next up previous contents
Next: The Tina Colour Segmentation Up: The Tina Colour Tool Previous: RGB Normalisation   Contents

Colour Segmentation

Tina provides an implementation of the colour segmentation algorithm described in more detail in Tina Memo no. 2001-015, as presented at the British Machine Vision Conference in 2001. The algorithm works by non-parametric density estimation in the JK colour space. The red, green and blue fields are converted to the IJK colour space, and the I field discarded to remove illumination effects to first order. The JK colour space is then mapped with points, where the distance between the points is specified by the "resolution" field. All measurements in the colour space are scaled by the local noise, in order to convert to first order into a variance-normalised space and so take proper account of the noise. Hill climbing is then performed on this map of nodes, moving from each pixel to the nearest node, and then across the nodes in the direction of steepest gradient, until a local peak is found. A list of peaks is generated, and each is assigned a label, in order of the number of pixels assigned to that peak. Then the labels are propagated to all pixels assigned to each peak, and an image of labels generated.

The novel aspect of the Tina colour segmentation algorithm is the way in which the peak climbing is performed. The points used to map the colour space can be envisaged as tesselating the space with a set of polygonal regions known as Voronoid cells: each point is at the centre of a cell, and the boundaries of the cell are defined by lines normal to the vector between each point and its immediate neighbours. During the hill climbing stage of the algorithm, each step from point to point is tested to ensure that it only steps to a Voronoid neighbour i.e. does not jump completely across a cell. This ensures that the hill climbing will not jump across local minima, without requiring any implicit assumption of scale (see the discussion in section 3.4). Combined with scaling all distance measurements in the colour space be the local noise, this ensures that proper statistical account is taken of the data accuracy.

The noise scaling in the colour segmentation operates by maintaining a matrix of errors on each pixel. This matrix is regenerated whenever an image is loaded, a colour field is popped from the stack, or a normalisation is performed. This error matrix is then used to perform error propagation through the colour segmentation algorithm. The noise estimation process operates on the assumption that the noise on each colour field is uniform when it is loaded into the tool. This should be a reasonably good approximation for most colour images. However, if you load a colour image and then manipulate a field within Imcalc, before popping it back into the colour tool, you could in theory transform the colour field such that the noise is no longer uniform. This will interfere with the operation of the colour segmentation function.



Subsections
next up previous contents
Next: The Tina Colour Segmentation Up: The Tina Colour Tool Previous: RGB Normalisation   Contents
root 2017-09-26