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<section id="frequently-asked-questions">
<h1>Frequently Asked Questions<a class="headerlink" href="#frequently-asked-questions" title="Link to this heading"></a></h1>
<section id="adjusting-the-options-for-different-reconstruction-scenarios-and-output-quality">
<h2>Adjusting the options for different reconstruction scenarios and output quality<a class="headerlink" href="#adjusting-the-options-for-different-reconstruction-scenarios-and-output-quality" title="Link to this heading"></a></h2>
<p>COLMAP provides many options that can be tuned for different reconstruction
scenarios and to trade off accuracy and completeness versus efficiency. The
default options are set to for medium to high quality reconstruction of
unstructured input data. There are several presets for different scenarios and
quality levels, which can be set in the GUI as <code class="docutils literal notranslate"><span class="pre">Extras</span> <span class="pre">></span> <span class="pre">Set</span> <span class="pre">options</span> <span class="pre">for</span> <span class="pre">...</span></code>.
To use these presets from the command-line, you can save the current set of
options as <code class="docutils literal notranslate"><span class="pre">File</span> <span class="pre">></span> <span class="pre">Save</span> <span class="pre">project</span></code> after choosing the presets. The resulting
project file can be opened with a text editor to view the different options.
Alternatively, you can generate the project file also from the command-line
by running <code class="docutils literal notranslate"><span class="pre">colmap</span> <span class="pre">project_generator</span></code>.</p>
</section>
<section id="extending-colmap">
<h2>Extending COLMAP<a class="headerlink" href="#extending-colmap" title="Link to this heading"></a></h2>
<p>If you need to simply analyze the produced sparse or dense reconstructions from
COLMAP, you can load the sparse models in Python and Matlab using the provided
scripts in <code class="docutils literal notranslate"><span class="pre">scripts/python</span></code> and <code class="docutils literal notranslate"><span class="pre">scripts/matlab</span></code>.</p>
<p>If you want to write a C/C++ executable that builds on top of COLMAP, there are
two possible approaches. First, the COLMAP headers and library are installed
to the <code class="docutils literal notranslate"><span class="pre">CMAKE_INSTALL_PREFIX</span></code> by default. Compiling against COLMAP as a
library is described <a class="reference internal" href="install.html#installation-library"><span class="std std-ref">here</span></a>. Alternatively, you can
start from the <code class="docutils literal notranslate"><span class="pre">src/tools/example.cc</span></code> code template and implement the desired
functionality directly as a new binary within COLMAP.</p>
</section>
<section id="share-intrinsics">
<span id="faq-share-intrinsics"></span><h2>Share intrinsics<a class="headerlink" href="#share-intrinsics" title="Link to this heading"></a></h2>
<p>COLMAP supports shared intrinsics for arbitrary groups of images and camera
models. Images share the same intrinsics, if they refer to the same camera, as
specified by the <cite>camera_id</cite> property in the database. You can add new cameras
and set shared intrinsics in the database management tool. Please, refer to
<a class="reference internal" href="tutorial.html#database-management"><span class="std std-ref">Database Management</span></a> for more information.</p>
</section>
<section id="fix-intrinsics">
<span id="faq-fix-intrinsics"></span><h2>Fix intrinsics<a class="headerlink" href="#fix-intrinsics" title="Link to this heading"></a></h2>
<p>By default, COLMAP tries to refine the intrinsic camera parameters (except
principal point) automatically during the reconstruction. Usually, if there are
enough images in the dataset and you share the intrinsics between multiple
images, the estimated intrinsic camera parameters in SfM should be better than
parameters manually obtained with a calibration pattern.</p>
<p>However, sometimes COLMAP’s self-calibration routine might converge in
degenerate parameters, especially in case of the more complex camera models with
many distortion parameters. If you know the calibration parameters a priori, you
can fix different parameter groups during the reconstruction. Choose
<code class="docutils literal notranslate"><span class="pre">Reconstruction</span> <span class="pre">></span> <span class="pre">Reconstruction</span> <span class="pre">options</span> <span class="pre">></span> <span class="pre">Bundle</span> <span class="pre">Adj.</span> <span class="pre">></span> <span class="pre">refine_*</span></code> and check
which parameter group to refine or to keep constant. Even if you keep the
parameters constant during the reconstruction, you can refine the parameters in
a final global bundle adjustment by setting <code class="docutils literal notranslate"><span class="pre">Reconstruction</span> <span class="pre">></span> <span class="pre">Bundle</span> <span class="pre">adj.</span>
<span class="pre">options</span> <span class="pre">></span> <span class="pre">refine_*</span></code> and then running <code class="docutils literal notranslate"><span class="pre">Reconstruction</span> <span class="pre">></span> <span class="pre">Bundle</span> <span class="pre">adjustment</span></code>.</p>
</section>
<section id="principal-point-refinement">
<h2>Principal point refinement<a class="headerlink" href="#principal-point-refinement" title="Link to this heading"></a></h2>
