ADAPTIVE LEVEL OF DETAIL WITH OCCLUSION CULLING

Hermann Birkholz

University of Rostock

Albert-Einstein-Str. 21, 18059 Rostock, Germany

Stefan Rahn

University of Rostock

Albert-Einstein-Str. 21, 18059 Rostock, Germany

Keywords:

Level of Detail, Occlusion Culling.

Abstract:

Many techniques have been developed in order to accelerate the visualization of large triangle meshes. Level

of Detail techniques can be used to create view-dependent approximations of wide range scenes with low

occlusion, such as landscapes. For highly occluded scenes there exist many occlusion culling techniques,

which discard occluded parts of the scene before rendering. This can drastically speed up the visualization

of such scenes but will not improve rendering of wide non-occluded scenes. In this paper we will combine

both acceleration methods. We present a new approach, which combines the beneﬁts of Level of Detail

rendering and of Occlusion Culling, in order to minimize their drawbacks. The technique adds occlusion as

a view-dependence criterion for Level of Detail rendering and is even able to optimize the reﬁnement of self

occluding meshes.

1 INTRODUCTION

Very large triangle meshes can easily be acquired by

current 3d-scanning hardware. Meshes consisting of

millions of triangles exceed the capabilities of modern

graphics hardware in interactive rendering. Interac-

tive frame rates can be achieved only by a reduction

of the number of triangles before rendering. Level

of Detail (LOD) algorithms create hierarchical de-

scriptions of meshes by iteratively simplifying them

locally. All local simpliﬁcations are stored together

with an error estimation and thus form the hierarchy.

This enables view-dependent approximations of the

original mesh. The approximation process uses the

hierarchy in order to reﬁne the parts of the mesh that

are situated within the view frustum and that do not

face away from the viewer. The mesh complexity is

locally adopted to the viewpoint distance. All these

parameters enable interactive frame rates for large tri-

angle meshes but produce poor approximations for

highly occluded views. Due to the lack of occlusion

information, occluded parts of a mesh are reﬁned in

the same way as non-occluded ones. This problem

could be solved by an additional occlusion criterion

during the approximation process, which forbids fur-

ther reﬁnement of occluded parts of the mesh. This

paper will show recent work regarding this problem

and presents an new integration technique for LOD

and occlusion culling.

2 RELATED WORK

The technique that is presented in this paper was inﬂu-

enced by miscellaneous papers, which are reviewed in

this section.

2.1 Level of Detail Rendering

Many algorithms have been proposed to create LOD

hierarchies, which can be used to approximate tri-

angle meshes but do not consider occlusion. Hoppe

(Hoppe, 1997) shows how to use his earlier intro-

duced ”progressive meshes”(Hoppe, 1996) for view-

dependent rendering. The progressive meshes store

a edge-collapse sequence, which is used for the re-

construction of the original mesh. In each simpliﬁ-

cation step the edge whose simpliﬁcation causes the

least error is collapsed to a new vertex. This op-

eration removes one vertex and two triangles from

the surface. View-frustum and backface culling are

supported with bounding-spheres and normal-cones.

Another approach (Xia and Varshney, 1996) uses

”Merge Trees”, in order to present the LOD hierarchy.

279

Birkholz H. and Rahn S. (2006).

ADAPTIVE LEVEL OF DETAIL WITH OCCLUSION CULLING.

