Tutorial Part 5

Tutorial contents:

Creating, Training and Adding Markers

What are markers?

Markers are physical artifacts that BuildAR Pro tracks in order to align virtual objects with the real world. BuildAR Pro is able to locate, identify and track markers by examining the captured video image using computer vision techniques. In order for this process to work, markers must have distinguishing characteristics that allow them to be discovered quickly and efficiently by the tracking algorithm. The types of markers used by BuildAR Pro follow certain basic design guidelines:

  • The markers must be square
  • There should be a thick black border that is quarter as wide as the entire marker
  • There must be high contrast around the border (i.e. keep some white space free around the marker so the square clearly stands out)

Therefore, the markers all have generally the same appearance of a black frame. To differentiate between multiple markers, and to resolve the correct orientation of a marker, the marker must also have a unique pattern within the frame. Again, there are rules about how to design a pattern that will work:

  • The pattern should be bold (large obvious designs work better than fine details)
  • Use black and white, or at least aim for high contrast
  • Ensure the design is not rotationally symmetric (more on this below)
  • Ensure that no two markers share a similar pattern

The “not rotationally symmetric” requirement means that the pattern should not look the same when rotated. If you place the marker flat on a desk, and turn it in steps of ninety degrees, the appearance should be different at each step. Consider the case of a solid black circle in the middle of the marker. It appears identical no matter what angle it is rotated to, so you are unable to be sure what angle it is really at. Likewise, the software won’t be able to tell either. The design could be fixed by offsetting the circle from the center, or adding a second shape as a reference point.

Creating Markers

You can create markers in a number of ways. The most common method is to use a graphics application to design your marker and then print it out. However, you can also create markers manually out of craft materials. For example, creating markers out of adhesive-backed felt has the advantage that the felt is a matte surface, so you don’t get bright specular highlights which can interfere with tracking.

It is generally easier to work with a vector graphics application like Adobe Illustrator because then you can work on a fixed grid with shapes in real-world units. All you need to do is create a black square and then a white square half its size and centrally align them. This gives a basic frame that meets the requirements described in the previous section. You can now design a pattern to fill the frame, bearing in mind the requirements for patterns. Once finished you can print out the marker. It is a good idea to save your file (e.g. as a PDF) so that you can share it for others to print when they look at your AR scene.

BuildAR Pro provides a marker generator utility that simplifies this process by creating the frame for you. All you need to do is load in a picture of your pattern, choose the size you want your marker to be, and then print it out. The marker generator also creates the pattern file needed to track the new marker. This file is described in the next section.

Training Markers

In order to use the physical marker for tracking, BuildAR Pro needs a description of the pattern so that it knows what to look for. This description is stored in a pattern file which is simply a snapshot of the pattern. An accompanying pattern file is needed for each marker you create. If you use the marker generator built into BuildAR Pro, then the pattern file is automatically generated for you when you create a marker. However, if you create your marker in some other way, such as in a graphics application or by hand, then you need to train in the pattern.

In BuildAR Pro, marker training is very simple. On the toolbar, press the Marker Training button. If you have some markers and models already loaded then they will be temporarily hidden while in training mode. While in training mode, BuildAR Pro searches for candidate markers – that is, black frames which could possibly be markers. A green and red border will be drawn around the largest, most likely candidate currently in view. If you hold your marker up to the camera then this should be the one BuildAR Pro finds.

Orient the marker so that the red corner of the border is in the top left of your marker. This is simply to ensure that all markers are trained in a uniform manner. Next, click the mouse on the AR view to capture a snapshot of the pattern. A save dialog box will appear to allow you to save the pattern file. Give it a descriptive name.

Adding Markers

Now that you have a physical marker and its corresponding pattern file, you can add the marker to your scene in BuildAR Pro.

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