<p>By default, COLMAP keeps the principal point constant during the reconstruction,
as principal point estimation is an ill-posed problem in general. Once all
images are reconstructed, the problem is most often constrained enough that you
can try to refine the principal point in global bundle adjustment, especially
when sharing intrinsic parameters between multiple images. Please, refer to
<a class="reference internal" href="#faq-fix-intrinsics"><span class="std std-ref">Fix intrinsics</span></a> for more information.</p>
</section>
<section id="increase-number-of-matches-sparse-3d-points">
<h2>Increase number of matches / sparse 3D points<a class="headerlink" href="#increase-number-of-matches-sparse-3d-points" title="Link to this heading"></a></h2>
<p>To increase the number of matches, you should use the more discriminative
DSP-SIFT features instead of plain SIFT and also estimate the affine feature
shape using the options: <code class="docutils literal notranslate"><span class="pre">--SiftExtraction.estimate_affine_shape=true</span></code> and
<code class="docutils literal notranslate"><span class="pre">--SiftExtraction.domain_size_pooling=true</span></code>. In addition, you should enable
guided feature matching using: <code class="docutils literal notranslate"><span class="pre">--SiftMatching.guided_matching=true</span></code>.</p>
<p>By default, COLMAP ignores two-view feature tracks in triangulation, resulting
in fewer 3D points than possible. Triangulation of two-view tracks can in rare
cases improve the stability of sparse image collections by providing additional
constraints in bundle adjustment. To also triangulate two-view tracks, unselect
the option <code class="docutils literal notranslate"><span class="pre">Reconstruction</span> <span class="pre">></span> <span class="pre">Reconstruction</span> <span class="pre">options</span> <span class="pre">></span> <span class="pre">Triangulation</span> <span class="pre">></span>
<span class="pre">ignore_two_view_tracks</span></code>. If your images are taken from far distance with
respect to the scene, you can try to reduce the minimum triangulation angle.</p>
</section>
<section id="reconstruct-sparse-dense-model-from-known-camera-poses">
<h2>Reconstruct sparse/dense model from known camera poses<a class="headerlink" href="#reconstruct-sparse-dense-model-from-known-camera-poses" title="Link to this heading"></a></h2>
<p>If the camera poses are known and you want to reconstruct a sparse or dense
model of the scene, you must first manually construct a sparse model by creating
a <code class="docutils literal notranslate"><span class="pre">cameras.txt</span></code>, <code class="docutils literal notranslate"><span class="pre">points3D.txt</span></code>, and <code class="docutils literal notranslate"><span class="pre">images.txt</span></code> under a new folder:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>+── path/to/manually/created/sparse/model
│ +── cameras.txt
│ +── images.txt
│ +── points3D.txt
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">points3D.txt</span></code> file should be empty while every other line in the <code class="docutils literal notranslate"><span class="pre">images.txt</span></code>
should also be empty, since the sparse features are computed, as described below. You can
refer to <a class="reference internal" href="format.html#output-format"><span class="std std-ref">this article</span></a> for more information about the structure of
a sparse model.</p>
<p>Example of images.txt:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="mi">1</span> <span class="mf">0.695104</span> <span class="mf">0.718385</span> <span class="o">-</span><span class="mf">0.024566</span> <span class="mf">0.012285</span> <span class="o">-</span><span class="mf">0.046895</span> <span class="mf">0.005253</span> <span class="o">-</span><span class="mf">0.199664</span> <span class="mi">1</span> <span class="n">image0001</span><span class="o">.</span><span class="n">png</span>
<span class="c1"># Make sure every other line is left empty</span>
<span class="mi">2</span> <span class="mf">0.696445</span> <span class="mf">0.717090</span> <span class="o">-</span><span class="mf">0.023185</span> <span class="mf">0.014441</span> <span class="o">-</span><span class="mf">0.041213</span> <span class="mf">0.001928</span> <span class="o">-</span><span class="mf">0.134851</span> <span class="mi">2</span> <span class="n">image0002</span><span class="o">.</span><span class="n">png</span>
<span class="mi">3</span> <span class="mf">0.697457</span> <span class="mf">0.715925</span> <span class="o">-</span><span class="mf">0.025383</span> <span class="mf">0.018967</span> <span class="o">-</span><span class="mf">0.054056</span> <span class="mf">0.008579</span> <span class="o">-</span><span class="mf">0.378221</span> <span class="mi">1</span> <span class="n">image0003</span><span class="o">.</span><span class="n">png</span>
<span class="mi">4</span> <span class="mf">0.698777</span> <span class="mf">0.714625</span> <span class="o">-</span><span class="mf">0.023996</span> <span class="mf">0.021129</span> <span class="o">-</span><span class="mf">0.048184</span> <span class="mf">0.004529</span> <span class="o">-</span><span class="mf">0.313427</span> <span class="mi">2</span> <span class="n">image0004</span><span class="o">.</span><span class="n">png</span>
</pre></div>
</div>
<p>Each image above must have the same <code class="docutils literal notranslate"><span class="pre">image_id</span></code> (first column) as in the database (next step).
