
A key feature of modern ray-guided and visualizationdriven volume renderers is that they make full use of recent developments in GPU programmability. They usually
include a read-back mechanism to update the current working set, and traverse a multi-resolution hierarchy dynamically on the GPU. This flexibility was not possible on earlier
GPUs and is crucial for determining an accurate and tight(as small as possible) working set.
5.2. Evolution of GPU-Based Volume Rendering
GPUs have, over the last two decades, become very versatile and powerful parallel processors, succeeding the fixedfunction pipelines of earlier graphics accelerators. General purpose computing on GPUs (GPGPU)—now also
called GPU Compute—leverages GPUs for non-graphics related and compute-intensive computations [OLG∗ 07], suchas simulations or general linear algebra problems. Increased programmability has been made possible by APIs
like the OpenGL Shading Language (GLSL) [Ros06] andCUDA [NVI13].

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J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

working set
determination

volume data
representation(storage)rendering(ray traversal)

scalability

full volume
linear (non-bricked)
volume storage[CN93] [CCF94] [WE98][RSEB∗ 00] [HBH03][LMK03]† [RGW∗ 03][KW03] [SSKE05][BG05] [MHS08][KGB∗ 09]† [MRH10]

texture slicing[CN93] [CCF94] [WE98][RSEB∗ 00] [HBH03][LMK03]†

basic culling(global, view frustum)single-resolution gridoctree[HSSB05] [BHWB07][LHJ99] [WWH∗ 00][GGSe∗ 02] [GS04]
grid with octree[PHKH04] [HFK05]
per brickkd-tree[RV06][FK10]
multi-resolution grid
[Lju06a] [BHMF08][JBH∗ 09]
CPU octree traversal (multi-pass)[LHJ99] [WWH∗ 00] [GGSe∗ 02][GS04] [PHKH04] [HFK05] [RV06]
CPU kd-tree traversal (multi-pass)[FK10]

non-bricked ray-casting(multi-pass)[RGW∗ 03] [KW03](single-pass)[SSKE05] [BG05] [MHS08][KGB∗ 09]† [MRH10]
low

ray-guided /visualization-driven
octree[GMG08]‡[CNLE09] [Eng11][RTW13]
multi-resolution grid
[HBJP12] [BAaK∗ 13][FSK13]GPU octree traversal(single-pass)[GMG08]‡[CNLE09] [Eng11][RTW13]bricked/virtual texture
ray-casting (single-pass)[HSSB05] [Lju06a] [BHWB07][BHMF08] [JBH∗ 09]

multi-level virtual texture
ray-casting (single-pass)[HBJP12] [BAaK∗ 13][FSK13]

mediumhigh

Table 2: Categorization of GPU-based volume visualization techniques based on the type of working set determination mechanism and the resulting scalability in terms of data size, as well as according to the volume data representation employed,
and the actual rendering technique (type of ray traversal; except in the case of texture slicing). † [LMK03, KGB∗ 09] performculling for empty space skipping, but store the entire volume in linear (non-bricked) form. ‡ [GMG08] is not fully ray-guided,
but utilizes interleaved occlusion queries with similar goals (see the text).

However, GPU on-board memory sizes are much more
limited than those of CPUs. Therefore, large-scale volume
rendering on GPUs requires careful algorithm design, memory management, and the use of out-of-core approaches.
Before discussing current state-of-the-art ray-guided volume renderers, we review traditional GPU volume rendering approaches. We start with 2D and 3D texture slicing
methods, before continuing with GPU ray-casting. This will
give us the necessary context for categorizing and differentiating between the more traditional and the more modern
approaches. A detailed comparison of different GPU-based
volume rendering techniques is shown in Table 3.
Texture slicing. The earliest GPU volume rendering approaches were based on texture mapping [Hec86] using 2Dand 3D texture slicing [CN93, CCF94]. Westermann and
Ertl [WE98] extended this approach to support arbitrary clipping geometries and shaded iso-surface rendering. For correct tri-linear interpolation between slices, Rezk-Salama etal. [RSEB∗ 00] made use of multi-texturing. Hadwiger et
al. [HBH03] described how to efficiently render segmented
volumes on GPUs and how to perform two-level volume
rendering on GPUs, where each labeled object can be ren-dered with a different render mode and transfer function.
This approach was later extended to ray-casting of multiple segmented volumes [BHWB07]. Engel et al. [ESE00]
were among the first to investigate remote visualization using hardware-accelerated rendering.
Texture slicing today. In general, the advantage of texture
slicing-based volume renderers is that they have minimal
hardware requirements. 2D texture slicing, for example, canbe implemented in WebGL [CSK∗ 11] and runs efficiently on
mobile devices without 3D texture support. However, a disadvantage is that they often exhibit visual artifacts and lessflexibility when compared to ray-casting methods.
Ray-casting. Röttger et al. [RGW∗ 03] and Krüger andWestermann [KW03] were among the first to perform raycasting on GPUs, using a multi-pass approach. Ray-casting(also called ray-marching) is embarrassingly parallel andcan be implemented on the GPU in a fragment shaderor compute kernel, where each fragment or thread typically casts one ray through the volume. Ray-casting easily admits a wide variety of performance and quality enhancements such as empty space skipping and early raytermination. Hadwiger et al. [HSSB05] and Stegmaier etsubmitted to COMPUTER GRAPHICS Forum (10/2014).

