me XXXIX-B3, 2012
' check points uniformly
4km x 4km. The average
pixel size of these large
' image size is 12096 rows
' average groundel size is
PC with the CPU of Intel
\M of 3GB, as well as the
fessional SP3, where all
selected for testing the
extraction, AFTP and QF,
ction for 11 images spent
AFTP is 0.2, there are 28
loop number is reduced
AFTP" means the distance
hreshold in order to select
put points to the AFTP.
6 seconds. QF for all key
conds. Figure 7 illustrates
iatically by the AFTP for
dvantage of the proposed
ud matching and aero
need on image overlap
ilation time (seconds)
6575
116
567
n time
y AFTP for the 11 test
int density for these 108
pyramid, where a stricter
IFT is adopted to extract
3 illustrates the key point
rently, a higher level of
, owns a denser cloud of
density of key points
0 threshold of SIFT but
ation as well as image
tes the locations of key
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B3, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
points extracted from two images where red +, green © , blue 9,
magenta X and cyan 4 denote key points extracted on the
level 0, 1, 2, 3, and 4, respectively. The image 10123 (left) has
better quality than the image 60185 (right) so that it has much
denser cloud of key points. Both images 10123 and 60185 have
the same image size of 12096 x 11200 pixels. The same
distance ratio of 0.2 determines 359404 and 55269 key points
on the image 10123 and 60185, respectively.
Moreover, Figure 10 illustrates the location of key points after
quality filtering on different pyramid level. Apparently, they are
located at those local feature points with good contrast and
textures. Table 3 shows summarily the number and density of
key points without and with QF for these 108 test images. For
example, Figure 11 illustrates the location of key points without
and with quality filtering operation for both good image 10123
and bad image 60185. They show clearly that QF reserves less
number of best key points.
Level Level Level Level Level
0 1 2 3 4
Avg. 1.38 2.34 339 4.86 7.29
Max 2.72 4.08 5.97 8.15 12.01
Min 0.41 0.57 0.86 1.41 2.24
RMSD 0.61 0.90 1.32 1.88 2.68
Image 12096 | 6048 3024 1512 756
size X x 5600 | x 2800 | x 1400 | x 700
(pixels) | 11200
Table 2. Key point density (unit: points / 1000pixels) for the
108 test images on different level of image pyramid
key point density
m Max
aan xs
TEX
^m AVG
300 -
A RMSE
i die
MIN
Ba
level d igval 1 level 2 levet 3 devel 4
Figure 8. Key point density on each pyramid level (unit: points
/1000 pixels)
To investigate the operational efficiency of the method, twelve
test cases are also done, and the results are briefly shown in
Table 4. In Table 4, Cases I, II and III denote the operation with
only QF, with only AFTP, with both QF and AFTP,
respectively. The cases 1,2,3 and 4 denote the operation with
the distance ratio of 0.20, 0.25, 0.30 and 0.35, respectively. The
number of skipped points denotes the number of image points
eliminated in the free network adjustment of AT. Thus, the ratio
of the number of skipped points divided by the number of
matched points, denoted as the skip rate, describes the goodness
of matching in each case. As shown in Figure 12, when distance
ratio is set to the threshold less than 0.3, Case III, namely
operation with both QF and AFTP, maintains better matching
efficiency and best matching with lowest rate of skipped points.
In general, the larger the distance ratio is, more points are
matched per second, but the more the skipped points become.
Moreover, Table 4 shows apparently that the calculation time is
almost the same for the same case (I, II, or III).
i i 2 2m. 23 : TE
Figure 9. Extracted key points (bottom) from two original
images (top) 10123 (left) and 60185 (right)
ocation of key points after QF
Figure 10.
Number of key Density (key points /
points 1000pixels)
Level 0 | After QF Level 0 After QF
AVG 186939 27751 1.38 0.20
MAX 368206 53042 2.72 0.39
MIN 55269 7344 0.41 0.05
RMSD 82744 12186 0.61 0.09
71
Table 3. Number and density of key points without and with QF