This database can be inspected either in the GUI (under <code class="docutils literal notranslate"><span class="pre">Database</span> <span class="pre">management</span> <span class="pre">></span> <span class="pre">Processing</span></code>),
or, one can create a reconstruction with colmap and later export it as text in order to see
the images.txt file it creates.</p>
<p>To reconstruct a sparse map, you first have to recompute features from the
images of the known camera poses as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap feature_extractor \
--database_path $PROJECT_PATH/database.db \
--image_path $PROJECT_PATH/images
</pre></div>
</div>
<p>If your known camera intrinsics have large distortion coefficients, you should
now manually copy the parameters from your <code class="docutils literal notranslate"><span class="pre">cameras.txt</span></code> to the database, such
that the matcher can leverage the intrinsics. Modifying the database is possible
in many ways, but an easy option is to use the provided
<code class="docutils literal notranslate"><span class="pre">scripts/python/database.py</span></code> script. Otherwise, you can skip this step and
simply continue as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap exhaustive_matcher \ # or alternatively any other matcher
--database_path $PROJECT_PATH/database.db
colmap point_triangulator \
--database_path $PROJECT_PATH/database.db \
--image_path $PROJECT_PATH/images
--input_path path/to/manually/created/sparse/model \
--output_path path/to/triangulated/sparse/model
</pre></div>
</div>
<p>Note that the sparse reconstruction step is not necessary in order to compute
a dense model from known camera poses. Assuming you computed a sparse model
from the known camera poses, you can compute a dense model as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap image_undistorter \
--image_path $PROJECT_PATH/images \
--input_path path/to/triangulated/sparse/model \
--output_path path/to/dense/workspace
colmap patch_match_stereo \
--workspace_path path/to/dense/workspace
colmap stereo_fusion \
--workspace_path path/to/dense/workspace \
--output_path path/to/dense/workspace/fused.ply
</pre></div>
</div>
<p>Alternatively, you can also produce a dense model without a sparse model as:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap image_undistorter \
--image_path $PROJECT_PATH/images \
--input_path path/to/manually/created/sparse/model \
--output_path path/to/dense/workspace
</pre></div>
</div>
<p>Since the sparse point cloud is used to automatically select neighboring images
during the dense stereo stage, you have to manually specify the source images,
as described <a class="reference internal" href="#faq-dense-manual-source"><span class="std std-ref">here</span></a>. The dense stereo stage
now also requires a manual specification of the depth range:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap patch_match_stereo \
--workspace_path path/to/dense/workspace \
--PatchMatchStereo.depth_min $MIN_DEPTH \
--PatchMatchStereo.depth_max $MAX_DEPTH
colmap stereo_fusion \
--workspace_path path/to/dense/workspace \
--output_path path/to/dense/workspace/fused.ply
</pre></div>
</div>
</section>
<section id="merge-disconnected-models">
<span id="faq-merge-models"></span><h2>Merge disconnected models<a class="headerlink" href="#merge-disconnected-models" title="Link to this heading"></a></h2>
<p>Sometimes COLMAP fails to reconstruct all images into the same model and hence
produces multiple sub-models. If those sub-models have common registered images,
they can be merged into a single model as post-processing step:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">colmap</span> <span class="n">model_merger</span> \
<span class="o">--</span><span class="n">input_path1</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">sub</span><span class="o">-</span><span class="n">model1</span> \
<span class="o">--</span><span class="n">input_path2</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">sub</span><span class="o">-</span><span class="n">model2</span> \
<span class="o">--</span><span class="n">output_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">merged</span><span class="o">-</span><span class="n">model</span>
</pre></div>
</div>
<p>To improve the quality of the alignment between the two sub-models, it is
recommended to run another global bundle adjustment after the merge:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">colmap</span> <span class="n">bundle_adjuster</span> \
<span class="o">--</span><span class="n">input_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">merged</span><span class="o">-</span><span class="n">model</span> \
<span class="o">--</span><span class="n">output_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">refined</span><span class="o">-</span><span class="n">merged</span><span class="o">-</span><span class="n">model</span>
</pre></div>
</div>
</section>
<section id="geo-registration">
<h2>Geo-registration<a class="headerlink" href="#geo-registration" title="Link to this heading"></a></h2>
<p>Geo-registration of models is possible by providing the 3D locations for the
camera centers of a subset or all registered images. The 3D similarity
transformation between the reconstructed model and the target coordinate frame
of the geo-registration is determined from these correspondences.</p>
<p>The geo-registered 3D coordinates can either be extracted from the database
(tvec_prior field) or from a user specified text file.
For text-files, the geo-registered 3D coordinates of the camera centers for
images must be specified with the following format:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">image_name1</span><span class="o">.</span><span class="n">jpg</span> <span class="n">X1</span> <span class="n">Y1</span> <span class="n">Z1</span>
<span class="n">image_name2</span><span class="o">.</span><span class="n">jpg</span> <span class="n">X2</span> <span class="n">Y2</span> <span class="n">Z2</span>
<span class="n">image_name3</span><span class="o">.</span><span class="n">jpg</span> <span class="n">X3</span> <span class="n">Y3</span> <span class="n">Z3</span>
<span class="o">...</span>
</pre></div>
</div>
<p>The coordinates can be either GPS-based (lat/lon/alt) or cartesian-based (x/y/z).
