200 lines
7.1 KiB
HTML
200 lines
7.1 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<head>
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<title>ALE User Manual: Alignment</title>
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</head><body>
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<table align=right valign=top width=160>
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<td valign=top height=600 width=160>
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<a href="http://auricle.dyndns.org/ALE/">
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<big>ALE</big>
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<br>
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Image Processing Software
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<br>
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<br>
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<small>Deblurring, Anti-aliasing, and Superresolution.</small></a>
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<br><br>
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<big>
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Local Operation
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</big>
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<hr>
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localhost<br>
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2684415918<br>
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</table>
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<p><b>[ <a href="../">Up</a> ]</b></p>
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<h1>Alignment</h1>
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Alignment compares each supplemental frame in the sequence with an intermediate
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rendering known as the accumulated image. Based on this comparison, a
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transformation is assigned to the supplemental frame. Many different
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transformations can be assigned before alignment of the frame is complete.
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<h2>Match statistics</h2>
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The match statistic is a single number indicating how well two frames are
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aligned. After alignment is complete, ALE displays the final match statistic.
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If the value is close to 100%, then the frames are well aligned. Very low
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values can indicate misalignment. However, even frames that are very well
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aligned do not usually achieve 100% alignment, and for improving image quality,
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ALE works most effectively when match values are less than 100%.
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<h3>Match threshold</h3>
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A match threshold can be specified; no images with final match statistics
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falling below this threshold will contribute to the final output.
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<pre>
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--threshold=x Min. match threshold; a perfect match is 100. (0 is default)
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</pre>
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<h3>Error metric exponent</h3>
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The function calculated at each pixel to determine the match statistic is known
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as the error metric, and is of the form <i>(a-b)<sup>x</sup></i>. The value <i>x</i> is
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known as the error metric exponent, and is 2 by default. Larger numbers
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usually mean that alignment will be more influenced by smaller image features.
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<pre>
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--metric=x Set the error metric exponent. (2 is default)
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</pre>
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<h3>Alignment Channel Options</h3>
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In calculating the per-pixel error metric, there are three ways in which ALE
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can handle color channels. By default, ALE adds the channels before
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calculating the match. However, ALE can also rely solely on the green color
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channel or use all three channels separately.
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<pre>
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--align-all Align images using all color channels
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--align-green Align images using the green channel
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--align-sum Align images using a sum of channels [default]
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</pre>
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<h3>Monte Carlo Alignment</h3>
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Aligning large images can take a very long time if all pixels are examined in
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determining the match statistic, so it is often desirable to examine a smaller
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number of pixels. The Monte Carlo alignment option allows this. The number of
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pixels used is specified as a percentage, and smaller numbers usually mean
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faster, but less precise, alignment.
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<pre>
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--mc <x> Align using, on average, x% of available pixels (0 < x < 100)
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--no-mc Align using all pixels. [default]
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</pre>
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<h2>Perturbation</h2>
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Every dot displayed after an input filename indicates a change in perturbation
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size. The size is initially set to be high, allowing large changes in
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alignment to be evaluated, and is reduced as the locally optimal alignment is
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found for each size. When this size drops below a specified lower bound, the
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frame is considered to be aligned.
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<h3>Perturbation bounds</h3>
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These options determine the upper and lower bounds for perturbation size. The
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perturb-upper and perturb-lower bounds apply to rotation (in degrees),
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translation (in pixels), and the movement of the boundaries of a projected
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frame (in pixels). The rot-upper bound disables rotational perturbation above
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a certain perturbation size (in degrees). To disable alignment, set
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perturb-upper to zero.
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<pre>
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--perturb-upper=x Perturbation upper bound in pixels/degrees (32.0 is default)
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--perturb-lower=x Perturbation lower bound in pixels/degrees (.125 is default)
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--rot-upper=x Rotation-specific perturbation upper bound (32.0 is default)
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</pre>
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<h3>Level of detail</h3>
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<p>Alignment at large perturbation sizes is usually carried out on
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reduced-detail images. To disable this, set lod-max to
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log<sub>2</sub>(perturb-upper).
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<pre>
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--lod-max=x LOD scale factor is max(1, (2^floor(x))/perturb) (1 is def.)
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</pre>
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<h2>Transformations</h2>
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The variables used to adjust alignment are called transformations, and map
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points from a source image to a target image.
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<p>There are three transformation classes that can be used by ALE. The
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translational class applies only translations to the inputs, changing only the
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position of images; the Euclidean class applies translations and rotations; and
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the projective class applies general projective transformations, which are the
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most general type of transformation supported by ALE.
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<p>The Euclidean class is most appropriate for use with scanners and as a first
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pass for projective transformations. The projective class is most appropriate
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for use with cameras.
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<p>Except when capturing flat scenes, ALE does not correct for perspective
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changes, so movement of cameras should ideally be constrained so that no large
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translations occur.
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<h3>Transformation Class Options</h3>
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<pre>
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--translation Only adjust the position of images
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--euclidean Adjust the position and orientation of images [default]
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--projective Use projective transformations. Best quality, but slow.
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</pre>
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<h3>Transformation data file options</h3>
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Transformations used in alignment can be loaded from a file or saved to a file.
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This can be useful when performing alignment in several passes, or when
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refining rendering options.
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<p>Note that projective transformation data saved to a file cannot be used to
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initialize other transformation types.
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<pre>
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--trans-load=x Load initial transformation settings from file x
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--trans-save=x Save final transformation data in file x
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</pre>
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<h3>Alignment following</h3>
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When frames are more closely aligned with adjacent frames in the sequence than
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they are with the first frame in the sequence, alignment success can be
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improved by specifying the --follow option. This option can be effective even
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when initial alignment information is loaded from a file.
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<pre>
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--identity Frames align closely with the original frame. [default]
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--follow Frames align closely with their immediate predecessor.
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</pre>
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<br>
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<hr>
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<i>Copyright 2003 <a href="mailto:dhilvert@auricle.dyndns.org">David Hilvert</a></i>
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<p>Verbatim copying and distribution of this entire article is permitted in any medium, provided this notice is preserved.
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</body>
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</html>
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