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<title>ALE User Manual: Alignment</title>
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<table align=right valign=top width=160>
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<a href="http://auricle.dyndns.org/ALE/">
<big>ALE</big>
<br>
Image Processing Software
<br>
<br>
<small>Deblurring, Anti-aliasing, and Superresolution.</small></a>
<br><br>
<big>
Local Operation
</big>
<hr>
localhost<br>
2684415918<br>
</table>
<p><b>[ <a href="../">Up</a> ]</b></p>
<h1>Alignment</h1>
Alignment compares each supplemental frame in the sequence with an intermediate
rendering known as the accumulated image. Based on this comparison, a
transformation is assigned to the supplemental frame. Many different
transformations can be assigned before alignment of the frame is complete.
<h2>Match statistics</h2>
The match statistic is a single number indicating how well two frames are
aligned. After alignment is complete, ALE displays the final match statistic.
If the value is close to 100%, then the frames are well aligned. Very low
values can indicate misalignment. However, even frames that are very well
aligned do not usually achieve 100% alignment, and for improving image quality,
ALE works most effectively when match values are less than 100%.
<h3>Match threshold</h3>
A match threshold can be specified; no images with final match statistics
falling below this threshold will contribute to the final output.
<pre>
--threshold=x Min. match threshold; a perfect match is 100. (0 is default)
</pre>
<h3>Error metric exponent</h3>
The function calculated at each pixel to determine the match statistic is known
as the error metric, and is of the form <i>(a-b)<sup>x</sup></i>. The value <i>x</i> is
known as the error metric exponent, and is 2 by default. Larger numbers
usually mean that alignment will be more influenced by smaller image features.
<pre>
--metric=x Set the error metric exponent. (2 is default)
</pre>
<h3>Alignment Channel Options</h3>
In calculating the per-pixel error metric, there are three ways in which ALE
can handle color channels. By default, ALE adds the channels before
calculating the match. However, ALE can also rely solely on the green color
channel or use all three channels separately.
<pre>
--align-all Align images using all color channels
--align-green Align images using the green channel
--align-sum Align images using a sum of channels [default]
</pre>
<h3>Monte Carlo Alignment</h3>
Aligning large images can take a very long time if all pixels are examined in
determining the match statistic, so it is often desirable to examine a smaller
number of pixels. The Monte Carlo alignment option allows this. The number of
pixels used is specified as a percentage, and smaller numbers usually mean
faster, but less precise, alignment.
<pre>
--mc &lt;x> Align using, on average, x% of available pixels (0 < x < 100)
--no-mc Align using all pixels. [default]
</pre>
<h2>Perturbation</h2>
Every dot displayed after an input filename indicates a change in perturbation
size. The size is initially set to be high, allowing large changes in
alignment to be evaluated, and is reduced as the locally optimal alignment is
found for each size. When this size drops below a specified lower bound, the
frame is considered to be aligned.
<h3>Perturbation bounds</h3>
These options determine the upper and lower bounds for perturbation size. The
perturb-upper and perturb-lower bounds apply to rotation (in degrees),
translation (in pixels), and the movement of the boundaries of a projected
frame (in pixels). The rot-upper bound disables rotational perturbation above
a certain perturbation size (in degrees). To disable alignment, set
perturb-upper to zero.
<pre>
--perturb-upper=x Perturbation upper bound in pixels/degrees (32.0 is default)
--perturb-lower=x Perturbation lower bound in pixels/degrees (.125 is default)
--rot-upper=x Rotation-specific perturbation upper bound (32.0 is default)
</pre>
<h3>Level of detail</h3>
<p>Alignment at large perturbation sizes is usually carried out on
reduced-detail images. To disable this, set lod-max to
log<sub>2</sub>(perturb-upper).
<pre>
--lod-max=x LOD scale factor is max(1, (2^floor(x))/perturb) (1 is def.)
</pre>
<h2>Transformations</h2>
The variables used to adjust alignment are called transformations, and map
points from a source image to a target image.
<p>There are three transformation classes that can be used by ALE. The
translational class applies only translations to the inputs, changing only the
position of images; the Euclidean class applies translations and rotations; and
the projective class applies general projective transformations, which are the
most general type of transformation supported by ALE.
<p>The Euclidean class is most appropriate for use with scanners and as a first
pass for projective transformations. The projective class is most appropriate
for use with cameras.
<p>Except when capturing flat scenes, ALE does not correct for perspective
changes, so movement of cameras should ideally be constrained so that no large
translations occur.
<h3>Transformation Class Options</h3>
<pre>
--translation Only adjust the position of images
--euclidean Adjust the position and orientation of images [default]
--projective Use projective transformations. Best quality, but slow.
</pre>
<h3>Transformation data file options</h3>
Transformations used in alignment can be loaded from a file or saved to a file.
This can be useful when performing alignment in several passes, or when
refining rendering options.
<p>Note that projective transformation data saved to a file cannot be used to
initialize other transformation types.
<pre>
--trans-load=x Load initial transformation settings from file x
--trans-save=x Save final transformation data in file x
</pre>
<h3>Alignment following</h3>
When frames are more closely aligned with adjacent frames in the sequence than
they are with the first frame in the sequence, alignment success can be
improved by specifying the --follow option. This option can be effective even
when initial alignment information is loaded from a file.
<pre>
--identity Frames align closely with the original frame. [default]
--follow Frames align closely with their immediate predecessor.
</pre>
<br>
<hr>
<i>Copyright 2003 <a href="mailto:dhilvert@auricle.dyndns.org">David Hilvert</a></i>
<p>Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
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