In case of GPS coordinates, a conversion will be performed to turn those into
cartesian coordinates. The conversion can be done from GPS to ECEF
(Earth-Centered-Earth-Fixed) or to ENU (East-North-Up) coordinates. If ENU coordinates
are used, the first image GPS coordinates will define the origin of the ENU frame.
It is also possible to use ECEF coordinates for alignment and then rotate the aligned
reconstruction into the ENU plane.</p>
<p>Note that at least 3 images must be specified to estimate a 3D similarity
transformation. Then, the model can be geo-registered using:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">colmap</span> <span class="n">model_aligner</span> \
<span class="o">--</span><span class="n">input_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">model</span> \
<span class="o">--</span><span class="n">output_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">geo</span><span class="o">-</span><span class="n">registered</span><span class="o">-</span><span class="n">model</span> \
<span class="o">--</span><span class="n">ref_images_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">text</span><span class="o">-</span><span class="n">file</span> <span class="p">(</span><span class="ow">or</span> <span class="o">--</span><span class="n">database_path</span> <span class="o">/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">database</span><span class="o">.</span><span class="n">db</span><span class="p">)</span> \
<span class="o">--</span><span class="n">ref_is_gps</span> <span class="mi">1</span> \
<span class="o">--</span><span class="n">alignment_type</span> <span class="n">ecef</span> \
<span class="o">--</span><span class="n">robust_alignment</span> <span class="mi">1</span> \
<span class="o">--</span><span class="n">robust_alignment_max_error</span> <span class="mf">3.0</span> <span class="p">(</span><span class="n">where</span> <span class="mf">3.0</span> <span class="ow">is</span> <span class="n">the</span> <span class="n">error</span> <span class="n">threshold</span> <span class="n">to</span> <span class="n">be</span> <span class="n">used</span> <span class="ow">in</span> <span class="n">RANSAC</span><span class="p">)</span>
</pre></div>
</div>
<p>By default, the robust_alignment flag is set to 1. If this flag is set, a 3D similarity
transformation will be estimated with a RANSAC estimator to be robust to potential outliers
in the data. In such case, it is required to provide the error threshold to be used in the
RANSAC estimator.</p>
</section>
<section id="manhattan-world-alignment">
<h2>Manhattan world alignment<a class="headerlink" href="#manhattan-world-alignment" title="Link to this heading"></a></h2>
<p>COLMAP has functionality to align the coordinate axes of a reconstruction using
a Manhattan world assumption, i.e. COLMAP can automatically determine the
gravity axis and the major horizontal axis of the Manhattan world through
vanishing point detection in the images. Please, refer to the
<code class="docutils literal notranslate"><span class="pre">model_orientation_aligner</span></code> for more details.</p>
</section>
<section id="mask-image-regions">
<h2>Mask image regions<a class="headerlink" href="#mask-image-regions" title="Link to this heading"></a></h2>
<p>COLMAP supports masking of keypoints during feature extraction two different ways:</p>
<p>1. Passing <code class="docutils literal notranslate"><span class="pre">mask_path</span></code> to a folder with image masks. For a given image, the corresponding
mask must have the same sub-path below this root as the image has below
<code class="docutils literal notranslate"><span class="pre">image_path</span></code>. The filename must be equal, aside from the added extension
<code class="docutils literal notranslate"><span class="pre">.png</span></code>. For example, for an image <code class="docutils literal notranslate"><span class="pre">image_path/abc/012.jpg</span></code>, the mask would
be <code class="docutils literal notranslate"><span class="pre">mask_path/abc/012.jpg.png</span></code>.</p>
<ol class="arabic simple" start="2">
<li><p>Passing <code class="docutils literal notranslate"><span class="pre">camera_mask_path</span></code> to a single mask image. This single mask is applied to all images.</p></li>
</ol>
<p>In both cases no features will be extracted in regions,
where the mask image is black (pixel intensity value 0 in grayscale).</p>
</section>
<section id="register-localize-new-images-into-an-existing-reconstruction">
<h2>Register/localize new images into an existing reconstruction<a class="headerlink" href="#register-localize-new-images-into-an-existing-reconstruction" title="Link to this heading"></a></h2>
<p>If you have an existing reconstruction of images and want to register/localize
new images within this reconstruction, you can follow these steps:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap feature_extractor \
--database_path $PROJECT_PATH/database.db \
--image_path $PROJECT_PATH/images \
--image_list_path /path/to/image-list.txt
colmap vocab_tree_matcher \
--database_path $PROJECT_PATH/database.db \
--VocabTreeMatching.vocab_tree_path /path/to/vocab-tree.bin \
--VocabTreeMatching.match_list_path /path/to/image-list.txt
colmap image_registrator \
--database_path $PROJECT_PATH/database.db \
--input_path /path/to/existing-model \
--output_path /path/to/model-with-new-images
colmap bundle_adjuster \
--input_path /path/to/model-with-new-images \
--output_path /path/to/model-with-new-images
</pre></div>
</div>
<p>Note that this first extracts features for the new images, then matches them to
the existing images in the database, and finally registers them into the model.
