VLFeat - Tutorials > Quick shift superpixels

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Tutorials> Quick shift superpixels

Quick shift is a mode seeking algorithm (like mean shift) which instead of iteratively shifting each point towards a local mean instead forms a tree of links to the nearest neighbor which increases the density. For a more in-depth description of the algorithm, see our API reference for quick shift

# Using quick shift to find superpixels

This demo shows quick shift in a simple superpixelization problem where we seek to segment this image:

The image we wish to segment

As a feature vector, we choose the LAB colorspace representation of the image augmented with the x,y location of the pixel. `` vl_quickseg is a convenient wrapper function which takes care of the transformation of the image to LAB and performs segmentation, making our job as easy as:

    ratio = 0.5;
    kernelsize = 2;
    Iseg = vl_quickseg(I, ratio, kernelsize, maxdist);

where ``ratio is the tradeoff between color importance and spatial importance (larger values give more importance to color), ``kernelsize is the size of the kernel used to estimate the density, and ``maxdist is the maximum distance between points in the feature space that may be linked if the density is increased.

The effect of ``maxdist on the superpixelization. As we increase ``maxdist, superpixels become larger and larger since we can link less similar points. Top: ``maxdist=10. Bottom: ``maxdist=20.

# Multiple segmentations

Quick shift arranges all of the data points into a tree where parents in the tree are the nearest neighbors in the feature space which increase the estimate of the density. By imposing a limit on the distance between nearest neighbors (``maxdist), we decrease the amount of computation required to search for the nearest neighbors. However, we also break our tree into a forest, because local modes of the density will now have no neighbor which is close enough in the feature space to form a link.

In the previous section, we created a superpixel segmentation by taking each of the trees in this forest as a distinct cluster. However, since ``maxdist simply prevents new links from forming, the segmentation formed by every ``dist < maxdist is contained in the result. `` vl_quickvis lets us visualize this by running quick shift once and forming multiple segmentations by cutting links in the tree which are smaller and smaller.

    maxdist = 50;
    ndists = 10;
    Iedge = vl_quickvis(I, ratio, kernelsize, maxdist, ndists)
    imagesc(Iedge);
    axis equal off tight;
    colormap gray;

A visualization of multiple ``maxdist thresholds on a single image. Here, boundaries are colored by the largest ``maxdist where the boundary is preserved.