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.