The image list text file contains a list of images to extract and match,
specified as one image file name per line. The bundle adjustment is optional.</p>
<p>If you need a more accurate image registration with triangulation, then you
should restart or continue the reconstruction process rather than just
registering the images to the model. Instead of running the
<code class="docutils literal notranslate"><span class="pre">image_registrator</span></code>, you should run the <code class="docutils literal notranslate"><span class="pre">mapper</span></code> to continue the
reconstruction process from the existing model:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap mapper \
--database_path $PROJECT_PATH/database.db \
--image_path $PROJECT_PATH/images \
--input_path /path/to/existing-model \
--output_path /path/to/model-with-new-images
</pre></div>
</div>
<p>Or, alternatively, you can start the reconstruction from scratch:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>colmap mapper \
--database_path $PROJECT_PATH/database.db \
--image_path $PROJECT_PATH/images \
--output_path /path/to/model-with-new-images
</pre></div>
</div>
<p>Note that dense reconstruction must be re-run from scratch after running the
<code class="docutils literal notranslate"><span class="pre">mapper</span></code> or the <code class="docutils literal notranslate"><span class="pre">bundle_adjuster</span></code>, as the coordinate frame of the model can
change during these steps.</p>
</section>
<section id="available-functionality-without-gpu-cuda">
<h2>Available functionality without GPU/CUDA<a class="headerlink" href="#available-functionality-without-gpu-cuda" title="Link to this heading"></a></h2>
<p>If you do not have a CUDA-enabled GPU but some other GPU, you can use all COLMAP
functionality except the dense reconstruction part. However, you can use
external dense reconstruction software as an alternative, as described in the
<a class="reference internal" href="tutorial.html#dense-reconstruction"><span class="std std-ref">Tutorial</span></a>. If you have a GPU with low compute power
or you want to execute COLMAP on a machine without an attached display and
without CUDA support, you can run all steps on the CPU by specifying the
appropriate options (e.g., <code class="docutils literal notranslate"><span class="pre">--SiftExtraction.use_gpu=false</span></code> for the feature
extraction step). But note that this might result in a significant slow-down of
the reconstruction pipeline. Please, also note that feature extraction on the
CPU can consume excessive RAM for large images in the default settings, which
might require manually reducing the maximum image size using
<code class="docutils literal notranslate"><span class="pre">--SiftExtraction.max_image_size</span></code> and/or setting
<code class="docutils literal notranslate"><span class="pre">--SiftExtraction.first_octave</span> <span class="pre">0</span></code> or by manually limiting the number of
threads using <code class="docutils literal notranslate"><span class="pre">--SiftExtraction.num_threads</span></code>.</p>
</section>
<section id="multi-gpu-support-in-feature-extraction-matching">
<h2>Multi-GPU support in feature extraction/matching<a class="headerlink" href="#multi-gpu-support-in-feature-extraction-matching" title="Link to this heading"></a></h2>
<p>You can run feature extraction/matching on multiple GPUs by specifying multiple
indices for CUDA-enabled GPUs, e.g., <code class="docutils literal notranslate"><span class="pre">--SiftExtraction.gpu_index=0,1,2,3</span></code> and
<code class="docutils literal notranslate"><span class="pre">--SiftMatching.gpu_index=0,1,2,3</span></code> runs the feature extraction/matching on 4
GPUs in parallel. Note that you can only run one thread per GPU and this
typically also gives the best performance. By default, COLMAP runs one feature
extraction/matching thread per CUDA-enabled GPU and this usually gives the best
performance as compared to running multiple threads on the same GPU.</p>
</section>
<section id="feature-matching-fails-due-to-illegal-memory-access">
<h2>Feature matching fails due to illegal memory access<a class="headerlink" href="#feature-matching-fails-due-to-illegal-memory-access" title="Link to this heading"></a></h2>
<p>If you encounter the following error message:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">MultiplyDescriptor</span><span class="p">:</span> <span class="n">an</span> <span class="n">illegal</span> <span class="n">memory</span> <span class="n">access</span> <span class="n">was</span> <span class="n">encountered</span>
</pre></div>
</div>
<p>or the following:</p>
<blockquote>
<div><dl class="simple">
<dt>ERROR: Feature matching failed. This probably caused by insufficient GPU</dt><dd><p>memory. Consider reducing the maximum number of features.</p>
</dd>
</dl>
</div></blockquote>
<p>during feature matching, your GPU runs out of memory. Try decreasing the option
<code class="docutils literal notranslate"><span class="pre">--SiftMatching.max_num_matches</span></code> until the error disappears. Note that this
might lead to inferior feature matching results, since the lower-scale input
features will be clamped in order to fit them into GPU memory. Alternatively,
you could change to CPU-based feature matching, but this can become very slow,
or better you buy a GPU with more memory.</p>
<p>The maximum required GPU memory can be approximately estimated using the
following formula: <code class="docutils literal notranslate"><span class="pre">4</span> <span class="pre">*</span> <span class="pre">num_matches</span> <span class="pre">*</span> <span class="pre">num_matches</span> <span class="pre">+</span> <span class="pre">4</span> <span class="pre">*</span> <span class="pre">num_matches</span> <span class="pre">*</span> <span class="pre">256</span></code>.
