Thanks for the explanation. This kind of background is essential
to understand the methods and thus use them right.
Just to fully clarify, for a given keypoint in image1, is the
ratio of "the distance to the closest to the distance to the
second closest" computed
to all keypoints of image 2 or just to those within a radius
equal to precision only? If the ratio is computed to all
keypoints of image 2 we could often have
points with similar radiometry, thus would get a high ratio and
erroneously discard a correct matching. While the probability of
having a non-homologous point with similar radiometry within the
radius of precision would be low and thus the erroneous rejection
would be rare.
Also, why is "the signature is a 128 dimension vector" ? Are you
using a window instead of the actual pixel of the keypoint?
Agus
On Mon, Aug 31, 2015 at 3:12 PM, Julien Michel
<[email protected] <mailto:[email protected]>> wrote:
Le 31/08/2015 14:29, Agustin Lobo a écrit :
Actually, I do not quite understand:
Is mfilter stating whether homologous points are filtered
according to the value
of "precision"?
yes
What do you mean by " padded according to the user defined
*precision*.?
I mean that in geobins mode, we consider square tiles in
reference image and predict the location of the same tile on
the secondary image. We take a slightly larger region on the
secondary image, to account for sensor model precision (given
by parameter "precision"). The idea is the following : the
naive matching cost is O(n1xn2), where n1 and n2 are the
number of keypoints found in each image. Therefore if you try
to match large images, it will take forever ... But if you
divide your reference image in bins, and predict roughly
where the bin is in the second image, you only match a few
point at a time, and therefore gain speed (and robustness,
since it will constrain the range within which false matches
occur).
No padding is done in full mode (obviously), which is the one
you use.
What is threshold? Is it a radiometric distance or an
spatial distance?
Threshold is used during matching. Each keypoint has a
signature, and the matching algorithm works as follows :
- Euclidean distance is computed between the signatures of
each keypoint of the reference image and each keypoint of the
secondary image (hence the O(n1xn2) cost).
- A naive matching approach would consider that the correct
match has the minimum distance
- However, this does not allow to discard matches (each point
will have a match). But we know that some points should not
have matches (because they were not detected in both images).
We therefore need a way to discard them.
- We could filter them by considering that the distance
should be minimum AND bellow a given threshold. However, the
signature is a 128 dimension vector, and setting such a
threshold is not easy,
- A much more robust way of finding matches is by
thresholding the ratio from the distance to the closest to
the distance to the second closest. In fact, the more this
ratio is close to 1 the more the match is ambiguous (because
there exist a second point in about the same distance). The
thresold parameter is exactly this : the thresold above with
the match will be discarded by the matching algortihm.
Let me explain. 0.6 is a fair default value and used in many
papers / implementaiton. By setting threshold to 10, you
discard the filter on distance ratio (remember the max is 1).
Therefore, you have one match for each of the detected point
in image 1.
You therefore end up with a lot of false matches, and this is
where mfilter comes : since you are using this option, all
matches that are more that "precision" away of the the
homologous point predicted by sensor modelling get discarded
(therefore, you discard 11469 points).
To cut it short : with threshold = 0.6, you have a few
homologous points, but none of them get discarded by the
posteriori filtering, probably because they are good.
With threshold = 10, you have a lot of homologous points, but
most of them get discareded by the a posteriori filter.
I have tested and by increasing threshold I get more
homologous points but also more of them get discarded (all
other parameters equal):
$ otbcli_HomologousPointsExtraction -in1 IMG_0062.tif -band1
2 -in2 IMG_0062.tif -band2 1 -mode full -precision 50
-mfilter 1 -algorithm surf -out IMG_0062homSURF2.txt
-threshold 0.6
2015 Aug 31 13:58:34 : Application.logger (INFO) Elevation
management: setting default height above ellipsoid to 0 meters
2015 Aug 31 13:58:34 : Application.logger (INFO) Doing update
2015 Aug 31 13:58:40 : Application.logger (INFO) Found
15650 surf points in image 1.