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

al. [SSKE05] were among the first to perform GPU raycasting using a single-pass approach, taking advantage ofdynamic looping and branching in then-recent GPUs. Proxy
geometries for efficient empty space skipping can be basedon bricks [HSSB05, SHN∗ 06], spheres [LCD09], or occlusion frustums [MRH08].
With the introduction of CUDA as a higher-level GPU
programming language, CUDA-based ray-casters were introduced [MHS08, KGB∗ 09, MRH10]. They make use of
CUDA’s thread/block architecture, and possibly sharedmemory model.
Bricked ray-casting. To support volumes that are larger
than GPU memory, in principle each brick can be raycast in a separate rendering pass, but it is much more efficient to render the bricked volume via single-pass raycasting [HSSB05, BHWB07, JBH∗ 09]. Single-pass raycasters usually store the current working set of bricks in alarge brick cache (or brick pool) texture. This requires someform of address translation (Section 6) from “virtual” (volume) to “physical” (cache) texture coordinates on-the-fly
during ray-casting.
To some extent, this is similar to the early idea of adaptive
texture maps [KE02], but usually the brick cache is managed fully dynamically. In volume rendering, the two main
reasons why the brick cache has to be managed dynamicallyare to be able to accommodate changes to (a) the current
view, but also—very importantly—to (b) the current transfer
function.
Multi-resolution rendering. There are several motivations
for multi-resolution rendering. In addition to the obvious advantages of data reduction and rendering speed-ups, choosing a resolution that matches the current screen resolution
reduces aliasing artifacts due to undersampling [Wil83].
For rendering large data, several multi-resolution octree rendering methods have been proposed, most ofthem based on texture-slicing [LHJ99,WWH∗ 00,GGSe∗ 02,
GS04, PHKH04]. Hong et al. [HFK05] used a min-max octree structure for ray-casting the Visible Human CT data set.
A multi-resolution data structure requires level-of-detail(LOD) or scale selection [LB03] for rendering. Weiler et
al. [WWH∗ 00] us a focus point oracle based on the distance from the center of a brick to a user-defined focus
point to select a brick’s LOD. Other methods to selecta level of detail include estimating the screen-space error [GS04], using a combined factor of data homogeneity and importance [BNS01] or using the predicted visualsignificance of a brick [Lju06b]. A common method estimates the projected screen space size of the corresponding
voxel/brick [CNLE09]. Whereas LOD selection is often performed on a per-brick basis, Hadwiger et al. [HBJP12] selectthe LOD on a per-sample basis for finer LOD granularity(see Figure 9).
Ljung et al. [Lju06a] used a multi-resolution bricking
submitted to COMPUTER GRAPHICS Forum (10/2014).

11

structure and adaptive sampling in image- and object-space
to render large data. Beyer et al. [BHMF08] proposed a technique for correct interpolation between bricks of two different resolution levels.
The most common data refinement strategy (e.g., whenquickly zooming-in on the data) consists of a “greedy” approach that iteratively loads the next higher-resolution of thebrick until the desired resolution is reached [CNLE09]. A
different approach, where the highest resolution is loaded directly and intermediate resolutions are skipped was proposedin [HBJP12]. Most recently, Fogal et al. [FSK13] found thatthe “greedy” approach converges in the fewest number offrames in their ray-guided ray-caster.
Parallel and remote volume rendering methods. A lot of
research has focused on remote, parallel, or distributed visualization for rendering large data, which we cannot all cover
here.
Texture slicing has been used in many distributed andparallel volume rendering systems [MHE01, CMC∗ 06,
MWMS07, EPMS09, FCS∗ 10]. Magallon et al. [MHE01]
used sort-last rendering on a cluster, where each cluster node
renders one volume brick before doing parallel compositing for final image generation. For volume rendering on
small to medium GPU clusters, Fogal et al. [FCS∗ 10] introduced a load-balanced sort-last renderer integrated into
VisIt [CBB∗ 05], a parallel visualization and data analysis
framework for large data sets. Moloney et al. [MWMS07]
proposed a sort-first technique using eight GPUs, where therender costs per pixel are used for dynamic load balancing
between nodes. They later extended their method to support
early ray termination and volume shadowing [MAWM11].
Equalizer [EPMS09] is a GPU-friendly parallel rendering
framework that supports both sort-first and sort-last approaches.
Ray-casting for distributed and parallel volume rendering first focused on CPU methods before first GPU methods
were developed. Wang et al. [WGL∗ 05] use a parallel CPU
ray-caster on a PC cluster for rendering large time varying
volumes represented as a wavelet-based time space partitioning tree. Müller et al. [MSE06] used GPU ray-castingin a sort-last parallel rendering system.
A method for rendering large remote micro-CT scans using an octree was proposed by Prohaska et al. [PHKH04].
Other large data methods. A different approach to dealing
with large data was proposed by Turlington et al. [THM01],
who introduced sliding thin slab (STS) visualization to limit
the amount of data needed for any current view. Knoll etal. [KTW∗ 11] optimized CPU ray-casting, achieving interactive rates using a bounding volume hierarchy (BVH)
min/max acceleration structure and SIMD optimizations.

12

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

(b)

(c)

(d)

original volume bricked volume

resolution

multi-resolution
bricked volume

octreepage table level

resolution

address translation(a)multi-res

resolution

data representation

single-res

direct accesspage table look-up

tree traversal

single-level
multi-res page table

multi-level
multi-res page table

Figure 5: Address translation for different data representations. The red dot represents a given sample to be accessed.
The figure illustrates both single-resolution (left) and multi-resolution (right) data structures. (a) direct access in non-brickedvolume; (b) page table look-up for bricked volume; (c) octree traversal, proceeding from the root downward; (d) multi-resolution
page table look-up for multi-resolution bricked volume—page tables enable accessing volume bricks directly, without having totraverse different resolution levels. Multi-level page tables handle large volumes by also virtualizing the page tables (Fig. 7).

6. Address Translation
One of the crucial properties of rendering large bricked volumes in an out-of-core fashion is that not all bricks have tobe resident in on-board GPU memory for rendering. In order
to be able to do this, it must be possible to address the volume data stored in individual bricks as required by any given
sampling location, even if the brick storage order or format
are not directly related to the original (non-bricked) volume.
For ray-casting, it is beneficial to specify the sample locations in a space that covers the whole bounding box of thevolume and that is independent of individual bricks. For example, all sample locations can be specified via normalized
coordinates in a [0, 1]3 unit cube. In order to be able to sample the corresponding volume data independently of where
each brick’s data are actually located, the volume coordinates of a given sample need to be translated to actual texture coordinates, taking the “physical” storage of the brick’s
data into account. The latter most commonly are the texture
coordinates of a box-shaped sub-region in a specific texture
that stores the data of multiple different bricks, for example
all the bricks comprising the current working set.
Terminology. We refer to this process as address translation, and denote the volume coordinates used for specifying
sample locations along viewing rays as virtual volume coordinates or virtual addresses, and the corresponding coordinates in actual texture space as physical addresses. We often
use the term page table to refer to the concept as well as thedata structure that actually stores the current mapping fromvirtual addresses to physical addresses in some form, in analogy to standard CPU virtual memory terminology [HP11].

