Image & shape analysis for (re)construction, illustration and stylization.

 


Growing Least Squares

Last modified on 2013-05-16 15:23:57 GMT. 0 comments. Top.

Approach: 2D or 3D point sets equipped with normals are analyzed using local regressions (algebraic spheres) over point neighborhoods of increasing support (hence the name growing least squares). Pertinent scales are those where fitted spheres exhibit small or zero variation along scale; such pertinence is used in a variety of applications ranging from noise reduction to shape matching and feature detection.

Contributions: Our approach provides a fully continuous solution for scale-space analysis. It provides accurate curvature information at arbitrary positions and scales, and it is robust to noise. Contrary to scale-space methods that track extrema of a signal through scales, ours characterizes all pertinent regions of scale-space and is robust to changes in input. Moreover, it is easily implemented on the GPU because of its locality.

Limitations: Sphere fitting only provides mean curvature information at a given scale. Although more complex shapes are disambiguated by taking into account fitted spheres at multiple scales, this added complexity cannot be used for characterizing scale pertinence. Computation times could also be reduced by tolerating approximations at large scales.

Continuous feature detection

Publications

Growing Least Squares for the Continuous Analysis of Manifolds in Scale-Space.Nicolas Mellado, Pascal Barla, Gaël Guennebaud, Patrick Reuter, Christophe Schlick.  Computer Graphics Forum, Proceedings of Symposium on Geometry Processing 2010.
Screen-Space Curvature for Production-Quality Rendering and Compositing. Nicolas Mellado, Pascal Barla, Gaël Guennebaud, Patrick Reuter, Gregory Duquesne. Siggraph Talk program, 2013.  movie_icon

Software

Working on it…

 


Surface Relief Analysis

Last modified on 2013-05-16 15:24:54 GMT. 0 comments. Top.

Approach: Relief data is analyzed at each surface point at multiple scales and directions by fitting 1D cubic polynomials. Feature values of these polynomials (e.g., distance to inflexion) are combined over directions and scales, and used in a variety of illustrative shading techniques.

Contributions: One-dimensional fitting permits to identify non-local features in the neighborhood of a surface point, while avoiding artifacts raised by fitting in a single direction along a flow (around singularities). It provides natural multi-scale combination and novel filtering possibilities at render time, and works in real-time after a short pre-process.

Limitations: The method requires a preliminary base/relief decomposition, which may differ depending on input data. It is also not able to identify meaningful scales of a surface but rather averages them in a reasonnable manner.

Relief features depicted through shading

Publications

Surface Relief Analysis for Illustrative Shading. Ammann Lucas, Pascal Barla, Xavier Granier, Gael Guennebaud, Patrick Reuter. Computer Graphics -New York- Association for Computing Machinery- Forum, Wiley, 2012, 31 (4)

 


Implicit Brushes

Last modified on 2013-05-16 15:32:36 GMT. 0 comments. Top.

Approach: Various feature lines are described in terms of a height function and direction field. These are used to perform a local analysis of a feature profile using a fitting technique. Brush footprints are then convolved along features based on profile parameters to let a stylized line drawing emerge.

Contributions: The method is temporally coherent, it creates natural simplifications of various line definitions, and deals with inherent topological changes of surface features once projected in the picture plane. It works in real-time in post process with 3D data or videos.

Limitations: The method is limited by flow singularities, which occur more likely with features of higher order. The stylization is also limited to the convolution of a brush at all pixels in parallel; hence there is no explicit 2D line parametrization.

Feature lines rendered on top of a video

Publications

Implicit Brushes for Stylized Line-based Rendering, Romain Vergne, David Vanderhaeghe, Jiazhou Chen, Pascal Barla, Xavier Granier, Christophe Schlick, Computer Graphics Forum 30, 2 (proceedings of Eurographics 2011)