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11, pp. 184
TRIANGLE-BASED VISIBILITY ANALYSIS AND TRUE ORTHOIMAGE GENERATION
KAZUO ODA WEILU OSAMU UCHIDA TAKESHI DOIHARA
ASIA AIR SURVEY CO., LTD.
Commission III, PS WG 111/8
KEY WORDS: Orthorectification, Photogrammetry, Algorithms, Orthoimage, Transformation, Visibility Analysis, Triangular
Prism Model, 3-D space description
ABSTRACT:
We propose a new method that can realize rapid generation of true orthoimages from area-type sensor images, accelerated by
triangle-based visibility analysis. A Triangular-Prism Model (TPM) for 3-D space description and triangle-based visibility analysis
is introduced. TPM, considered as an extension of TIN model, consists of triangular prisms and every top triangle is the surface of
the ground, buildings, and other objects on the ground. Vertical walls also can be expressed with the side faces of prisms.
TPM simplifies visibility analysis between elements of surface model. The visibility among a group of triangular prisms can be
related with that of a group of 2-D triangles that are the projection of the prisms onto 2-D space. This paper introduces visibility
sorting, where triangular prisms are sorted according to the visibility from the viewpoint.
The visibility sorting has been applied to true orthoimage processing where occlusions by buildings are essential. Tests with 30
aerial images of Shinjuku area shows that occluded area around building can be extracted in each true orthoimage. True orthoimages
can be synthesized into one composite true orthoimage where occluded areas of one image are compensated with other true
orthoimages.
1. INTRODUCTION (1) All side faces are vertical and vertical projection of the top
face and the bottom face are the same. This triangle is called
Many of the existing methods of true orthoimage generation are "base triangle".
facilitated by separation of DSM into DTM and DBM (Digital (2) All prisms have no common parts except for side faces.
Building Model). These types of true-orthoimage generation — Similarly, all base triangles have no common parts except for
simplify visibility analysis by counting the cases where some edge line segments.
parts of DBM may occlude DTM, and omit the cases of
Mode
Figure 1. Triangular prism model (TPM) and base triangles.
2.0 TPM Creation from Digital Maps
TPM can be constructed from ordinary 3-D digital maps.
Before creation, features in digital maps should be categorized
into two types of feature groups: one includes features on the
ground, such as roads, vegetations, or elevation contours. The
other group includes features above the ground, typically
building polygons which have elevation at roofs. Then 2-D TIN
is created from all feature points and line segments. Triangle
prisms are generated for each triangle in the TIN. Top faces of
the prisms for the features on the ground lay on ground surface,
while those for the features above the ground are the roof tops.
Bottom faces of all prisms may have a certain elevation value
that is equal to or lower than the lowest elevation of the features.
occlusion between DBM and DBM, or DTM and DTM. Other
types of methods perform Z-buffering for each pixel of
Our method adopts Triangular Prism model (vertical faces like
building walls are allowed) for DSM and does not distinguish
limitation in the target of visibility analysis. The theory for bobby wean
triangle-based visibility analysis utilizes the fact that triangle :
except the case that triangles contact at their edges or surfaces. : NS EN [y \ LN
TPM simplifies visibility analysis between elements of surface
related with visibility of a group of 2-D triangles that are s EN NU NN / Bas Triangles
projection of the prisms onto 2-D space. This paper also * Ra baa
sorted according to the visibility from the viewpoint. Such
visibility sorting can be applied to true orthoimage processing
This paper first introduces the definition of TPM and describes
how to create TPM from a digital map. The following section
introduces visibility sorting. Section 4 treats the procedure for
true orthoimage generation based on visibility sorting of TPM.
orthoimage and requires large amount of processing time.
between DTM and DBM. This means that this method has no = “um xe
planar surfaces in the model never intersect with each other, ‘
model. Visibility among a group of triangular prisms can be ANT A
introduces visibility sorting, by which triangular prisms are
where occlusions by buildings are essential.
explains visibility analysis of TPM and its base triangles, and
The procedure of orthorectification is demonstrated in Section 5.
2. DEFINITION OF TRIANGULAR PRISM MODEL
2.1 Definition
Triangle Prism Model (TPM) is a group of triangular prisms of
finite number, satisfying the following conditions (Figure 1).
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