For example, if you set <code class="docutils literal notranslate"><span class="pre">--SiftMatching.max_num_matches</span> <span class="pre">10000</span></code>, the maximum
required GPU memory will be around 400MB, which are only allocated if one of
your images actually has that many features.</p>
</section>
<section id="trading-off-completeness-and-accuracy-in-dense-reconstruction">
<h2>Trading off completeness and accuracy in dense reconstruction<a class="headerlink" href="#trading-off-completeness-and-accuracy-in-dense-reconstruction" title="Link to this heading"></a></h2>
<p>If the dense point cloud contains too many outliers and too much noise, try to
increase the value of option <code class="docutils literal notranslate"><span class="pre">--StereoFusion.min_num_pixels</span></code>.</p>
<p>If the reconstructed dense surface mesh model using Poisson reconstruction
contains no surface or there are too many outlier surfaces, you should reduce
the value of option <code class="docutils literal notranslate"><span class="pre">--PoissonMeshing.trim</span></code> to decrease the surface are and
vice versa to increase it. Also consider to try the reduce the outliers or
increase the completeness in the fusion stage, as described above.</p>
<p>If the reconstructed dense surface mesh model using Delaunay reconstruction
contains too noisy or incomplete surfaces, you should increase the
<code class="docutils literal notranslate"><span class="pre">--DenaunayMeshing.quality_regularization</span></code> parameter to obtain a smoother
surface. If the resolution of the mesh is too coarse, you should reduce the
<code class="docutils literal notranslate"><span class="pre">--DelaunayMeshing.max_proj_dist</span></code> option to a lower value.</p>
</section>
<section id="improving-dense-reconstruction-results-for-weakly-textured-surfaces">
<h2>Improving dense reconstruction results for weakly textured surfaces<a class="headerlink" href="#improving-dense-reconstruction-results-for-weakly-textured-surfaces" title="Link to this heading"></a></h2>
<p>For scenes with weakly textured surfaces it can help to have a high resolution
of the input images (<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.max_image_size</span></code>) and a large patch window
radius (<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.window_radius</span></code>). You may also want to reduce the
filtering threshold for the photometric consistency cost
(<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.filter_min_ncc</span></code>).</p>
</section>
<section id="surface-mesh-reconstruction">
<h2>Surface mesh reconstruction<a class="headerlink" href="#surface-mesh-reconstruction" title="Link to this heading"></a></h2>
<p>COLMAP supports two types of surface reconstruction algorithms. Poisson surface
reconstruction <a class="reference internal" href="bibliography.html#kazhdan2013" id="id1"><span>[kazhdan2013]</span></a> and graph-cut based surface extraction from a
Delaunay triangulation. Poisson surface reconstruction typically requires an
almost outlier-free input point cloud and it often produces bad surfaces in the
presence of outliers or large holes in the input data. The Delaunay
triangulation based meshing algorithm is more robust to outliers and in general
more scalable to large datasets than the Poisson algorithm, but it usually
produces less smooth surfaces. Furthermore, the Delaunay based meshing can be
applied to sparse and dense reconstruction results. To increase the smoothness
of the surface as a post-processing step, you could use Laplacian smoothing, as
e.g. implemented in Meshlab.</p>
<p>Note that the two algorithms can also be combined by first running the Delaunay
meshing to robustly filter outliers from the sparse or dense point cloud and
then, in the second step, performing Poisson surface reconstruction to obtain a
smooth surface.</p>
</section>
<section id="speedup-dense-reconstruction">
<h2>Speedup dense reconstruction<a class="headerlink" href="#speedup-dense-reconstruction" title="Link to this heading"></a></h2>
<p>The dense reconstruction can be speeded up in multiple ways:</p>
<ul class="simple">
<li><p>Put more GPUs in your system as the dense reconstruction can make use of
multiple GPUs during the stereo reconstruction step. Put more RAM into your
system and increase the <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.cache_size</span></code>,
<code class="docutils literal notranslate"><span class="pre">--StereoFusion.cache_size</span></code> to the largest possible value in order to
speed up the dense fusion step.</p></li>
<li><p>Do not perform geometric dense stereo reconstruction
<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.geom_consistency</span> <span class="pre">false</span></code>. Make sure to also enable
<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.filter</span> <span class="pre">true</span></code> in this case.</p></li>
<li><p>Reduce the <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.max_image_size</span></code>, <code class="docutils literal notranslate"><span class="pre">--StereoFusion.max_image_size</span></code>
values to perform dense reconstruction on a maximum image resolution.</p></li>
<li><p>Reduce the number of source images per reference image to be considered, as
described <a class="reference internal" href="#faq-dense-memory"><span class="std std-ref">here</span></a>.</p></li>
<li><p>Increase the patch windows step <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.window_step</span></code> to 2.</p></li>
<li><p>Reduce the patch window radius <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.window_radius</span></code>.</p></li>
