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<title>ALE User Manual: Rendering</title>
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<table align=right valign=top width=160>
<td valign=top height=600 width=160>
<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>ALE Rendering</h1>
Rendering determines how an output image is constructed from a series of
aligned input frames. The incremental renderers, merging and drizzling, update
the output image after reading each new frame from the series, whereas the
non-incremental renderers wait until the last frame has been loaded to produce
output. More than one renderer can be involved in producing the final output image.
Available rendering options are outlined below.
<h2>Merging</h2>
Merging, the default incremental renderer, works most effectively on images
that have no large changes in scale (e.g. camera zooming). Since it uses
bilinear interpolation, output images are approximately convolved with the
triangle, or Bartlett, filter, and may appear to have reduced contrast and
resolution in comparison with other renderers.
<h2>Drizzling</h2>
The drizzling renderer is an incremental renderer that can usually produce
better images than merging, but it works most effectively with devices having
very little blur and optical distortion. Devices sampling very small angles of
the scene at each pixel are ideal; hence, if very fine staircase aliasing is
visible in the input frames, drizzling should work very well. (For creation of
higher resolution images with drizzling, use also the --scale option described
below.)
<pre>
--drizzle-diam=x Drizzle with input pixel diameter x (where 0 &lt; x &lt;= 1).
--drizzle-only If drizzling, output black for pixels with no drizzle data.
</pre>
<p>For more information about drizzling, see the paper by Richard Hook and
Andrew Fruchter, "Variable-Pixel Linear Combination", or the following URL:
<p><a href="http://www.cv.nrao.edu/adass/adassVI/hookr.html">http://www.cv.nrao.edu/adass/adassVI/hookr.html</a>
<h2>High-frequency enhancement</h2>
<p>This renderer uses an unsharp mask to enhance the contrast of high
frequencies. It can be used to compensate for systematic blurriness introduced
by merging, drizzling, or the image capture device. However, better results
will generally be achievable by using a separate postprocessing tool, such as
<a href="http://www.gimp.org">the GIMP</a> or <a
href="http://cinepaint.sourceforge.net/">CinePaint</a> (formerly called Film
GIMP). In addition to the built-in unsharp mask, the GIMP can make use of
Ernest Lippe's <a href="http://refocus.sourceforge.net/">refocus</a> plugin.
<pre>
--hf-enhance=x Enhance high frequency details by factor x. (0.0 is default)
</pre>
<h2>Irani-Peleg iterative image reconstruction</h2>
This renderer is by far the most general, and can be used to compensate for
blurring introduced by the image capture device. In order to use it most
effectively, an approximation of the projection function for the capture device
should be known. Two general classes of projection functions are supported: box
filters and custom filters.
<p>If rendering with the box filter option, manual experimentation can be used
to find a diameter that works well. Alternatively, ALE includes a script that
calibrates custom filters for devices, based on images captured from known
scenes.
<p>Two custom filter options are included: one for the IBM PC Camera Pro
(XVP610) in 320x240 mode, and one for interactive filter specification.
<p>Note that this option can take a long time to produce output, especially on
large images. If incremental output is enabled (see below), output is written after
each iteration is complete. Using more iterations will generally improve
output, up to the point of convergence, when additional iterations have no
effect. A dot is displayed to indicate the completion of each iteration.
<pre>
--ip &lt;d> &lt;i> Solve for a box filter with diameter &lt;d> over &lt;i> iterations.
--ipc &lt;c> &lt;i> Solve for device config &lt;c> over &lt;i> iterations.
</pre>
The available device configurations are as follows:
<pre>
xvp610_320x240
stdin
</pre>
<p>For more information on the backprojection technique of Michal Irani and Shmuel
Peleg, see their paper, "Improving Resolution by Image Registration", at
<p><a href="ftp://ftp.wisdom.weizmann.ac.il/pub/irani/PAPERS/SR_CVGIP91.pdf">ftp://ftp.wisdom.weizmann.ac.il/pub/irani/PAPERS/SR_CVGIP91.pdf</a>
<h2>Scale factor</h2>
Rendering with a larger scale factor causes internal data structures to store
more information, resulting in image output of the specified larger scale.
Larger scales can improve alignment precision and output image quality, but can
also increase alignment and rendering times.
<pre>
--scale=x Scale images by the factor x (where x is at least 1.0)
</pre>
<h2>Image extents</h2>
The --extend option records pixel data that falls outside of the region of the
first frame in the sequence. This can be useful for providing additional
alignment constraints or creating panoramic image mosaics. (See also the
description of the --follow option in the <a href="../alignment">alignment</a>
section.)
<pre>
--extend Increase image extents to accommodate all pixel data.
--no-extend Don't increase extents; crop to original frame. [default]
</pre>
<h2>Incremental output</h2>
By default, the output file is written every time a renderer completes an
update cycle. In order to suppress output until the final image is rendered,
specify the --no-inc option.
<pre>
--inc Produce incremental output. [default]
--no-inc Don't produce incremental output.
</pre>
<h2>Pixel replacement</h2>
<p>When using the merging renderer, --replace indicates that each new frame
should replace the corresponding region of the rendering, instead of being
merged with data from previous frames. This option was originally suggested by
Jeff Treece for use in stabilization of video streams. His approach to
stabilization is outlined below.
<h3>Video Stabilization</h3>
<p>If the files
<pre>0.ppm, 1.ppm, ..., 9.ppm</pre>
represent frames from an unstabilized video stream, then the following sequence
of commands may result in a stabilized sequence:
<pre>
ale --replace 0.ppm 1.ppm 1.stable.ppm
ale --replace 1.stable.ppm 2.ppm 2.stable.ppm
ale --replace 2.stable.ppm 3.ppm 3.stable.ppm
.
.
.
ale --replace 8.stable.ppm 9.ppm 9.stable.ppm
</pre>
<p>The resulting stabilized sequence would be:
<pre>0.ppm, 1.stable.ppm, 2.stable.ppm, ..., 9.stable.ppm</pre>
<h3>Option Syntax</h3>
<pre>
--replace Replace overlapping areas rather than merging.
--no-replace Do not replace. [default]
</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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