A page in this context most commonly refers to a volume
brick, i.e., a page table references physical brick storage.
6.1. Categorization
We categorize the different address translation methods according to the volume data representation that they workwith, and according to how their “page table” is organized
and accessed. Figure 5 illustrates the major categories, given
different volume data representations and the corresponding
ways of performing address translation in volume rendering.
Conceptual framework. For a presentation and categorization that is as uniform as possible over all important volume data representations and the corresponding rendering
techniques, we will use the unified conceptual frameworkof address translation for all representations and rendering
techniques that we discuss, even if they are not traditionally
thought of as employing address translation.
For example, octree volume rendering is commonly
thought of as a tree traversal process that renders volumedata stored in bricks attached to the individual octree nodes.
However, this process can also be thought of as a particularform of address translation, where the octree itself contains
the information that is required to translate virtual to physical addresses, and the address translation itself is built on
tree traversal. From this viewpoint, the octree itself represents the “page table,” just in a very particular, hierarchicalform, and the tree traversal traverses this page table.
We make a major distinction between single-resolution
vs. multi-resolution approaches, and between approaches
employing single-level vs. multi-level page tables.
submitted to COMPUTER GRAPHICS Forum (10/2014).

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

Single- vs. multi-resolution. A subtle but important distinction must be made between data of a single or multiple
resolutions, and page tables of a single or multiple resolutions, respectively. In our categorization (see Figure 5 andSection 6.2) of address translation methods, we categorize
according to the properties of the page table. An example forthis subtle difference that is described below would be adaptive texture maps [KE02]. They employ a single-resolution
page table, but can nevertheless reference data of different resolutions. We put such a technique into the singleresolution category (Section 6.2.2), although we also mention it again in the multi-resolution category to try to minimize confusion.
Single- vs. multi-level. Most published work that uses
page tables (or, equivalently, index textures) employs a single level of page table, i.e., each page table entry references
data. For very large volumes, however, it is beneficial to employ multiple levels of page tables, where page table entries
can again reference page tables, leading to a page table hierarchy [HBJP12]. The latter approaches we denote as using
multi-level page tables (Figures 5 and 7, and Section 6.2.5).
6.2. Address Translation Methods
This section categorizes and discusses the major methodsof address translation in volume rendering that have beenreported in the literature according to the discussion above.
6.2.1. Direct Access
For completeness, we start with the simplest form of “address translation,” which is essentially an identity operation.
As illustrated in Figure 5 (a), the volume is stored linearly
without bricking, e.g., in a single 3D texture for the entire
volume. Therefore, essentially no address translation is necessary, except mapping volume coordinates to actual texture
coordinates. If the volume is addressed in a [0, 1]3 unit cube,
and texture coordinates are also given in [0, 1]3 as they commonly are, this mapping is an identity operation. No pagetables or similar structures are needed, but out-of-core rendering is not possible and hence scalability is very limited.
6.2.2. Single-Level, Single-Resolution Page Tables
The first really interesting form of address translation is performed for bricked, but single-resolution, volumes. As illustrated in Figure 5 (b), the original volume is bricked forout-of-core rendering, but no multi-resolution hierarchy iscomputed. This allows for out-of-core volume rendering, but
only with volume bricks from the original resolution and nodown-sampling. Address translation can be performed witha single page table look-up per sample. The page table can be
stored in a linear texture, e.g., a single 3D texture, with one
entry (texel) per page (brick) that contains the informationnecessary for address translation. In this context, the pagetable texture is sometimes simply called an index texture.
submitted to COMPUTER GRAPHICS Forum (10/2014).

13

Adaptive texture maps. Kraus and Ertl [KE02] were thefirst to introduce adaptive texture maps for GPUs, where animage or volume can be stored in a bricked fashion withadaptive resolution and accessed via a look-up in a small
index texture. This index texture can be seen as a page table. However, in adaptive texture maps the index texture wasstatically computed and not updated according to parameterchanges such as a changing transfer function. Moreover, theindex texture itself only has a single resolution, although it
can refer to data of different resolutions. For this reason, andfor a better order of introducing the different approaches, wehave included adaptive texture maps in this section. However, we also mention this approach again below in Section 6.2.4.
Dynamically managed page tables. One of the earliest
uses of dynamically managed page table textures for rendering volume data was introduced in the context of isosurface ray-casting [HSSB05]. The working set of pages(bricks) potentially intersected by the iso-surface are dynamically paged (downloaded) into a cache texture, whichis the “physical” storage. The cache texture is referenced
by a page table texture that is likewise updated dynamically.
This approach can easily be adapted to direct volume rendering [SHN∗ 06,BHMF08]. The working set of active bricks isthen simply determined according to the transfer function
instead of the iso-value. However, in these early approaches,
this was done independently of view-dependent occlusions.
Virtual texturing. Recent hardware support for partially resident textures [BSH12] (e.g., the OpenGL
GL_ARB_sparse_texture extension) allow the GPU to
manage both the page table and the corresponding physicalstorage, as well as performing the actual address translationin the GPU hardware texture unit. For non-mipmapped textures, this essentially implements what we have describedabove. For mipmapped textures, it implements what we describe in Section 6.2.4. However, current hardware limitations still limit the size of partially resident textures to 16K
pixels/voxels and do not allow for automatic page fault handling. In the context of game engines, virtual texturing approaches have also been used successfully [vW09, OVS12].
6.2.3. Tree Traversal
Tree data structures and traversal algorithms (e.g., kdtrees, octrees) have been employed since the earliest multiresolution volume renderers. Although usually not described
as a form of address translation, in our conceptual framework we view tree traversal as a particular kind of addresstranslation, and the tree itself as a particular kind of pagetable, as illustrated in Figure 5 (c).
Trees as “page tables.” In order to highlight the differences and commonalities between tree traversal and otherapproaches, we can view the tree data structure as a kind of
hierarchical page table, where each tree node contains theinformation where the corresponding physical volume data