In Proceedings of the First International Conference on Computer Graphics Theory and Applications, pages 279-284

DOI: 10.5220/0001351302790284

 SciTePress

The ”Merge Tree” is constructed by repeated edge-

collapses in the original mesh. Backface and view-

frustum culling is supported with normal-cones and

bounding spheres, again. Garland and Heckbert (Gar-

land and Heckbert, 1997) extended the edge-collapse

hierarchies to vertex-contraction hierarchies, which

can also merge vertices without a shared edge. Fur-

thermore they introduced a new quadric error met-

rics in their paper. Newer techniques enable ren-

dering of very large LOD hierarchies from external

memory. Hoppe presented a hierarchical terrain man-

agement (Hoppe, 1998) that was extended to arbi-

trary meshes by Prince (Prince, 2000). An external

memory approach based on vertex-clustering was pre-

sented by Lindstrom (Lindstrom, 2003). With the

”adaptive TetraPuzzles” (Cignoni et al., 2004) a hi-

erarchy of tetrahedral cells is constructed from the

bounding box around the mesh. Each tetrahedral

cell is associated with a precomputed simpliﬁed part

of the original model. The tetrahedra-hierarchy is

adaptively reﬁned and replaced with the precomputed

geometry of the tetrahedral cells.

2.2 Occlusion Culling

Another method to reduce the number of triangles that

has to be processed by the graphics hardware is to

detect and to cull occluded regions. In highly oc-

cluded scenes most triangles will not be visible and

the frame rate would signiﬁcantly beneﬁt from occlu-

sion culling. For wide and less occluded views how-

ever, there will be no improvements.

There exist several different approaches to deter-

mine the occluded parts of a scene. Some divide

the scene into disjoint cells and compute a set of

potentially visible triangles (PVS) for each cell in a

preprocess (Chrysanthou et al., 1998; Durand et al.,

2000; Schauﬂer et al., 2000). While rendering, only

triangles of the actual PVS are taken into account.

Due to the preprocess such techniques are only suited

for the visibility of static scenes.

Other algorithms determine occlusion in screen-

space and can be accelerated by graphics hard-

ware. The hierarchical z-buffer visibility, presented

by Greene (Greene et al., 1993), is meanwhile avail-

able in current graphics hardware and enables fast z-

tests for bounding objects. The z-buffer values are

hierarchically merged 4 by 1 and the deepest value

is chosen for each group. This allows to cull small

objects with only few z-tests in the upper hierarchy

levels. Greene recommends to store the scene in an

octree, whose cells are visibility-tested against the hi-

erarchical z-buffer and only processed further when

they are visible. The ”hierarchical occlusion maps”

used by Zhang (Zhang et al., 1997) allow hardware

supported occlusion culling with so-called occlusion

maps. He uses alpha-map hierarchies, whose opacity

values represent the opacity of selected occluders. Hi-

erarchy levels of the occlusion maps are generated by

subsampling the previous hierarchy level 4 by 1 (hard-

ware accelerated). These map hierarchies can then be

used in order to determine the visibility of occludees.

Screen space techniques have the advantage of build-

in occluder fusion. Because all occluders share the

screen space, they are merged implicitly.

Finally there exist algorithms that cull occluded

objects in the object-space. Hudson (Hudson et al.,

1997) constructs shadow frusta from all occluders and

tests the occludees against them. To accelerate the

selection of proper occluders, a preprocess assigns a

set of potentially good occluders to each viewpoint.

Coorg (Coorg and Teller, 1997) also determines po-

tentially good occluders for each viewpoint in a pre-

process. The occlusion test is then done with the help

of supporting and separating planes.

2.3 Integration of LOD and OC

In order to overcome the disadvantages of both Level

of Detail and occlusion culling, they were combined

in some approaches. Andujar (Andujar et al., 2000)

introduces ”Hardly-Visible Sets” in order to differen-

tiate partial occluded objects in the scene. He sug-

gests the use of LOD approximations as occluders to

determine occlusion and to use coarser approxima-

tions for the rendering of ”Hardly-Visible” objects in

the scene. He uses discrete LODs for the approxima-

tion and thus can guarantee neither image quality nor

the amount of geometry, which ist sent to the graphics

hardware.

