Optimization - Revision history

Timnugent: /* Definition */

2009-04-16T13:52:28Z

‎Definition

Erik Krause: wikified, glossarified, added some info

2009-01-10T13:02:18Z

wikified, glossarified, added some info

Erik Krause: added link to John's tutorial

2008-12-21T11:25:11Z

added link to John's tutorial

Mick: New page: John Houghton posted this clear explanation of optimization on the ptgui mailing list. Assume for the moment that your images were taken with a perfectly positioned camera (no parallax) a...

2008-12-20T22:50:10Z

New page: John Houghton posted this clear explanation of optimization on the ptgui mailing list. Assume for the moment that your images were taken with a perfectly positioned camera (no parallax) a...

New page

John Houghton posted this clear explanation of optimization on the ptgui mailing list.

Assume for the moment that your images were taken with a perfectly
positioned camera (no parallax) and a perfect lens (no distortions).
Each image is placed on the spherical stitching surface and warped
(transformed) such that, as viewed from the centre of the stitching
sphere, it looks exactly the same as the original scene viewed from
the camera position. The lens fov determines the size of the images.
It's then possible to simply slide the images around so that they
align exactly in the overlapped areas. To enable the optimizer to
perform this alignment, you assign control points on matching features
in the images. (The optimizer does not "see" the images themselves).
To align the images perfectly, a minimum of only two accurately placed
contol points per overlap is sufficient, though a few more will do no
harm.

Alas, a real world lens is not perfect and commonly suffers from
barrel or pincushion distortion. If uncorrected in the warping
process, this will prevent images aligning perfectly in the overlap
areas. This issue is addressed by the lens parameters a,b and c,
which together control the amount of distortion correction applied.
When enabled, the optimizer will try different values of these
parameters in its efforts to get all the control points to align as
well as possible. Note that to enable the optimizer to align the
images accurately all over an overlap area, it needs control points
nicely spread over that whole area - or at least all along the
anticipated line where the blender will position the seam. If you
already know the optimum values for the parameters a,b and c (from
previous projects), these can be entered via the lens database or a
template or manually. They don't then need to be included in the
optimization and the wide spread of control points is no longer so
important.

Two further factors that cause problems in optimization are object
movement and parallax:

Control points should not be assigned on anything likely to have moved
between shots. The moving object will appear to be in different
positions relative to the background in successive shots. Clearly,
the optimizer will be unable to simultaneously align the stationary
background and the moving object. Aligning the moving object will
compromise the alignment of the background. E.g control points on
moving clouds might be nicely aligned, but the horizon then might
become misaligned or bent. The automatic control point generator
won't avoid moving objects, so you need to be vigilant and delete
points unwisely assigned.

The problems arising from parallax (due to not rotating the camera
about the entrance pupil of the lens) are similar. Near objects appear
to move relative to the background in sucessive shots and so should
not have control points assigned to them. A distant background will
be largely unaffected by parallax and should be aligned well by the
optimizer. Bear in mind that in the case of fisheye lenses, parallax
effects can't generally be avoided completely: the entrance pupil
position varies for light rays entering the lens at different angles
to the lens axis, so it's not a single point.

When the optimizer gives a bad result, with large control point
distances reported, check the placement of the bad points. Try to
account for why the cp distances are so big. If a point is not
assigned accurately on the same feature in both images, correct it so
that it is. If it's accurately positioned already, did the feature
move between shots or is it likely to be affected by parallax? If so,
delete the point. If not, look for another explanation. Maybe the
lens parameters are bad or were not included in the optimization when
they should have been. The shift parameters d and e should be
optimized in the case of fisheye lenses, but be on guard for silly,
unlikely values in any of the lens parameters. Be guided by
experience. You can always manually reset the lens parameters to zero
and try another run.

Hopefully, you will be able to achieve an optimzation in which the
control point distances are very low (less than 2, say), with control
points nicely spread, and the stitched result should then be fine.
Misalignments due to parallax or movement may need correction in
postprocessing, Smartblend can often be helpful in disguising these
errors too, by routing the seams around objects during the blending.
--------------
and then in a follow up post
--------------
The optimizer will never change the positions of the points in the
images. They should be assigned on identical features, but the
optimizer does not access the images and cannot therefore know whether
they are or are not accurately placed.

The optimzer positions the images so as to minimise the separation of
the control points, ideally reducing the cp distances all to 0 when
the points are perfectly aligned (if that is possible). If you have a
lot of bad points and you correct the placement of one point, the
optimizer will change the positions of the images to take account of
this. In doing so, some points may move closer and others further
apart, but the average cp distance should be reduced somewhat. Large
control point distances can be due to inaccurate placement but can
also result from other causes, e.g. poor lens parameters, parallax,
movement as explained in the previous post.

