3.2 Test 2: real case in a cloudy day
Figure 6 shows the base image. In this case a costrained
Delaunay triangulation has been performed and the homologous
points of the vertices are found in the new image to compare.
The preprocessing previously described is performed. Figure 7
shows the new image with the triangulation. It must be
underlined that the triangles obtained have been extracted after
the resampling (affine transformation) of a wider area, to avoid
misregistration on the borders. The comparison will be
performed for every triangle; figure 10 shows the upper-right
triangle after the affine transformation, with the contour of the
detected anomalies. The histogram of the images difference
shows a high peak (figure 8).
The 3D representation of the differences (the triangle of figure 7
has been subtracted) is shown in figure 9 (after shift and non
maxima suppression).
Figure 7. The new image of test 2 with the triangulation
Figure 8. Histogram of images difference of test 2
3.3 Test 3: real case in a sunny day
Figure 11 shows the new image. Figure 12 shows the upper-
right triangle after the affine transformation, which has been
subtracted. The histogram of the images difference shows three
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
humps similar to a Gaussian distribution (figure 13): the sun
causes a false change.
The 3D representation of the differences (figure 14) shows both
positive (true) and negative (false) differences. In the shadowed
zone no differences are detected.
To better understand this case, we perform the differences
limited to the single humps. Figures 15 and 16 show these
differences. It is evident that the judgement of an operator is
necessary, to distinguish the real changes from the false ones.
Figure 9. 3D view of image difference of test 2 after
radiometric shift and non maxima suppression
Figure 10. The upper-right triangle of new image of test 2: the
stain is well detected and isolated.