An integration of approximate visibility and con-

tinuous LOD was presented by El-Sana (El-Sana

et al., 2001). They divide the space with a uniform

grid and store an opacity value for each cell, which

depends on the area of the projected geometry in the

cell. The split- and merge-criteria for the nodes in the

LOD hierarchy is now expanded by occlusion. For

each node a visibility value is estimated by tracing

the ray between viewer and hierarchy node through

the grid. The opacity values of the traversed cells are

combined, in order to determine which nodes to split

or to collapse. Due to the approximate estimation and

the discrete grid, the visibility might appear incorrect.

Furthermore, the traverse time for the visibility test

could consume much time in hardly occluded scenes.

The approach that is presented in this paper, com-

bines view-dependent LOD and occlusion culling in

another way. It uses coarse approximations of the ﬁ-

nal mesh itself to determine occlusion during the re-

ﬁnement.

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3 OCCLUSION CONTROLLED

REFINEMENT

In this section we describe our algorithm for the

integration of view-dependent LOD and occlusion

culling. Multiple LOD objects can be approximated

in one scene and be freely moved in the scene. Vis-

ibility data is extracted online and does not demand

precomputed data.

We use a edge-collapse hierarchy, which stores

bounding volumes and normal cones for each node

in the hierarchy. The occlusion tests are done in the

z-buffer in image space resolution. Occluder fusion

is enabled implicitly, due to the use of the z-buffer.

Depth values of different occluders always merge to

a common occluder in z-buffer. We assume that the

LOD extraction process takes much more time of the

complete rendering procedure, than the time for the

actual rendering of the extracted geometry. This al-

lows us to render different versions of the occluders

with increasing detail into the z-buffer without signif-

icant effects to the frame rate. Our LOD extraction is

controlled by the number of triangles in the approx-

imation. The extraction stops when a given triangle

threshold is reached. The reﬁnement always starts

with a coarse base mesh that is reﬁned by vertex-split

operations. The reﬁnement sequence is controlled by

the viewpoint distance and the visibility. The visibil-

ity of reﬁnement candidates is determined with view-

frustum culling (bounding spheres), backface culling

(normal cones) and occlusion culling. Occlusion tests

are only initiated when the other culling tests fail.

All visible vertices are placed in a reﬁnement prior-

ity queue. During the reﬁnement process, always the

ﬁrst vertex is removed from the queue, processed, and

its child-vertices are added when they are visible.

3.1 Occlusion Estimation

Occlusion culling is implemented with hardware oc-

clusion queries. These queries test given geometry

against the z-buffer of the graphics hardware with-

out modifying it and return the number of pixels that

would be visible if the geometry had been really ren-

dered. A coarse approximation of the mesh itself is

used as the occluder. It is rendered to the z-buffer and

the bounding volumes of vertices, which will poten-

tially be split, are tested against it. As the ﬁrst oc-

cluder, a very coarse approximation of the mesh is

chosen, which of course was produced without occlu-

sion culling. For a better adoption this occluder mesh

can be updated several times during the reﬁnement

process. As bounding volumes for the vertices, we

use simple shapes, that conservatively approximate all

reﬁnement levels, which are stored in the subtree of

the vertex.

3.2 Algorithm

The complete reﬁnement process consists of the fol-

lowing steps:

• Creation of a coarse approximation without occlu-

sion culling.

• Render the coarse occluder to z-buffer.

• While the triangle threshold is not exceeded, test

the visibility for the next reﬁnement step and exe-

cute it, if it is visible. Otherwise proceed with the

next reﬁnement step.

• Render the ﬁnal mesh to the frame-buffer.

In the reﬁnement loop, the occluder should be updated

periodically for a better approximation. On each up-

date, all vertices culled due to occlusion should be

tested against the new occluder again.This ensures

that the geometry, which was occluded by the coarse

approximated occluder can be reﬁned, if it is visible

in the ﬁnal mesh.

Figure 1: Reﬁnement process.

The whole reﬁnement process is shown in ﬁgure 1,

where a ”Dragon” mesh is reﬁned to a coarse level.