<li><p>Reduce the number of patch match iterations <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.num_iterations</span></code>.</p></li>
<li><p>Reduce the number of sampled views <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.num_samples</span></code>.</p></li>
<li><p>To speedup the dense stereo and fusion step for very large reconstructions,
you can use CMVS to partition your scene into multiple clusters and to prune
redundant images, as described <a class="reference internal" href="#faq-dense-memory"><span class="std std-ref">here</span></a>.</p></li>
</ul>
<p>Note that apart from upgrading your hardware, the proposed changes might degrade
the quality of the dense reconstruction results. When canceling the stereo
reconstruction process and restarting it later, the previous progress is not
lost and any already processed views will be skipped.</p>
</section>
<section id="reduce-memory-usage-during-dense-reconstruction">
<span id="faq-dense-memory"></span><h2>Reduce memory usage during dense reconstruction<a class="headerlink" href="#reduce-memory-usage-during-dense-reconstruction" title="Link to this heading"></a></h2>
<p>If you run out of GPU memory during patch match stereo, you can either reduce
the maximum image size by setting the option <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.max_image_size</span></code> or
reduce the number of source images in the <code class="docutils literal notranslate"><span class="pre">stereo/patch-match.cfg</span></code> file from
e.g. <code class="docutils literal notranslate"><span class="pre">__auto__,</span> <span class="pre">30</span></code> to <code class="docutils literal notranslate"><span class="pre">__auto__,</span> <span class="pre">10</span></code>. Note that enabling the
<code class="docutils literal notranslate"><span class="pre">geom_consistency</span></code> option increases the required GPU memory.</p>
<p>If you run out of CPU memory during stereo or fusion, you can reduce the
<code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.cache_size</span></code> or <code class="docutils literal notranslate"><span class="pre">--StereoFusion.cache_size</span></code> specified in
gigabytes or you can reduce <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.max_image_size</span></code> or
<code class="docutils literal notranslate"><span class="pre">--StereoFusion.max_image_size</span></code>. Note that a too low value might lead to very
slow processing and heavy load on the hard disk.</p>
<p>For large-scale reconstructions of several thousands of images, you should
consider splitting your sparse reconstruction into more manageable clusters of
images using e.g. CMVS <a class="reference internal" href="bibliography.html#furukawa10" id="id2"><span>[furukawa10]</span></a>. In addition, CMVS allows to prune
redundant images observing the same scene elements. Note that, for this use
case, COLMAP’s dense reconstruction pipeline also supports the PMVS/CMVS folder
structure when executed from the command-line. Please, refer to the workspace
folder for example shell scripts. Note that the example shell scripts for
PMVS/CMVS are only generated, if the output type is set to PMVS. Since CMVS
produces highly overlapping clusters, it is recommended to increase the default
value of 100 images per cluster to as high as possible according to your
available system resources and speed requirements. To change the number of
images using CMVS, you must modify the shell scripts accordingly. For example,
<code class="docutils literal notranslate"><span class="pre">cmvs</span> <span class="pre">pmvs/</span> <span class="pre">500</span></code> to limit each cluster to 500 images. If you want to use CMVS
to prune redundant images but not to cluster the scene, you can simply set this
number to a very large value.</p>
</section>
<section id="manual-specification-of-source-images-during-dense-reconstruction">
<span id="faq-dense-manual-source"></span><h2>Manual specification of source images during dense reconstruction<a class="headerlink" href="#manual-specification-of-source-images-during-dense-reconstruction" title="Link to this heading"></a></h2>
<p>You can change the number of source images in the <code class="docutils literal notranslate"><span class="pre">stereo/patch-match.cfg</span></code>
file from e.g. <code class="docutils literal notranslate"><span class="pre">__auto__,</span> <span class="pre">30</span></code> to <code class="docutils literal notranslate"><span class="pre">__auto__,</span> <span class="pre">10</span></code>. This selects the images
with the most visual overlap automatically as source images. You can also use
all other images as source images, by specifying <code class="docutils literal notranslate"><span class="pre">__all__</span></code>. Alternatively, you
can manually specify images with their name, for example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">image1</span><span class="o">.</span><span class="n">jpg</span>
<span class="n">image2</span><span class="o">.</span><span class="n">jpg</span><span class="p">,</span> <span class="n">image3</span><span class="o">.</span><span class="n">jpg</span>
<span class="n">image2</span><span class="o">.</span><span class="n">jpg</span>
<span class="n">image1</span><span class="o">.</span><span class="n">jpg</span><span class="p">,</span> <span class="n">image3</span><span class="o">.</span><span class="n">jpg</span>
<span class="n">image3</span><span class="o">.</span><span class="n">jpg</span>
<span class="n">image1</span><span class="o">.</span><span class="n">jpg</span><span class="p">,</span> <span class="n">image2</span><span class="o">.</span><span class="n">jpg</span>
</pre></div>
</div>