14

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization(partial) tree needs to be stored on the GPU. (The latter being
the equivalent of a page table that is only partially resident).
Gobbetti et al. [GMG08] use a spatial index structure to
store the current subtree with neighbor information based onrope trees [HBZ98]. Each octree node stores pointers to its
eight children and its six neighbors (the “ropes” [HBZ98,
PGS∗ 07]), and a pointer to the actual brick data. Traversal
leverages the rope pointers.

Figure 6: The Gigavoxels system [CN09] uses an N 3 treewith node and brick pools storing the set of active nodes andbricks, respectively. Usually, N = 2, resulting in an octree.

are located (the brick data attached to the node), and how todescend in the multi-resolution hierarchy represented by thetree, from coarser to finer resolutions (which can be viewed
as a particular kind of page table hierarchy).
Trees vs. multi-level, multi-resolution page tables. Using the viewpoint of a tree being a kind of hierarchical pagetable, in principle it could also be seen as a multi-level, multiresolution page table. However, we make the crucial distinction that in the case of a standard tree structure, the way
in which the resolution of the data is reduced, and the way
in which the resolution of the “page table” is reduced, arecompletely coupled. This means that their parameters cannot be chosen independently. In contrast, in our categorization we use the term multi-level, multi-resolution page table
for a structure where the hierarchy of the different data resolutions (the resolution hierarchy), and the hierarchy of thedifferent page table resolutions (the page table hierarchy)
are completely decoupled. This leads to a structure that cannot be described as a single tree, but that is comprised of two
orthogonal hierarchies [HBJP12], as shown in Figure 7.
This leads to crucial differences in terms of scalability,
which are discussed in more detail in Section 6.2.5.
Traversal algorithms for efficiently navigating andtraversing trees, such as kd-trees or octrees have been wellresearched in the ray-tracing community. Amanatides andWoo [AW87] were the first to introduce a fast regular gridtraversal algorithm. Recently, stackless traversal methods
such as kd-restart [FS05] have received a lot of attention
in practical implementations [PGS∗ 07], as they are wellsuited for GPU implementation. Other variants include the
kd-shortstack [HSHH07] and kd-jump [HL09] algorithms.
Octree-based systems. To traverse an octree directly on theGPU, not only the current working set of bricks, but also a

Crassin et al. [CN09, CNLE09] use an N 3 tree (although
they usually use N = 2, i.e., a regular octree), whose current
subtree is stored in a node pool and a brick pool, respectively.
Each node stored in the node pool contains one pointer to
its N 3 children, and one pointer to the corresponding brickdata in the brick pool (see Figure 6). Using a single childpointer is possible because all N 3 children are always stored
together in a single node pool entry. Tree traversal is basedon an adapted kd-restart algorithm [FS05]. Engel [Eng11]
and Reichl et al. [RTW13] use the same basic structure andtraversal method.
6.2.4. Single-Level, Multi-Resolution Page Tables
The simplest form of supporting multiple resolutions withpage tables is to employ a separate page table per resolution level, as illustrated in Figure 5 (d, left). Everythingelse is then very similar to the single-resolution case (Section 6.2.2). For each sample, first the desired resolution levelis determined (e.g., according to projected screen size of avoxel or brick), which then allows accessing the corresponding page table. Usually, all pages (bricks) have the same
physical resolution (in terms of voxels), independently ofwhich resolution level they correspond to. This is very beneficial in practice, because the working set can mix bricks
from different resolution levels easily, and all active bricks
can be stored in the same cache texture (because they are allof exactly the same size). Such approaches have been employed for relatively large data (e.g., [JBH∗ 10]), but better
scalability can be achieved by using multiple page table levels per resolution level instead of just a single one [HBJP12](Section 6.2.5).
Virtual texturing. As already described in Section 6.2.2,
virtual texturing approaches often also employ page tables.
If mipmapped textures are used, at least conceptually there
is a separate page table for each mipmap level, i.e., for eachresolution level. However, in this case all page tables together can also be stored as a single mipmapped page table.
6.2.5. Multi-Level, Multi-Resolution Page Tables
For very large volume data, the concept of single-level,
multi-resolution page tables described above starts to become impractical in terms of page table storage. However,it can be extended in a relatively simple, “recursive” fashionin order to achieve much higher scalability. The resulting
multi-level, multi-resolution structure is illustrated in Figure 5 (d, right), and in much more detail in Figure 7. (Notesubmitted to COMPUTER GRAPHICS Forum (10/2014).

15

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

VirtualPage Tables

l=0

resolution hierarchy
l=1
l=2

virtualized

323 pagetable entries
single pagetable entry
Virtual Voxel
Volumes
323 voxel
brick

Virtual Memory Architecture

page table hierarchy

Multi-Resolution
Page Directory l=0
single page
directory
entry

l=1

Multi-Resolution
l=2 Page Directory

Page Table
Cache

Brick Cache
Virtual Volume
Cache Miss
Hash Table

Ray-Casting

Figure 7: Multi-level, multi-resolution page tables [HBJP12]. Left: The virtual memory architecture comprises two orthogonalhierarchies: the resolution hierarchy, and the page table hierarchy. Right: Address translation during ray-casting starts in themulti-resolution page directory (one directory per resolution level), and then references cache textures shared for all resolutions.