This coarse representation is used as occluder in the

further reﬁnement steps. The ﬁnal mesh will have a

much lower triangle resolution in occluded regions,

compared to the visible parts, after the reﬁnement is

done.

3.3 Reduction of Occlusion Queries

Latencies

Due to the time necessary for the occlusion test, the

algorithm would make the reﬁnement very slow. Es-

pecially large bounding volumes (in screen space) can

require long testing time. This latency-time can be

reduced by making use of parallel threads on CPU

and GPU. We can send multiple queries to the GPU,

which are queued there and are processed one by one.

We choose the number of occlusion queries that are

ADAPTIVE LEVEL OF DETAIL WITH OCCLUSION CULLING

281

queued in the GPU large enough to signiﬁcantly de-

crease latency times. When one occlusion result is

read from GPU, the further processing of the vertex is

determined with it. A visible bounding volume means

that the occlusion queries for the bounding volumes

of the child vertices are queued in the GPU. When the

number of queued queries on the GPU becomes too

small, we simply add queries from the global reﬁne-

ment queue.

To improve the frame rate, we do not send all

queries to the graphics hardware. To decrease the

number of occlusion queries, we only send queries for

odd hierarchy levels. This hardly lowers the reﬁne-

ment quality but saves half of the occlusion queries.

4 RESULTS

The LOD reﬁnement with occlusion culling has been

tested with different meshes. We determined the ratio

between visible and invisible triangles in the scene,

in order to measure the efﬁciency. The scene can

be composed of multiple LOD hierarchies, in order

to test the occlusion among meshes. All test scenes

were approximated with a number of 10,000 triangles.

We used two different occluder steps during the re-

ﬁnement process. After 300 reﬁnement steps, a very

coarse occluder is rendered to the z-buffer and after

2500 steps, it is updated with a ﬁner version. The us-

age of further occluders was tested, but it did not im-

prove the distribution of triangles in the visible and in-

visible areas. The measured frame rates for the LOD

rendering without occlusion culling were between 26

and 27 fps, while the use of occlusion culling de-

creased the frame rate to slightly above 15 fps (due

to the occlusion query overhead).

The LOD hierarchies of all meshes were created

by half-edge collapses using the Quadric Error Met-

ric (Garland and Heckbert, 1997). The meshes that

were used in the test scenes can be found in table 1.

Table 1: Test meshes.

Name Armadillo Armadillo Rough

Group Planet

Vertices 172,974 518,922 16,777,218

Triangles 345,944 1,037,832 33,554,432

Disc Size 19,7 MB 59,1 MB 1920 MB

The different bounding volumes were tested with

the ”Armadillo” mesh. Due to the queued parallel

processing, the complexity of the tested bounding vol-

umes did not affect the frame rate. The box is ren-

dered with 12 triangles while the sphere is approxi-

mated with 32 triangles and the cylinder with 64 tri-

angles. The best ratio between visible and invisible

triangles was achieved with a cylindrical bounding

volume. The cylinder is oriented along the surface

normal of the according vertex. All geometry that is

associated with the vertex is enclosed by the cylin-

der. The box and the sphere volume always resulted

in more conservative approximations and poorer visi-

bility ratios.

A comparison between view-dependent LOD with

and without occlusion culling can be found in table

2. For each view the the number of visible trian-

gles without (VT

LOD

) and with (VT

) occlusion

culling is shown together with the reached improve-

ment ratio (IR). The total number of triangles which

is used to approximate the whole scene is always

10,000. The results show signiﬁcant improvements

Table 2: Test meshes.

Name VT

LOD

Armadillo 2,277 3,642 59,95%

Armadillo group 2,231 3,733 67,32%

Planet full 3,546 4,781 34,83%

Planet near 1,888 4,623 144,86%

in the distribution of triangles. Many more triangles

are used to approximate the visible parts of the scene

when occlusion culling is integrated. Of course the

improvement ratio depends on the amount of occlu-

sion in the view to be rendered. Views with low oc-

clusion, such as the ”Planet full” view (Figure 2) show

only little improvement.

