CSS – Curvature Scale Space in OpenCV

Description

The goal of this project is to implement algorithm that creates curvature scale space (CSS) image of given shape using OpenCV library. “The CSS image consists of several arch-shape contours representing the inflection points of the shape as it is smoothed. The maxima of these contours are used to represent a shape. The CSS representation is robust with respect to scale, noise and change in orientation.”[1]

Process

1. Find contour coordinates of given shape

findContours(im, contours, CV_RETR_LIST, CV_CHAIN_APPROX_NONE );

Following steps are repeated with increased sigma until there are no zero-crossing points:

2. Gaussian kernel is the base for upcoming steps:

   transpose(getGaussianKernel(width, sigma, CV_64FC1), G);
   

3. Curve evolution can be computed by convolution of contour points with Gaussian kernel. Smoothed contour is not needed for CSS computation; it is used only to visualize the process:

   filter2D(X, Xsmooth, X.depth(), G);
   filter2D(Y, Ysmooth, Y.depth(), G);
   

   

4. To compute 1^st and 2^nd derivation of contour points, Gaussian kernel derivations will be needed:

   Sobel(G, dG, G.depth(), 1, 0, 3);
   Sobel(G, ddG, G.depth(), 2, 0, 3);
   

5. Convolution of contour points using derivatives of Gaussian kernel. According to OpenCV documentation: filter2D does actually computes correlation, not the convolution. That is, the kernel is not mirrored around the anchor point. If you need a real convolution, flip the kernel using flip() and set the new anchor to (kernel.cols – anchor.x – 1, kernel.rows – anchor.y – 1) :

   flip(dg, dg, 0);
   flip(ddg, ddg, 0);
   Point anchor(dg.cols - fwhm -1, dg.rows - 0 - 1);
   filter2D(X, dX, X.depth(), dG, anchor);
   filter2D(Y, dY, Y.depth(), dG, anchor);
   filter2D(X, ddX, X.depth(), ddG, anchor);
   filter2D(Y, ddY, Y.depth(), ddG, anchor);
   

6. Finally, we calculate the curvature and find zero crossings:

   

7. Zero-crossing points are plotted to the final CSS image. X-axis represents position of point on the curve; Y-axis represents the value of sigma:
   

   

Practical Applications

• Finding similar shapes  (Used as shape descriptor in MPEG-7 standard)
• Corner detection

References

[1] Sadegh Abbasi, Farzin Mokhtarian, Josef Kittler: Curvature Scale Space Image in Shape Similarity Retrieval. Multimedia Syst. 7(6): 467-476 (1999)

[2] Farzin Mokhtarian, Alan K. Mackworth: A Theory of Multiscale, Curvature-Based Shape Representation for Planar Curves. IEEE Trans. Pattern Anal. Mach. Intell. 14(8): 789-805 (1992)