that the resolution and page table hierarchies, respectively,
are mapped differently to the horizontal/vertical axes in Figures 5 and 7, respectively.)
The basic problem of using single-level page tables is thatfor large volumes and small brick sizes (such as 323 voxel
bricks, for good culling granularity), the page tables themselves become very large. That is, the page tables virtualize
the volume data, but the page tables themselves are not virtualized. For example, the page table corresponding to thefinest resolution level of a 128K3 volume would itself be a
4K3 texture. However the basic idea of using page tables
to virtualize large volumes can easily be extended “recursively” to the page tables themselves, leading to the pagetable hierarchy illustrated in Figure 7. Such multi-level pagetable architectures have been shown to scale well to volumedata of extreme scale [BHL∗ 11, HBJP12], exhibiting better
scalability than tree hierarchies such as standard octrees.
In a multi-level page table architecture, one or more additional levels of page tables are added to each resolution
level [BHL∗ 11, HBJP12]. Each top level page table is calleda page directory, in analogy to CPU virtual memory terminology [HP11]. That is, in order to be able to address multiple resolution levels, each resolution level has its own corresponding page directory. Collectively, the page directoriesof all resolution levels are refered to as the multi-resolution
page directory. Only this multi-resolution directory structure
must be resident in GPU memory, i.e., is not virtualized. Allother page table hierarchy levels are completely virtualized.
Now, in addition to the cache texture storing the current
working set of bricks, an additional page table cache texture
is required for storing the current working set of page tablebricks. The brick sizes for voxel (data) bricks and for pagetable bricks can be chosen completely independently.
submitted to COMPUTER GRAPHICS Forum (10/2014).

Hadwiger et al. [HBJP12] describe multi-level, multiresolution page tables as a (conceptually) orthogonal 2D
structure (see Figure 7, left). One dimension corresponds
to the page table hierarchy, consisting of the page directories (the top-level page tables) and the page tables “below.”
The second dimension corresponds to the different resolution levels of the data. Each resolution level conceptually
has its own independent page table hierarchy. However, theactual cache textures can again be shared between all resolution levels. Multi-level page tables scale very well tolarge data. For example, just two page table levels have beenshown to already support volumes of up to several hundred
terabytes, and three levels would in principle be sufficient foreven exa-scale data in terms of “addressability” [HBJP12].
Address translation. The right part of Figure 7 depicts
address translation during ray-casting in a multi-level, multiresolution page table architecture. Hadwiger et al. [HBJP12]
use this approach for rendering extreme-scale electron microscopy data. Their approach starts with computing a LOD
for the current sample, which is then used to look up the
page directory corresponding to that resolution. Next, address translation traverses the page table hierarchy from thepage directory through the page table levels below.
Look-up overhead. A property of this approach for address translation that is crucial in order to obtain high volume
rendering performance is that when marching from sample
to sample along a given ray, the page directory and pagetable look-ups from the previous sample can trivially becached (remembered) in order to exploit spatial coherence.
Because the result of these look-ups changes less and lessfrequently the higher up in the hierarchy they come from,the number of actual texture look-ups that is required for address translation in practice is very low. Because of this fact,16

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

the performance difference between a single-level page tablearchitecture and a multi-level one is very low [HBJP12].
Multi-level, multi-resolution page tables vs. trees. Expanding on the previous discussion in Section 6.2.3, thecrucial property why an orthogonal multi-level, multiresolution page table hierarchy scales better to very large
volume data than tree structures such as octrees is that theformer completely decouple the hierarchy that is required
for scalable address translation (the page table hierarchy)
from the hierarchy that is required in order to be able toperform multi-resolution volume rendering (the resolution
hierarchy). While either of these hierarchies could also beviewed as a tree (without pointers), the orthogonal structure
depicted in Figure 7 cannot be described by a single tree.
When a hierarchical tree structure is built, a crucial decision is how many children each tree node should have. In
standard tree-based volume rendering approaches, this number determines the resolution reduction or down-sampling
ratio from resolution level to resolution level. In an octree(or an N 3 tree with N = 2), this down-sampling ratio isfixed to a factor of two per level in each dimension. This is
the most common down-sampling ratio that yields relatively
good results. However, for volume rendering it corresponds
to an already relatively strong jump in data quality from level
to level, because the total number of samples is reduced injumps by a factor of eight. This fact has been observed andtackled by very few researchers [EMBM06]. This makes theuse of an N 3 tree with N > 2 impractical for high-quality
volume rendering.
On the other hand, the number of look-ups that are required for address translation is in principle a completely
separate consideration from the desired down-sampling ratio of the volume data. Traversal performance for large datacrucially depends on the number of hierarchy (tree) levels
that must be traversed. For this reason, an octree (N = 2)
leads to a relatively large number of traversal steps for large
data [HBJP12]. In order to obtain traversal performance with
better scalability characteristics, a tree with more children
per node than 23 could be used, i.e., an N 3 tree with N > 2.
The results reported in [HBJP12] can to some extent be compared on a conceptual level to using an N 3 tree with N = 32
for the page table structure (not for data down-sampling).
This discussion shows that two opposing considerations
must be balanced for high-quality rendering and good scalability to large data—choosing N as small as possible forhigh-quality rendering, and choosing a rather large N for better scalability to large data. Because a single tree structure
cannot choose these two factors independently, they are inherently coupled. In contrast, a multi-level, multi-resolution
page table hierarchy allows completely decoupling these two
considerations. This allows choosing a down-sampling ratio
according to quality requirements, and a ratio for page table“size reduction” according to scalability requirements.

