Defining coordinate systems

MatchMover manages a set of user-defined coordinate systems with respect to which the cameras and 3D points are expressed. If no coordinate system is specified, MatchMover chooses an arbitrary one. You can define a coordinate system in order to facilitate the manipulation of your exported project in a 3D package. If no point relations have been set up, MatchMover aligns the coordinate system on the computed position of the camera for the first frame; the default is the camera looking towards Z and Y as Up axis, but this changes according to your project’s 3D parameters if you selected a different up axis.

If no coordinate system is defined, MatchMover tries to create one from the point relations or the survey points you defined (it needs at last 4 non colinear survey points to do so).

There are two main advantages in defining a particular coordinate system, described below.

Advantage 1 - It makes manipulation easier and more intuitive in the next stages, when virtual objects are inserted in the scene. For example, if a virtual object such as a car has to be placed on a flat surface, e.g., a road, it is very helpful to have two coordinate axes, e.g., X and Y, in the plane of that surface.

Advantage 2 - It allows you to impose strong constraints on the computed 3D points and the camera move, based on clear alignments in the scene inferred from the image sequence.

For example, two points that are at the same height above the floor, where the X and Y coordinate axes lie, have the same Z coordinate. These constraints help the system to compute data that is more accurate and closer to reality. In practice this applies to a lot of cases such as house walls and floors, any flat surface etc.

To set a 3D coordinate system, you specify two directions. Each direction is defined as one of the following:

Passing through the origin and one point, selected from the tracked points.
Passing through two points, selected from the tracked points.
Normal to three points, selected from the tracked points.

The coordinate system is then defined in 3 steps.

The first direction defines the first coordinate axis.
The second direction defines, with the first direction, the half-plane in which the second axis is chosen.
Using the two axes, MatchMover finds the third and final axis that is used to build a direct orthogonal coordinate system.

You can define a coordinate system to the scene using two methods:

Defining points for the coordinate system in the coordinate label’s Parameters Window. See Defining the coordinate system using the Parameters Window.
Snapping the axes of the Coordinate System Manipulator to 3D points in the 3D View. See Defining a coordinate system using the Coordinate System Manipulator.
NoteYou can move, rotate, and scale the Grid like any 3D object to manually adjust the coordinate system. See Editing 3D primitives and objects.

Defining the coordinate system using the Parameters Window

Select the points you want to include in your coordinate system.
See Selecting tracks.
Do one of the following:
- Select 3D Tracking > New Coordinate System.
- Click the New Coordinate System icon in the Toolbar.
- Right-click Coordinate System in the Project window to open the pop-up menu and select New Coordinate System. MatchMover creates a new coordinate label.
NoteOnly the points you have selected when you created the coordinate system will appear in the Parameters Window drop-down list later. See Creating a point relation.

By default, no points are selected and all the points will appear in the list. However, if there is a long list of points, you might find it difficult to select from such a long list. That’s why there is a way to pre-select points.
In the Parameters Window, select the point track that serves as the center of your coordinate system from the Origin drop-down list.
In Distance, enter the distance between two tracks.
From the other two drop-down lists (below Distance), choose the two tracks for which you have defined the distance.
Select either X, Y or Z from the Axis1 and Axis2 drop-down list.
From the drop-down list to the right of Axis1, select either:
- from origin to - Defines an axis from the origin to the selected track point.
- through 2 points - Defines an axis from the first point to the second point.
- normal to 3 points - Defines an axis as the normal to the plane defined by three points.
From the activated drop-down lists, below, select the appropriate tracks to define the axis.
Repeat the above process for Axis2.
Click the Apply Coordinate System button.

The coordinate system is now defined and the coordinates you set are listed in the Coordinate Systems folder in the Project window.

NoteIf you have deleted a point track that is used by a coordinate system, you will be given a warning message during the calculation process and offered the option of redefining the coordinate system.

Defining a coordinate system using the Coordinate System Manipulator

The Coordinate System Manipulator makes creating the coordinate system easy. The manipulator is displayed in the 3D View whenever a coordinate system is selected or a new coordinate system is created.

You can drag and snap the vertices of the manipulator’s three axes (red: X-axis, green: Y-axis, blue: Z-axis) to 3D points in the scene and use the independent, light blue distance line for determining the scale of the scene by measuring the distance between two points.

To create a coordinate system:

Click and drag the origin of the manipulator and snap it to a 3D Helper.
Click and drag the sphere at the end of an axis and snap it to a 3D Helper. The sphere changes color when it overlaps a 3D Helper. The first axis to be snapped to a 3D point is assigned as the “1st axis” and is equivalent to the first axis defined in the coordinate label’s properties in the Parameters Window. See Defining the coordinate system using the Parameters Window.
Click and drag another axis and snap it to a 3D Helper. The second axis that you snap to a 3D point is assigned as the “2nd axis”, defining a plane with the 1st axis. The 2nd axis is defined as the normal to the first axis that lies in that plane.
Resize the light blue distance line by snapping its ends to 3D Helpers. This scales the scene to using the arbitrary distance between two tracks. The first point of the line is linked to the origin by a thin, dashed line.
NoteYou must snap both ends of the distance line to a 3D Helper to properly define the coordinate system; otherwise it will not be taken into account. The only way to change the distance (or to define an axis using three points) is to edit it in the coordinate system’s properties in the Parameters Window. See Defining the coordinate system using the Parameters Window.

To change the color of the manipulator axes and distance line, select Edit > Preferences, click the Color tab in the User Preferences window, and change the color of the corresponding sample box.

Understanding locked axes

When an axis is defined as “1st axis” or “2nd axis”, it is locked, meaning that although it can be moved and snapped to another point, it still retains its quality of being first or second axis.

The origin can also be moved and snapped to a different point, and although the manipulator’s axes remain in the same direction, they are now defined between two points instead of being defined from the former origin to their snap point.

Deleting a coordinate system

To delete a coordinate system, do one of the following:

Select a coordinate system in the Coordinate Systems folder in the Project window and either:
- Select Edit > Delete.
- Press the Delete key.
Right-click a coordinate system in the Coordinate Systems folder in the Project window and select Delete Coordinate System from the pop-up menu.

The coordinate system is deleted from the project.

Setting the world reference

When you have created several coordinate systems, you can select one to define the world reference, shown by the three axes in the bottom left corner of the Workspace.

To set the world reference:

Select a coordinate system from the Project window.
Do one of the following:
- Select 3D Scene > Set World Reference.
- Right-click and select Set World Reference from the pop-up menu.

Mapping the coordinate system to a camera

You can also center the coordinate system on a camera at the current frame by selecting 3D Scene > Map World on Camera. This defines the coordinate system from the computed camera at the current frame so that the origin is at the optical center and the axes are that of the camera (Z is the optical axis and Y is the up axis).