@@ Line 1: / Line 1: @@
 == Definition ==
-{{Glossary|Optimization ist the process of determining the necessary warping for an image pair in order to align given [[control points|control point]] pairs|1}}
+{{Glossary|Optimization is the process of determining the necessary warping for an image pair in order to align given [[control points|control point]] pairs|1}}
 == Optimizer variants ==

← Older revision		Revision as of 11:25, 21 December 2008
Line 94:		Line 94:
	also result from other causes, e.g. poor lens parameters, parallax,		also result from other causes, e.g. poor lens parameters, parallax,
	movement as explained in the previous post.		movement as explained in the previous post.
		+
		+	== External links ==
		+	* Good tutorial on optimization: http://www.johnhpanos.com/optitute.htm

@@ Line 1: / Line 1: @@
-John Houghton posted this clear explanation of optimization on the ptgui mailing list.
+== Definition ==
 Assume for the moment that your images were taken with a perfectly
 positioned camera (no parallax) and a perfect lens (no distortions).
 Each image is placed on the spherical stitching surface and warped
-(transformed) such that, as viewed from the centre of the stitching
+(transformed) such that, as viewed from the center of the stitching
 sphere, it looks exactly the same as the original scene viewed from
-the camera position.  The lens fov determines the size of the images.
+the camera position. The lens fov determines the size of the images.
 It's then possible to simply slide the images around so that they
 align exactly in the overlapped areas.  To enable the optimizer to
@@ Line 12: / Line 20: @@
 in the images.  (The optimizer does not "see" the images themselves).
 To align the images perfectly, a minimum of only two accurately placed
-contol points per overlap is sufficient, though a few more will do no
+control points per overlap is sufficient, though a few more will do no
 harm.
 Alas, a real world lens is not perfect and commonly suffers from
-barrel or pincushion distortion.  If uncorrected in the warping
+[[barrel distortion|barrel]] or [[pincushion distortion]].  If uncorrected in the warping
 process, this will prevent images aligning perfectly in the overlap
-areas.  This issue is addressed by the lens parameters a,b and c,
+areas. This issue is addressed by the [[Lens correction model|lens parameters a,b and c]],
 which together control the amount of distortion correction applied.
 When enabled, the optimizer will try different values of these
-parameters in its efforts to get all the control points to align as
+parameters in its efforts to get all the [[control points]] to align as
 well as possible.  Note that to enable the optimizer to align the
 images accurately all over an overlap area, it needs control points
@@ Line 35: / Line 44: @@
 movement and parallax:
-Control points should not be assigned on anything likely to have moved
+[[Control points]] should not be assigned on anything likely to have moved
 between shots.  The moving object will appear to be in different
 positions relative to the background in successive shots.  Clearly,
@@ Line 46: / Line 55: @@
 points unwisely assigned.
-The problems arising from parallax (due to not rotating the camera
+The problems arising from [[parallax]] (due to not rotating the camera
 about the entrance pupil of the lens) are similar. Near objects appear
-to move relative to the background in sucessive shots and so should
+to move relative to the background in successive shots and so should
 not have control points assigned to them.  A distant background will
 be largely unaffected by parallax and should be aligned well by the
-optimizer. Bear in mind that in the case of fisheye lenses, parallax
+optimizer. Bear in mind that in the case of fisheye lenses, [[parallax]]
-effects can't generally be avoided completely:  the entrance pupil
+effects can't generally be avoided completely:  the [[entrance pupil]]
 position varies for light rays entering the lens at different angles
 to the lens axis, so it's not a single point.
@@ Line 64: / Line 73: @@
 delete the point.  If not, look for another explanation.  Maybe the
 lens parameters are bad or were not included in the optimization when
-they should have been.  The shift parameters d and e should be
+they should have been.  The [[Lens_correction_model#Lens_or_image_shift_d_.26_e_parameters|shift parameters d and e]] should be
 optimized in the case of fisheye lenses, but be on guard for silly,
 unlikely values in any of the lens parameters.  Be guided by
@@ Line 70: / Line 79: @@
 and try another run.
-Hopefully, you will be able to achieve an optimzation in which the
+Hopefully, you will be able to achieve an optimization in which the
 control point distances are very low (less than 2, say), with control
 points nicely spread, and the stitched result should then be fine.
-Misalignments due to parallax or movement may need correction in
+Misalignment due to parallax or movement may need correction in
-postprocessing,  Smartblend can often be helpful in disguising these
+post processing,  [[Smartblend]] can often be helpful in disguising these
 errors too, by routing the seams around objects during the blending.
---------------
-and then in a follow up post
---------------
 The optimizer will never change the positions of the points in the
-images.  They should be assigned on identical features, but the
+images. They should be assigned on identical features, but the
 optimizer does not access the images and cannot therefore know whether
 they are or are not accurately placed.
-The optimzer positions the images so as to minimise the separation of
+The optimizer positions the images so as to minimize the separation of
-the control points, ideally reducing the cp distances all to 0 when
+the [[control points]], ideally reducing the cp distances all to 0 when
-the points are perfectly aligned (if that is possible).  If you have a
+the points are perfectly aligned (if that is possible). If you have a
 lot of bad points and you correct the placement of one point, the
 optimizer will change the positions of the images to take account of
-this.  In doing so, some points may move closer and others further
+this. In doing so, some points may move closer and others further
-apart, but the average cp distance should be reduced somewhat.  Large
+apart, but the average cp distance should be reduced somewhat. Large
 control point distances can be due to inaccurate placement but can
 also result from other causes, e.g. poor lens parameters, parallax,
@@ Line 97: / Line 104: @@
 == External links ==
 * Good tutorial on optimization: http://www.johnhpanos.com/optitute.htm