6.2.6. Hash Tables
An alternative data structure that can be employed for address translation on GPUs are hash tables. However, theiruse for this purpose has not yet received a lot of attention in
the context of large-scale volume rendering.
Hastings et al. [HMG05] used spatial hashing to optimize collision detection in real-time simulations. Nießner et
al. [NZIS13] use voxel hashing for real-time 3D reconstruction of iso-surfaces in dynamically constructed distance field
volumes.
The crucial property of hashing approaches are how theyhandle collisions, i.e., how often collisions can occur in practice and how complicated and costly they are to resolve.
Without the need for resolving collisions, hash tables would
be rather straightforward to implement. However, proper
collision handling makes their implementation much more
involved, e.g., [NZIS13].
6.3. GPU Memory Management
Efficient GPU data structures for storing the working set
should be fast to access during ray traversal (i.e., addresstranslation), and should also support efficient dynamic updates of the working set. Recent approaches usually store
volume bricks (actual voxel data) in a singe large 3D cachetexture (or brick pool).
In addition to that, a address translation helper structures(e.g., page table, octree pointers) are required for translating from virtual to texture space. If ray traversal needs tofollow tree nodes (as in octree-based renderers), the working set of tree nodes must be stored, e.g., in a node pool(e.g., [CNLE09, Eng11]). If ray traversal is built on virtualto physical address translation (as in page table-based renderers), the working set of page table entries must be stored,e.g., in a page table cache (e.g., [BHL∗ 11, HBJP12]).
Finally, additional meta-data that is required by the renderer has to be stored on the GPU as well.
6.3.1. Texture Cache Management
Texture allocation. Early tree-based volume renderers often
employed one texture per brick, rendering one after the otherin visibility order using one rendering pass per brick/tree
node [LHJ99, WWH∗ 00, GGSe∗ 02, GS04]. However, multipass approaches are usually less performant than single-pass
approaches and are also limited in the number of passes theycan efficiently perform. To circumvent rendering bottlenecks
due to many rendering passes, Hong et al. [HFK05] cluster
bricks in layers (based on the manhattan distance) and render
all bricks of the same layer at the same time.
To support single-pass rendering, bricking approaches
and modern ray-guided renderers usually use a single large
3D cache texture (or brick pool) to store the working
submitted to COMPUTER GRAPHICS Forum (10/2014).

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

set [BHWB07, CN09, HBJP12], and often assume that theworking set will fit into GPU memory.
When the working set does not fit into GPU memory, either the level of detail and thus the number of bricks in theworking set can be reduced [HBJP12], or the renderer canswitch to a multi-pass fall-back [Eng11, FSK13].
Texture updates. Whenever the working set changes, thecache textures have to be updated accordingly. Hadwiger et
al. [HBJP12] compare texture update complexity between
octree-based and multi-level page table approaches. Octreebased approaches usually have to do a large number of updates of small texture elements, whereas hierarchical pagetables tend to perform fewer but larger updates.
To avoid cache thrashing [HP11], different brick replacement strategies have been introduced. Most common is the
LRU scheme which replaces the brick in the cache that was
least recently used [GMG08, CN09, FSK13]. It is also common to use a hybrid LRU/MRU scheme, where the LRU
scheme is used unless the cache is too small for the current working set. In the latter case, the scheme is switched to
MRU (most recently used) to reduce cache thrashing.
7. Working Set Determination (Culling)
Originally, the concept of culling was introduced in computer graphics for geometry rendering, where typically view
frustum culling and occlusion culling† are used to limit the
number of primitives (e.g., triangles) that have to be rendered. That is, in the former case culling everything that is
outside the view frustum, and in the latter case culling everything that is occluded within the view frustum, given thecurrent view direction and other relevant parameters. Ideally,
all occluded geometry should be culled before rendering. In
practice, for performance reasons a fast conservative estimate is computed, i.e., erring on the safe side by—to someextent—over-estimating the visible (non-occluded) geometry.
Performing culling to determine the current working setof bricks in volume rendering is crucial for being able tohandle large data at interactive frame rates and with a limited amount of memory (e.g., on-board GPU memory). Thatis, we want to cull everything that is not required to be in theworking set. All bricks that do not contribute to the output
image can be culled. Bricks that do not contribute are mainly
bricks that are either (a) outside the view frustum, (b) fullytransparent (i.e., have zero opacity), given the current transfer function, or (c) are occluded by bricks in front of them
(given the current view parameters as well as the transfer
function).
Different culling techniques can exhibit huge differences† Note that occlusion culling is sometimes also referred to as visibility culling (in an inverted sense).
submitted to COMPUTER GRAPHICS Forum (10/2014).

17

in their effectiveness, computational complexity, and flexibility, depending on how “tightly” they estimate the working
set (accurately vs. conservatively), how fast or complicated it
is to perform the actual culling, and which parameters (view,transfer function, etc.) can be changed interactively whilestill being able to update the culling.
7.1. View Frustum Culling
Removing primitives or volume bricks outside the current
view frustum is the most basic form of culling. The first stepof GPU ray-casting consists of computing the ray start points
and end points (often via rasterization), which already prevents sampling the volume in areas that are outside the view
frustum. However, in order to prevent volume bricks outside the frustum from being downloaded to the GPU, theindividual bricks have to be culled against the view frustum.
Naturally, if a brick lies completely outside the current view
frustum, it is not needed in GPU memory. Culling a view
frustum against a bounding box, a bounding volume hierarchy, or a tree, can be done very efficiently and has beenstudied extensively in several contexts [AM00, AMHH08].
7.2. Global, Attribute-Based Culling
Another way to cull bricks in volume rendering is based on
global properties like the current transfer function, iso value,or enabled segmented objects. Culling against the transfer
function is usually done based on min/max computations
for each brick [PSL∗ 98, HSSB05, SHN∗ 06]. The brick’s
min/max values are compared against the transfer function todetermine if the brick is invisible (i.e., only contains values
that are completely transparent in the transfer function). Invisible bricks are then culled. The downside of this approachis that it needs to be updated whenever the transfer functionchanges and usually needs pre-computed min/max values foreach brick that have to be available at runtime for all bricks.
A similar approach can be used for culling bricks against an
iso-surface [PSL∗ 98, HSSB05], or against enabled/disabled
objects in segmented volume rendering [BHWB07].
7.3. Occlusion Culling
Occlusion culling tries to cull primitives (bricks) that are
inside the view frustum but are nevertheless occluded byother primitives (bricks/volume data). This process usually requires a multi-pass rendering approach. While occlusion culling is easier to perform for opaque geometry, forsemi-transparent geometry—or for (semi-transparent) volume rendering—this process is more involved.
Greene et al. [GKM93] introduced hierarchical z-buffer
visibility. They used two hierarchical data structures—anoctree in object space, and a z-pyramid in image space—to quickly reject invisible primitives in a hierarchical manner. Zhang et al. [ZMHH97] proposed hierarchical occlusion maps (HOMs), where first a set of occluders is rendered18

