Fondazione Bruno Kessler - Technologies of Vision

contains material from
Template Matching Techniques in Computer Vision: Theory and Practice
Roberto Brunelli 2009 John Wiley & Sons, Ltd

2.6 Log-polar imaging

While the representation of image as rectangular arrays is the most widespread, alternative exists. Log-polar mapping, described in Section TM:2.4 is an example of space variant representation providing a spatial resolution decreasing from the center towards the boundary.

Log-polar mapping is usually obtained with specific sensors but can also be simulated via software to appreciate some of its characteristics. When we change the image representation from a (standard) rectangular lattice to a log-polar spatio-variant structure we are effectively performing a resampling task which is prone to aliasing if frequency content is not properly tuned. As log-polar sensor resolution is spatio-variant so must be the amount of smoothing applied before resampling. Usually, the two sensing structures are compared for a given maximum spatial resolution. In the case of a rectangular sensor the maximum resolution extends over the whole sensor while in the case of a log-polar sensor it is confined to the central (foveal) region.

1 # We select an image portion 
2 # 
3 A <- ia.get(i_orthoRed, animask(472,272,257,257)) 
4 tm.plot("figures/lpOrig", ia.show(A, main="Original")) 
5 # 
6 # and we smooth it, increasingly so towards the perifery, 
7 # preparing it for a log-polar mapping with an angular 
8 # resolution of 72 cells 
9 # 
10 sA <- tm.logPolarSmoothing(A, angularResolution = 72) 
11 tm.plot("figures/lpSmooth", ia.show(sA, main="Smoothed")) 
12 # 
13 # and project/back project to get a feeling of the log-polar mapping with 
14 # 3960 pixels instead of 51471 
15 # 
16 lpeA      <- tm.logPolarMapEquivalent(sA, R = 128, 72) 
17 # 
18 # We now create the 'real' log-polar map where the horizontal 
19 # coordinate corresponds to rings, while the vertical one 
20 # represents the angular 'sector' 
21 # 
22 lpmA      <- tm.logPolarMap(sA, 128, angularResolution = 72) 
23 # 
24 # and we plot it with a gamma correction of 2.2 
25 # 
26 tm.plot("figures/lpMap", 
27 ...    plot(pixmapGrey((lpmA)**(1.0/2.2)), 
28 ...    main="Log polar map with k=72")) 
29 # 
30 # comparing it to the cartesian back projection 
31 # (with a gamma correction of 2.2) 
32 # 
33 tm.plot("figures/lpEquiv", 
34 ...    plot(pixmapGrey((lpeA@data)**(1.0/2.2)), 
35 ...    main="Cartesian equivalent of a log-polar map with k=72"))


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Figure 2.9: Log-polar mapping, providing image representation with spatio-variant resolution, requires a complex smoothing pre-processing. The images in the bottom row present the resulting map (left) and its back-projection in the standard image representation.