2015 Aug 31 13:58:47 : Application.logger (INFO) Found
15722 surf points in image 2.
2015 Aug 31 13:59:14 : Application.logger (INFO) Found 740
homologous points.
2015 Aug 31 13:59:14 : Application.logger (INFO) 0 points
discarded
$ otbcli_HomologousPointsExtraction -in1 IMG_0062.tif -band1
2 -in2 IMG_0062.tif -band2 1 -mode full -precision 50
-mfilter 1 -algorithm surf -out IMG_0062homSURF2.txt
-threshold 10
2015 Aug 31 13:59:26 : Application.logger (INFO) Elevation
management: setting default height above ellipsoid to 0 meters
2015 Aug 31 13:59:26 : Application.logger (INFO) Doing update
2015 Aug 31 13:59:32 : Application.logger (INFO) Found
15650 surf points in image 1.
2015 Aug 31 13:59:38 : Application.logger (INFO) Found
15722 surf points in image 2.
2015 Aug 31 14:00:07 : Application.logger (INFO) Found
15650 homologous points.
2015 Aug 31 14:00:08 : Application.logger (INFO) 11469
points discarded
Thanks!
Agus
On Wed, Jun 10, 2015 at 2:52 PM, Julien Michel
<[email protected] <mailto:[email protected]>> wrote:
Hi Agus,
The readthedoc documentation export is experimental
(Rashad did this a while ago), and surely has some
unsolved issues for now. If you refer to the official
documentation here :
https://www.orfeo-toolbox.org/CookBook/CookBooksu103.html#x136-6690005.6.7
You will see that the table is right. For instance :
precision
Float
Estimated precision of the colocalisation function (in
pixels).
mfilter
Boolean
Filter points according to geographical or sensor based
colocalisation
Regarding the precision parameter documentation, it is
used at several steps of the algorithm, as described in
the Detailed Description of the application :
"In this mode, the corresponding spatial bin in the
second image is estimated using geographical transform
or sensor modelling, and is padded according to the user
defined *precision*."
"Last, in both modes the application can filter matches
whose colocalisation in first image exceed this
*precision*."
But we can update the documentation of the parameter if
you think this is not clear enough.
Regards,
Julien
Le 10/06/2015 10:00, Agustin Lobo a écrit :
Doc on Homologous Point Extraction
http://otbcb.readthedocs.org/en/latest/Applications/app_HomologousPointsExtraction.html
is incomplete.
For example, the only information provided on argument mfilter
is that it is a Boolean. No definition, no default value, no
example.
Instead, the command line help page is more informative:
"-mfilter <boolean> Filter points according to
geographical or sensor based colocalisation (optional, off by
default)"
although it is not clear if the user has to write "on" or "1" to
turn
it on. And it is not clear either what kind of filter (mean filter?)
and which radius
Regarding "precision", there is also no definition in the table (the
information on
Parameter Type is just repeated) but it is later defined as in the
command line help page:
"Estimated precision of the colocalisation function (in pixels).
Estimated precision of the colocalisation function in pixels."
I do not say this definition is wrong, but it is no appropriate. The
definition must
state the role of the argument in the process. In this case, I
understand that the
algorithm searches, for a given point image in1 band1, an
homologous point on
image in2 band2 within a radius equal to the value of precision in
pixels.
Correct?
Also, what happens if the user sets precision to i.e. 10 and
mfilter is off?
Thanks
Agus
--
Julien MICHEL
CNES - DCT/SI/AP - BPI 1219
18, avenue Edouard Belin
31401 Toulouse Cedex 09 - France
Tel: +33 561 282 894 - Fax: +33 561 283 109
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CNES - DCT/SI/AP - BPI 1219
18, avenue Edouard Belin
31401 Toulouse Cedex 09 - France
Tel: +33 561 282 894 - Fax: +33 561 283 109
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