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

into a low-resolution occlusion map that is then hierarchically downsampled and subsequently used to test primitives
for occlusion before actually rendering them.
In volume visualization, Li et al. [LMK03] introduced
occlusion clipping for texture-based volume rendering to
avoid rendering occluded parts of the volume. Gao et
al. [GHSK03] proposed visibility culling in large-scale parallel volume rendering based on pre-computing a plenoptic opacity function per brick. Visibility culling based on
temporal occlusion coherence has also been used for timevarying volume rendering [GSHK04]. The concept of occlusion culling has also been used in a parallel setting forsort-last rendering [MM10], by computing and propagating
occlusion information across rendering nodes.
7.4. Ray-Guided Culling
Ray-guided culling approaches are different in the sense thatinstead of culling away bricks from the set of all bricks, they
start with an empty working set that is grown by only adding
bricks that are actually required (intersected) during rendering.
That is, the goal is that only the bricks that are actually
visited during ray traversal are added to the working set of
active bricks. This naturally also implies that subsequently
only these bricks will be downloaded to GPU memory. In
this way, ray-guided culling approaches cull all occluded
bricks as well as implicitly also cull all bricks outside theview frustum. For this reason, they usually do not require
separate steps for view frustum and occlusion culling, respectively, because no rays are generated outside the view
frustum.
Gobbetti et al. [GMG08] used a mixture of traditionalculling and ray-guided culling. They first perform cullingon the CPU (using the transfer-function, iso value, and view
frustum), but refined only those nodes of the octree that weremarked as visible in the previous rendering pass. To determine if a node is visible they used GPU occlusion queries tocheck the bounding box of a node against the depth of thelast visited sample that was written out during ray-casting.
Crassin et al. [CN09] originally used multiple rendertargets to report which bricks were visited by the raycaster over the course of several frames, exploiting spatial
and temporal coherence. In a later reported implementation [CNSE10], the same information was constructed in amore efficient way using CUDA.
Hadwiger et al. [HBJP12] divide the viewport into smaller
tiles and use a GPU hash table per image tile to report alimited number of cache misses per frame. Over the courseof several frames, this ensures that all missing bricks will bereported and added to the working set.
Fogal et al. [FSK13] use a similar approach but use lockfree hash tables.

Figure 8: Ray-guided volume rendering [FSK13] of theMandelbulb data set. Colors indicate the amount of empty
space skipping and sampling that needs to be performed(green: skipped empty brick, red: densely sampled brick,blue: densely sampled but quickly saturated). Image courtesy of Tom Fogal and Jens Krüger.
7.4.1. Handling Missing Bricks
In contrast to traditional ray-casting approaches, where theworking set is computed prior to rendering on the CPU, rayguided volume renderers only build up the current working
set during ray traversal. This implies that ray-guided volume
renderers have to be able to deal with missing bricks in GPU
memory, because bricks are only requested and downloaded
once they have been hit during ray-casting, but not before.
Whenever the ray-caster detects a missing brick(i.e., either a missing octree node or a page table entry that is flagged as unmapped), a request
for that missing brick is written out. Crassin etal. [CN09] use multiple render targets to report missing nodes and then stop ray traversal. More recent
approaches [CNSE10, HBJP12, FSK13] use OpenGL extensions such as GL_ARB_shader_image_load_store
or CUDA, and often GPU hash tables, to report cache
misses.
During rendering, missing bricks can be either skipped, or
potentially be substituted by a brick of lower resolution. After missing bricks are detected and reported, the CPU takes
care of loading the missing data, downloading it into GPU
memory, and updating the corresponding GPU data structures.
7.4.2. Empty Space Skipping
In addition to skipping missing bricks, a common optimization strategy in volume rendering that is also easily implemented in ray-guided volume rendering is empty space skipping (sometimes also called empty space leaping), i.e., skipsubmitted to COMPUTER GRAPHICS Forum (10/2014).

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualization

Figure 9: Per-sample LOD selection as in [HBJP12]. Left:electron microscopy volume (90 GB). Middle and right: theLOD used for each sample is color-coded. Middle: discrete
LOD for each sample (tri-linear interpolation). Right: fractional LOD for each sample, with interpolation between dataof neighboring LODs (“quad-linear” interpolation).
ping (or “leaping over”) “empty space,” which is usually
identified at the granularity of individual bricks.
This optimization relies on knowing (essentially a priori) which bricks are completely empty, i.e., contain novalid/useful data or are completely mapped to zero opacity
according to the current transfer function, and can therefore
be safely skipped during ray-casting. During ray-casting,empty space (bricks) can be identified via “missing” subtrees or marked tree nodes [GMG08, Eng11], or via pagetable flags [HBJP12]. Figure 8 shows a rendering with colorcoded empty space skipping information.
8. Ray-Guided Volume Rendering
In this section we show how the combination of the previously discussed techniques has led to the advent of rayguided volume renderers that achieve much better scalability
and performance than previous techniques. The main novelty of ray-guided (and visualization-driven) volume rendering approaches is that they incorporate a feedback loop between the ray-caster and the culling mechanism, where theray-caster itself writes out accurate information on missing
bricks and brick usage. Thus, this type of culling mechanism
determines an accurate working set directly on the GPU.

