Sylvain Michelin

Thinning grayscale well-composed images

Usual approaches for constructing topological maps on discrete structures are based on cellular complexes topology. This paper aims to construct a coherent topological map defined on a square grid from a watershed transformation. The main idea behind the proposed approach is to impose some constraints on the original image in order to obtain good properties of the resulting watershed. We propose a definition of well-composed grayscale images based on the well-composed set theory and the cross-section topology. Properties of two different thinning algorithms are then studied and we show how to obtain a thin crest network. We derive an efficient algorithm that permits the construction of a meaningful topological map, resulting in a topological segmentation, i.e. a segmentation that describes in a coherent framework faces and contours. Finally, we demonstrate the usefulness of this algorithm for multilevel image segmentation.

Proximity radiosity: exploiting coherence to accelerate form factor computations

This paper introduces a new acceleration principle for the zonal method. The core concept resides in exploiting the coherence that exists between form factors of two close voxels (or patches). Primarily, we dissociate the radiometric part of the form factors form the geometrical part, the remaining geometrical expressions including volume integrals can be then developed with the Green-Ostrogradski theorem in terms of double surface integrals. These new expressions are less complex and allow us to divide computational time by a factor of about 4. Secondary, we show how all voxels in the neighborhood of a given “reference” voxel, have form factors (with another patch or voxel) that are weighted sums of the reference voxel form factor and a series of associated integrals of generalized orthogonal polynomials. Subsequently, calculation time decreases while a control of the generated error is maintained.

Form Factor Calculation: a New Expression with Implementations on a Parallel T.Node Computer

The radiosity technique uses the heat transfer theory to define a view‐independent illumination model. For complex scenes, calculation of this model is very time‐intensive, due largely to the determination of a geometric factor, defined with a double area integral, and named form factor. This paper presents a new expression of the form factor, which can be reformulated, in a computationally more efficient manner, as a simple line integral for planar polygonal convex surfaces that are perfectly diffuse.

A new radiosity approach for regular objects: application to ruled surfaces

This paper introduces a new approach in the radiosity method. The main principle applied here, is the improvement of form factor computation by the knowledge of the model properties. More precisely, if surfaces describing a scene are “regular”, the values of the form factor and its “derivatives” between any patch B and a patch A intervening in the meshing of a given surface, enable us to evaluate (with a required precision) the form factors values between B and the neighboring patches of A. We will also show that the mathematical relation we have obtained, 1) is especially efficient with ruled surfaces, 2) significantly decreases the computation time, 3) is well‐adapted to refinement or subdivision techniques and 4) gives us an interesting variety of surfaces. Calculation times are equivalent to those obtained with a projective method (hemi‐cube for example) but with an efficient control of the generated errors.

Fourier-transform computer-generated hologram: a variation on the off-axis principle

In this paper we present a new simple and efficient technique based on Fourier transform to generate holograms by computer. This technique does not take into account the classical techniques of phase codification. Referring to the holographic principle (off-axe holograms of Leigh & Upatniek) studied first by Burch or Huang & Prasada, a reference source has been included in the global calculation. In the second part of this paper, the production of the holograms using a commercial photographic film and a laser printer is described and experimental results are presented.

Thinning Grayscale Well-Composed Images: A New Approach for Topological Coherent Image Segmentation

Usual approaches for constructing topological maps on discrete structures are based on cellular complexes topology. This paper aims to construct a coherent topological map defined on a square grid from a watershed transformation. We propose a definition of well-composed grayscale images based on the well-composed set theory and the cross-section topology. Properties of two different thinning algorithms are studied within this scope, and we show how to obtain a thin crest network. We derive an efficient algorithm that permits the construction of a meaningful topological map. Finally, we demonstrate the usefulness of this algorithm for multilevel image segmentation.

A Graph Labelling Approach for Connected Feature Selection

Many authors have already proposed linear feature extraction algorithms. In most cases, these algorithms can not guarantee the extraction of adjacency relations between extracted features. Object contours appearing in the analyzed images are often fragmented into nonconnected features. Nevertheless, the use of some topological information enables to reduce substantially the complexity of matching and registration algorithms. Here, we formulate the problem of linear feature extraction as an optimal labelling problem of a topological map obtained from low level operations. The originality of our approach is the maintaining of this data structure during the extraction process and the formulation of the problem of feature extraction as a global optimization problem.

Realistic rendering with turbulent flows

This paper focuses on realistic rendering of scenes which include turbulent flows. In this respect we suggest solving the Navier-Stokes equations through a simple approach which offers better steadiness than classical methods. Then we adapt a ray-tracing algorithm to treat convection motions by deflecting ray paths: we obtain effects of rendering through heat. Copyright (C) 2000 John Wiley & Sons, Ltd.