Figure 2: Full view of the ”Rough Planet” mesh, without

(left) and with (right) occlusion culling.

The relatively high result in this example, however,

can be reasoned with the inefﬁcient backface culling.

Due to the rough surface, the backface culling is

hardly able to inﬂuence the reﬁnement process. The

normal cones of the mesh enclose large angles even

in the higher levels of the hierarchy and thus will

be unable to support visibility culling. Occlusion

GRAPP 2006 - COMPUTER GRAPHICS THEORY AND APPLICATIONS

282

culling detects the surface regions on the backside and

culls it efﬁciently. For smoother surfaces this differ-

ence would narrow itself. A view of the ”Armadillo”

mesh, for instance, shows only improvements below

4% when there is no self-occlusion in view.

Views with heavy occlusion such as in ”Planet

near”, show enormously high improvements. If many

parts of the mesh are occluded, LOD techniques with-

out occlusion culling distribute the available triangles

evenly in both visible and invisible regions. An early

detection of occluded regions during the reﬁnement

process will inﬂuence the arrangement of triangles,

and cause higher detail in parts of the view that are

visible.

Figure 3 shows a comparison of the view with and

without occlusion culling. Because the hill in the

front occludes most geometry in the view, standard

view-dependent LOD methods distribute many of the

triangles to invisible regions.

Figure 3: Near view of the ”Rough Planet” mesh, without

(top) and with (bottom) occlusion culling.

Our approach however detects the occluded regions

and uses only few triangles to approximate them. A

birds eye view in ﬁgure 4 illustrates the effect of the

occlusion culling. The left image corresponds to the

upper image of ﬁgure 3 and the right image to the bot-

tom image of ﬁgure 3.It is noticeable that the regions

that are occluded by the hill in the front of the view

(yellow lines), are approximated much coarser than

the visible parts.

A view on the ”Aramdillo” mesh as listed in table 2

is shown in ﬁgure 5. The hand in the front occludes

a large region of the mesh and allows us to distribute

more triangles in the visible regions of the view.

Figure 4: Effect of occlusion culling in the approximation

shown in ﬁgure 3.

Figure 5: Views of the ”Armadillo” mesh without (left) and

with (right) occlusion culling.

Our last example view is shown in ﬁgure 6. Three

instances of the ”Armadillo” mesh are placed one af-

ter another. The instance in the front occludes most

geometry of the two other instances such that many

triangles would be wasted without occlusion culling.

Figure 6: Views of the ”Armadillo Group” without (left)

and with (right) occlusion culling.

The effect of the occlusion culling is illustrated in ﬁg-

ure 7. The regions that are occluded in ﬁgure 6 are

coarsely approximated only in the right image. The

saved triangles are distributed to the visible parts of

the meshes.

5 CONCLUSION

In our paper we presented a new way to integrate oc-

clusion culling and view-dependent LOD. The results

show that signiﬁcant improvements in the distribution

of triangles are possible. Views that contain much oc-

clusion can drastically increase the amount of visible

ADAPTIVE LEVEL OF DETAIL WITH OCCLUSION CULLING

283

Figure 7: Side-views of the ”Armadillo Group” approxima-

tions in ﬁgure 6.

triangles. Even in situations with less occlusion, our

approach can support the culling of faces that are ori-

ented away from the viewer but cannot be culled with

backface culling.

The price for the improvement in the rendering-

quality, however, is a 42% drop in the frame rate. The

approximately 5000 occlusion queries, which have to

be done for a reconstruction with 10,000 triangles,

consume a large amount of the frame time, although

they mainly run in parallel to the reﬁnement process.

Our next work will be focussed on the acceleration of

the rendering process.

Another way to increase the frame rate would per-

haps be the use of time coherency. A frame to frame

update of the approximation would likely demand less

occlusion queries, when the view moves slowly.

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