19

Crassin et al. [CN09] introduced the Gigavoxels system
for GPU-based octree volume rendering with ray-guided
streaming of volume data. Their system can also make useof an N 3 tree, as an alternative to an octree (which would
be an N 3 tree with N = 2). The tree is traversed at run timeusing the kd-restart algorithm [FS05] and active tree nodes
stored in a node pool. Actual voxel data are fetched frombricks stored in a brick pool. Each node stores a pointer to its
child nodes in the node pool, and a pointer to the associatedtexture brick in the brick pool (see Figure 6). The focus of
the Gigavoxels system is volume rendering for entertainment
applications and as such it does not support dynamic transfer
function changes [CNSE10]. The more recent CERA-TVR
system [Eng11] targets scientific visualization applications
and supports fully dynamic updates according to the transfer
function in real time. It also uses the kd-restart algorithm foroctree traversal. Reichl et al. [RTW13] also employ a similar
ray-guided approach, but target large smooth particle hydrodynamics (SPH) simulations.
A different category of ray-guided volume renderers uses
hierarchical grids with bricking, which are accessed via
multi-level page tables instead of a tree structure. Hadwiger
et al. [HBJP12] proposed such a multi-resolution virtual
memory scheme based on a multi-level page table hierarchy(see Figure 7). This approach scales to petavoxel data andcan also efficiently handle highly anisotropic data, which isvery common in high-resolution electron microscopy volumes. They also compare their approach for volume traversal to standard octree traversal in terms of traversal complexity and cache access behavior, and illustrate the advantagesof multi-level paging in terms of scaling to very large data.
Fogal et al. [FSK13] have performed an in-depth analysisof several aspects of ray-guided volume rendering.

9. Discussion and Conclusions

This information about the working set is then used toload missing data, and to determine which bricks can beevicted from the GPU cache because they are no longer
needed. Additionally, rays automatically determine the (locally) required data resolution. This determination can be
performed either on a per-sample basis [HBJP12] (see Figure 9), or on a per-brick basis [FSK13].

In this survey we have discussed different large-scale GPUbased volume rendering methods with an emphasis on rayguided approaches. Over recent years, sophisticated scalable
GPU volume visualization methods have been developed,
hand in hand with the increased versatility and programmability of graphics hardware. GPUs nowadays support dynamic branching and looping, efficient read-back mechanisms to transfer data back from the GPU to the CPU, andseveral high-level APIs like CUDA or OpenCL to makeGPU programming more efficient and enjoyable.

Gobbetti et al. [GMG08] were among the first to implement a volume ray-caster with stackless GPU octree traversal. They used occlusion queries to determine, load, and possibly refine visible nodes. This approach already has similar
properties to later fully ray-guided approaches. However, it
is strictly speaking not fully ray-guided, because culling ofoctree nodes is performed on the CPU based on the occlusion query information obtained from the GPU.

Our discussion of scalability in volume rendering was
based on the notion of working sets. We assume that the data
will never fit into GPU memory in its entirety. Therefore, it
is crucial to determine, store, and render the working set ofvisible bricks in the current view efficiently and accurately.
The review of “traditional” GPU volume rendering methods
showed that these approaches have several shortcomings thatseverely limit their scalability. Traditionally, the working set

submitted to COMPUTER GRAPHICS Forum (10/2014).

20

J. Beyer & M. Hadwiger & H. Pfister / GPU-Based Large-Scale Volume Visualizationof active bricks is determined on the CPU and no read-backmechanism is used to refine this working set. Additionally,
due to previously limited branching or looping functionality
on GPUs, renderers often had to resort to multi-pass rendering approaches. Modern ray-guided approaches exhibit
better scalability, they support dynamic traversal of multiresolution structures on the GPU, and they allow determining the working set of active bricks based on actual visibility
by employing efficient read-back mechanisms from the GPU
to the CPU. Therefore, ray-guided approaches are promising
for the future, where data set sizes will continue to increase.
In this survey we have focused on GPU-based approaches
for single stand-alone workstations. However, there is a hugearea of parallel and distributed visualization research that focuses on clusters, in-situ setups and client/server systems.
Additionally, we expect web-based visualization to become
more and more important, which will make it necessary toresearch scalable algorithms for remote visualization andmobile devices. Finally, as data sets get larger and scalable volume rendering methods more mature, it will become more and more important to have efficient workflows
and integrated solutions that encompass the whole data flow
through a system, from data acquisition and pre-processing
to interactive visualization and analysis.

10. Acknowledgments
This work was partially supported by NSF grant OIA
1125087.

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Technique[CN93][CCF94][WE98][LHJ99][RSEB∗ 00][WWH∗ 00]
[GGSe∗ 02][HBH03][KW03][LMK03][RGW∗ 03]
[GS04][PHKH04][BG05][HFK05][HSSB05][SSKE05][Lju06a][RV06][BHWB07][BHMF08][GMG08][MHS08][CNLE09][JBH∗ 09]
[KGB∗ 09]
[FK10][MRH10][Eng11][HBJP12][BAaK∗ 13][FSK13][RTW13]nonbricked•••volume data representation
single-res multi-res multi-res
bricked(tree)(grid)

textureslicing•••

rendering
singlemultipass RC pass RC

octree
•

virt.
tex.

address translationtree
multitraversal
res

•

•

•
•

•
•

•
octreeoctree

•
•
•
•

•
•
•
•
•
•

octreeoctree•••••octree
•
•
•
•
•

•
•
•

◦†

•
•

•
octree•octree
•
•

•
•
•
•
•
•
•

•
•
•
•
•
•

•

kd-tree•octree•••octree

•
•

•
•
•
•
•
•

•
•

◦††

•
•
•
•

working set determinationnoglobal/rayculling view cull guided
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
◦
•
•
•
•
∗
◦†††
•
•
•
∗
•
•
•
•
•
•
•
•
•
•
•
∗
•
•
•
∗
•
•
•
∗
•
•
•
∗
•
•
•
∗
•
•: full support; ◦: partial support; ∗: implicit support.
GPU

Table 3: Comparison of GPU-based volume visualization techniques based on data representation, rendering, address translation and working set determination. † uses
an octree per brick, †† can fall back to multi-pass raycasting if the current working set is too large to fit into GPU memory. † † † is not fully ray-guided, but utilizes
interleaved occlusion queries with similar goals.