<p>Here, <code class="docutils literal notranslate"><span class="pre">image2.jpg</span></code> and <code class="docutils literal notranslate"><span class="pre">image3.jpg</span></code> are used as source images for
<code class="docutils literal notranslate"><span class="pre">image1.jpg</span></code>, etc.</p>
</section>
<section id="multi-gpu-support-in-dense-reconstruction">
<h2>Multi-GPU support in dense reconstruction<a class="headerlink" href="#multi-gpu-support-in-dense-reconstruction" title="Link to this heading"></a></h2>
<p>You can run dense reconstruction on multiple GPUs by specifying multiple indices
for CUDA-enabled GPUs, e.g., <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.gpu_index=0,1,2,3</span></code> runs the dense
reconstruction on 4 GPUs in parallel. You can also run multiple dense
reconstruction threads on the same GPU by specifying the same GPU index twice,
e.g., <code class="docutils literal notranslate"><span class="pre">--PatchMatchStereo.gpu_index=0,0,1,1,2,3</span></code>. By default, COLMAP runs one
dense reconstruction thread per CUDA-enabled GPU.</p>
</section>
<section id="fix-gpu-freezes-and-timeouts-during-dense-reconstruction">
<span id="faq-dense-timeout"></span><h2>Fix GPU freezes and timeouts during dense reconstruction<a class="headerlink" href="#fix-gpu-freezes-and-timeouts-during-dense-reconstruction" title="Link to this heading"></a></h2>
<p>The stereo reconstruction pipeline runs on the GPU using CUDA and puts the GPU
under heavy load. You might experience a display freeze or even a program crash
during the reconstruction. As a solution to this problem, you could use a
secondary GPU in your system, that is not connected to your display by setting
the GPU indices explicitly (usually index 0 corresponds to the card that the
display is attached to). Alternatively, you can increase the GPU timeouts of
your system, as detailed in the following.</p>
<p>By default, the Windows operating system detects response problems from the GPU,
and recovers to a functional desktop by resetting the card and aborting the
stereo reconstruction process. The solution is to increase the so-called
“Timeout Detection & Recovery” (TDR) delay to a larger value. Please, refer to
the <a class="reference external" href="https://goo.gl/UWKVs6">NVIDIA Nsight documentation</a> or to the <a class="reference external" href="http://www.microsoft.com/whdc/device/display/wddm_timeout.mspx">Microsoft
documentation</a>
on how to increase the delay time under Windows. You can increase the delay
using the following Windows Registry entries:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">HKEY_LOCAL_MACHINE</span>\<span class="n">SYSTEM</span>\<span class="n">CurrentControlSet</span>\<span class="n">Control</span>\<span class="n">GraphicsDrivers</span><span class="p">]</span>
<span class="s2">"TdrLevel"</span><span class="o">=</span><span class="n">dword</span><span class="p">:</span><span class="mi">00000001</span>
<span class="s2">"TdrDelay"</span><span class="o">=</span><span class="n">dword</span><span class="p">:</span><span class="mi">00000120</span>
</pre></div>
</div>
<p>To set the registry entries, execute the following commands using administrator
privileges (e.g., in <code class="docutils literal notranslate"><span class="pre">cmd.exe</span></code> or <code class="docutils literal notranslate"><span class="pre">powershell.exe</span></code>):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">reg</span> <span class="n">add</span> <span class="n">HKEY_LOCAL_MACHINE</span>\<span class="n">SYSTEM</span>\<span class="n">CurrentControlSet</span>\<span class="n">Control</span>\<span class="n">GraphicsDrivers</span> <span class="o">/</span><span class="n">v</span> <span class="n">TdrLevel</span> <span class="o">/</span><span class="n">t</span> <span class="n">REG_DWORD</span> <span class="o">/</span><span class="n">d</span> <span class="mi">00000001</span>
<span class="n">reg</span> <span class="n">add</span> <span class="n">HKEY_LOCAL_MACHINE</span>\<span class="n">SYSTEM</span>\<span class="n">CurrentControlSet</span>\<span class="n">Control</span>\<span class="n">GraphicsDrivers</span> <span class="o">/</span><span class="n">v</span> <span class="n">TdrDelay</span> <span class="o">/</span><span class="n">t</span> <span class="n">REG_DWORD</span> <span class="o">/</span><span class="n">d</span> <span class="mi">00000120</span>
</pre></div>
</div>
<p>and restart your machine afterwards to make the changes effective.</p>
<p>The X window system under Linux/Unix has a similar feature and detects response
problems of the GPU. The easiest solution to avoid timeout problems under the X
window system is to shut it down and run the stereo reconstruction from the
command-line. Under Ubuntu, you could first stop X using:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sudo</span> <span class="n">service</span> <span class="n">lightdm</span> <span class="n">stop</span>
</pre></div>
</div>
<p>And then run the dense reconstruction code from the command-line:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">colmap</span> <span class="n">patch_match_stereo</span> <span class="o">...</span>
</pre></div>
</div>
<p>Finally, you can restart your desktop environment with the following command:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sudo</span> <span class="n">service</span> <span class="n">lightdm</span> <span class="n">start</span>
</pre></div>
</div>
<p>If the dense reconstruction still crashes after these changes, the reason is
probably insufficient GPU memory, as discussed in a separate item in this list.</p>
</section>
</section>
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