Li Zhang’s research while affiliated with ETH Zurich and other places

Nowadays 3D modeling is generally performed using image or range data. Range sensors are getting a quite common source of data for modeling purposes due to their speed and ability to capture millions of points. In this paper we report about two surface measurement algorithms for precise and detailed object reconstruction from terrestrial images. Photogrammetry has all the potentialities to retrieve the same details of an object that range sensors can achieve. Using advanced measurement techniques, which combine area-based and feature-based matching algorithms we are able to generate dense point clouds of complex and free-form objects, imaged in closely or widely separated images. Different examples are reported to show the potentiality of the methods and their applicability to different close-range data sets.

In this paper we present the modeling and visualization of the cultural heritage area of Bamiyan, Afghanistan. The region is situated ca 200 km north-west of Kabul and it was one of the major Buddhist centres until the ninth century AD. The two standing Buddhas belonged to some of the most famous Buddhist monuments world-wide. In 2001 they were destroyed through an act of vandalism by the Taleban militia. In our previous reports we have already presented the 3D computer reconstruction of the Great Buddha of Bamiyan while in this contribution we will demonstrate D visualization products generated with different software packages and a first attempt at generating a spatial information system.

Photogrammetric processing in geocentric orthogonal coordinate system have potential superiority with more and more enrichment of multi source images and spatial data. This paper uses tangent orthogonal coordinate system based on the velocity direction of flight line as the reference datum of attitudes, infers and describes the transformation of attitude data in the reference datum, and for the first time builds the airborne optical image collinear equation and bundle adjustment model in geocentric orthogonal coordinate system. An experiment of block adjustment of frame airborne images based on the new model is given at the end of this paper, the result can achieve the similar precision as traditional method, which show that the given method is right and reliable.

Imaging equations are always considered as the most essential and basic part in photogrammetry and image mapping with remote sensing images. Rigourism and conciseness should be held as their basic characteristics.In this paper,using sensor exterior orientation including three lines and two attitude elements as the orientation parameters, new imaging equations for side-looking radar or SAR image positioning were derived. The model was based on the distance condition between sensor and object point and the azimuth condition of sensor scanning plane including antenna and radar beam center. Three forms of range-coplanarity equation were derived, the first form was in the tangent plane rectangular coordinate system, the second was in the geocentric rectangular coordinate system, and the third was the range-coplanarity equation with coordinates of image point as explicit function. The model could be easily used for side-looking radar and SAR image processing in photogrammetry field.

In this article developments and performance analysis of image matching for detailed surface reconstruction of heritage objects is discussed. Three dimensional image-based modeling of heritages is a very interesting topic with many possible applications. In this article we propose a multistage image-based modeling approach that requires only a limited amount of human interactivity and is capable of capturing the fine geometric details with similar accuracy as close-range active range sensors. It can also cope with wide baselines using several advancements over standard stereo matching techniques. Our approach is sequential, starting from a sparse basic segmented model created with a small number of interactively measured points. This model, specifically the equation of each surface, is then used as a guide to automatically add the fine details. The following three techniques are used, each where best suited, to retrieve the details: 1) for regularly shaped patches such as planes, cylinders, or quadrics, we apply a fast relative stereo matching technique. 2) For more complex or irregular segments with unknown shape, we use a global multi-image geometrically constrained technique. 3) For segments unsuited for stereo matching, we employ depth from shading (DFS). The goal is not the development of a fully automated procedure for 3D object reconstruction from image data or a sparse stereo approach, but we aim at the digital reconstruction of detailed and accurate surfaces from calibrated and oriented images for practical daily documentation and digital conservation of wide variety of heritage objects.

