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CCTAG scaling

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Simon edited this page Nov 21, 2020 · 36 revisions

CCTAGs can now be used to scale a model. (MR v.2020)

Notes on CCTAG Markers:

CCTAG scaling

  • Place two markers in your scene and capture the dataset (you should know the two marker IDs, as they can not be shown in the GUI at the moment) Measure the distance between the to Marker centre points. (Best use a distance of 10, 20,..., 100, ... mm)

  • Enable the DescriberType "CCTAG (3)" in FeatureExtraction (Do not disable SIFT here, it is still required)

  • Add the SfMTranform node:

sfmt1

OR

sfmt2

  • In SfMTranform only select cctag3

  • As Transformation Method select from_markers and add two markers. Insert the two marker IDs.

  • Let´s assume the distance between Marker0 and Marker1 is 100mm. To scale the Model to this known size, set the Marker1 x-coord to 1 (1 -> 100mm) and the Additional Scale to 1. (100*1). (The Coord. scale is 100mm per unit. In addition with the Additional Scale (multiplicand) other distances can be set). The origin of the coordinate system is set in the centre of the first Marker.

cctag

  • To apply the correct model orientation use a new SfMTranform node with Transformation Method manual transformation or Transformation Method "From_Single_Camera".

  • CCTAGs can also be used to align two models using the SFMAlignment node

https://groups.google.com/forum/#!topic/alicevision/t4kcSceAFD4 Similar to https://github.com/alicevision/meshroom/issues/1135

Using multiple markers for scaling and orientation

It is possible to use multiple markers for orientation and scaling. This is for advanced users.

It is required to select CCTAG describer in FeatureExtration, FeatureMatching and StructureFromMotion.

At the moment it is not possible to display the marker ID in the Meshroom GUI. It is recommended to print markers with centre point and id or manually add the marker id on the paper as reference.

For the scaling and orientation to work, the CCTAG markers need to be positioned in a known distance.

Example for four markers on a paper sheet, placed in a square, clockwise, with object of interest in the middle:

cctag3: id0, id1, id2, id3, placed in a square with an equal distance of 100mm

In the FromMarker settings you now set the XYZ coordinates for the markers. Since the markers are placed on a flat paper, placed on a desk, the workplane is the XZ plane, therefore Y=0

clockwise (id0-3)

id0 -> X1 Y0 Z1

id1 -> X-1 Y0 Z1

id2 -> X-1 Y0 Z-1

id3 -> X1 Y0 Z-1

(3D viewer axis: x=red y=green z=blue line=positive direction)

This will place the object in the middle of the area surrounded by the four cctags in the centre of the coordinate system.

cctag222

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