High-resolution satellite images (HRSI) at sub-5m footprint such as IKONOS and SPOT-5 HRG/HRS images are becoming increasingly available to the earth observation community and their respective clients. SPOT-5 HRS/HRG imagery is one of the main data sources for the Project of Western China Topographic Mapping (WChTM) which has been approved by the State Council of China in year 2006. In this paper, we present a matching approach for automatic DSM/DTM generation from SPOT-5 HRS/HRG imagery. It can provide dense, precise and reliable results. The method matches multiple images simultaneously and it uses a coarse-to-fine hierarchical solution with an effective combination of several image matching algorithms and automatic quality control. The DSMs/DTMs are generated by combination of matching results of feature points, grid points and edges. The new characteristics provided by the high-resolution imaging systems, i.e. the multiple-view terrain coverage and the high quality image data, are also efficiently utilized in this approach. The proposed approach has been applied to SPOT-5 HRS/HRG images over a testfield in Zone of headstream of Three rivers, eastern Tibet Plateau, China with variable terrain geomorphologic type. The accuracy study was based on the comparison between as many as 160 accurate GPS check points, more than 1400 manually measured check points and the automatically extracted DTMs. The RMS errors for the whole area are 2–7 m, while for flat/hilly areas the accuracy is about 2–3 m or even better. From these experiments, it’s shown that with the proposed automatic DTM generation approach, by using SPOT-5 HRS/HRG and IRS-P5 imagery, satisfactory 1:50000 DTMs can be completed with a better accuracy than those requirements from Chinese Surveying and Mapping regulations.

High-resolution satellite images (HRSI) at sub-5m footprint such as IKONOS and SPOT-5 HRG/HRS images are becoming increasingly available to the earth observation community and their respective clients. SPOT-5 imagery is one of the main data sources for the Project of West China Topographic Mapping (WChTM) which has been approved by the State Council of China in year 2006. In this paper, we present an approach for block-adjustment based on Rational Function Model (RFM) with sparse GCPs by using SPOT-5 Images. To test the proposed approach, it has been applied to SPOT-5 images over 2 test-fields, one is in Baoji City, Chanxi, China, and another covers eastern part of Tibet Plateau, China. All test-fields are with variable terrain geomorphologic type and several tens of GCPs and check points measured by DGPS. The block-adjustment results show that with SPOT-5 HRS images and small number of GCPs we can achieve 5-9m in planimetric and 2–3m in height direction. From these experiments, it’s shown that with the proposed block-adjustment approach, by using SPOT-5 HRS imagery with small number of GCPs, satisfactory image orientation results can be achieved with a little bit better accuracy than those requirements from the Chinese Surveying and Mapping regulations for 1:50000 topographic maps.

High-resolution satellite images at sub-5-m footprint, such as IKONOS and SPOT5 HRG/HRS images, are becoming increasingly available to the earth observation community and their respective clients. The related cameras all use linear array CCD technology for image sensing. The processing of these kinds of images provides a challenge for algorithmic redesign and this offers the possibility of reconsidering and improving many photogrammetric processing components. This contribution presents an advanced matching approach for automatic DSM generation from high-resolution satellite images. It can provide dense, precise and reliable results. The method matches multiple (more than two) images simultaneously and it uses a coarse-to-fine hierarchical solution with an effective combination of several image matching algorithms and automatic quality control. The DSMs are generated by a combination of matching results of feature points, grid points and edges.The proposed approach has been applied to IKONOS images over a testfield in Thun, Switzerland with accurate ground control points, a 1600-m height range and variable land cover, but with sub-optimal imaging conditions (snow, long shadows). The accuracy tests are based on the comparison between the reference data from an airborne laser scanner and the automatically extracted DSMs. The RMS errors for the whole area, excluding trees and bushes, are 2–3 m, while for bare ground the accuracy is about 1 m or even better.

A vision based method to recover human faces from video sequences is presented. Although video sequences acquired from multiple static synchronized CCD cameras have been used as a tool for 3-D reconstruction of the face before, the precision and reliability remain as concerning issues, which are addressed in this paper Moreover, the presented matching algorithm is invariant to scaling, rotation and insufficient calibration. A geometric primitive guides the matching approach through object space and therefore allows to compare gray values in an unique coordinate system

The paper reports about two projects for derivation of photorealisti c 3D models using modern techniques of analytical and digital photogrammetry. The first project aims at documentation of the oldest preserved large relief model of Switzerland requiring a high quality object reconstruction. The second application presents modeling and visualization of one of the complex towers of the famous Bayon temple of the ancient city of Angkor Thom, Cambodia based on the use of small format tourist-type photography. Described photogrammetric procedures include phototriangulation, digital surface model generation in manual or automated mode and automated techniques of texture mapping.