Track Node

The Track node enables you to set a point or points in an image, which the Track node then accurately follows throughout the sequence, creating an x,y value for every frame. The track position data can then be accessed from other nodes to perform various functions.

There is almost no limit to what you can do with this tracking data. Two common applications are to stabilize an image and to match the motion of a foreground element to an object in the background plate (i.e., match move). The Match Move and Stabilize nodes are described in chapter 17, "Movement Nodes ."

The Corner Pin node (see chapter 17, "Movement Nodes ") can access tracking data to control the position of corner pin points. Track data can also be used for other purposes. For example, the Pixel Meter node (described earlier in this chapter) can access track data to drive the position of a pixel meter.

Generating Track Data

First you need to create a track point for each area to be tracked in the image and position each point manually at the first frame of the range to be tracked (refer to step 1, below). Next you make any desired adjustments to the tracking parameters, such as the maximum correlation error to allow (step 2). Finally, you specify the frames to track and actuate the track function (step 3). The Track node will correlate the image data from frame to frame to generate the x,y values.

Once the track data has been generated, you can review and adjust it (see Re-tracking Points and Frames ). You can also track frames manually (see Manual Tracking ).

Step 1: Create Track Points

Start by opening the Track work monitor at the first frame to be tracked. (The work monitor button is located on the Tracker tab in the node panel.) The work monitor displays track points over the image as you create them. To actually create a track point, go to the Points tab and click the "Add Point" button. An entry will appear on the tab for every point you create.

Position the point interactively in the work monitor, or by using the corresponding entry in the Points tab to enter numerical values. The work monitor will update to reflect any changes in a track point entry.

Step 2: Adjust Tracking Parameters

If needed, adjust the parameters on the Tracker tab to get the best track for the current imagery. In particular, you will want to review the "Track On" parameter setting in the Tracker tab and the "Size" and "Speed" parameter settings in the Points tab:

The Track On parameter enables you to specify how Chalice will distinguish the areas defined by the track points from surrounding portions of the image in subsequent frames, based on luminance or color channels. The more detail, contrast, or color variance present in the area you select, the better Chalice will be able to track the object.

If your tracking point is in an area with high contrast and detail, luminance should provide good tracking. Alternatively, a point surrounded by pixels with considerably different color values may track effectively using the Red, Green, or Blue channels.

Tip:
Another method you can use to improve a track is to run the image through a filter node and adjust it until it is most suitable for tracking, regardless of whether it looks natural or suitable for final composition. You can then input the altered image to the Track node and send the original image through the rest of the network.
For example, if your imagery is noisier than the Track node's grain filter can handle, you might use the Gaussian filter in a Blur node to filter out high-frequencies and remove noise, grain, and aliasing. Or you could use a Contrast node to increase the contrast and get a more accurate track. (Be aware that raising the contrast too much would also increase any noise or aliasing problems.)

In the Points tab, the Size parameter enables you to adjust the size of the area to track and the Speed parameter controls the size of the surrounding search region.

In general, the smaller you set the Size and Speed parameters for a track point, the faster the Track node will process the data. On the other hand, larger Size and Speed parameters take longer to process but may produce significantly better track data for some imagery.

Step 3: Set Frame Range and Actuate Track

Set the range of frames to track in the Tracking Frame Range controller. The Track node requires that all frames in the specified range be sequential; that is, there may be no gaps between frames.

Tip:
If you need to do more complex resequencing to specify the appropriate frames to track, insert a Sequence node between the source node and the Track node. For example, if you wish to track every other frame, set the increment in the Sequence node to 2.

Then actuate the track function by clicking the actuator button that corresponds to the order in which the frames should be tracked, such as forward to the end of the specified range. Refer to Tracking Frame Range Parameters later in this node description for more information.

Re-tracking Points and Frames

When the node has finished tracking, you can review the track path and point markers in the work monitor and make any necessary adjustments. Track points can be repositioned manually at any frame, and you can re-track selected points and frames.

For example, you might want to adjust the Maximum Error and Track On parameters before re-tracking a subset of frames that have less image detail than the successfully tracked frames.

The work monitor can display the path of each track point, along with a marker for the point position at each frame. Refer to the Draw Path and Marker in Monitor description to find out how each marker is color-coded.

You can reposition any track point for the currently displayed frame. Clicking on a point marker for a different frame will move you to that frame automatically, without altering the position of the point. Then you will be able to change the position of the tracking point for the new current frame.

Each track point can be enabled or disabled before executing the track function by using the Enable/Disable button associated with the track point entry in the Points tab. This means that you can re-track a point or points without affecting existing track data for other points. Similarly, you can re-track a specific frame range for all enabled points without affecting existing track data for frames outside the new range setting.

Manual Tracking

The Track node provides the option of setting points manually at any frame in the sequence. Hand-set points appear blue in the work monitor display.

If there are frames in between two hand-set positions for a point, Chalice will linearly interpolate the positions for those frames (untracked points are red). This gives you the option of adjusting some or all of these positions manually. However, these interpolated point positions can also be auto-tracked using the normal track function (they will turn green after auto-tracking).

You can specify whether or not the node should re-track any hand-set positions by checking or unchecking the "Reset Points Set by User" parameter.

Tracker Parameter Tab

The Tracker tab provides parameters for:

  • controlling work monitor display options,
  • specifying how the image will be processed by the Track node during the tracking function,
  • setting the frame range to use for the track, and
  • actuating the track function.

Work Monitor Display Parameters

Work Monitor / At Resolution

First, select a display resolution for the work monitor from the At Resolution popup menu. Then click the plus icon to display the work monitor for this node, which enables you to position the points that specify the areas to be tracked.

Marker Size

This parameter enables you to increase or decrease the size of the markers that designate tracking points in the work monitor.

Draw Path and Marker in Monitor

This box, when checkmarked, instructs Chalice to display tracking data for all frames in a sequence. This option effectively draws the path of the tracking points in the Track work monitor. The color of the marker provides the following information about the tracking point:

  • Green indicates that the point was tracked by the node.
  • Blue indicates that the point was set manually (as at the first frame).
  • Red indicates that the point is an interpolated position between two manually set points. If the frames are subsequently tracked by the node, any interpolated points will be tracked, after which they will turn from red to green.
  • Yellow indicates that the node lost track for that point at that frame.

Tracking Parameters

Maximum Error

This parameter enables you to set the maximum allowable error value for the correlation of data between frames. If the error value rises above this value, the Track node will cease processing and generate an error message. The range is 0-1, with 0 being a perfect match. Any error value of 0.15 or less will be a good match. Typically this value is set at around 0.05.

The error value for each point at each frame tracked is displayed in the Track Points entry panel on the Points tab.

Track On

This menu allows you to select the type of image data Chalice will use to distinguish a tracking point from surrounding pixels. You can track on luminance (Film, NTSC, or PAL) or a color channel (Red, Green, or Blue). For more information on how Chalice computes luminance types, see the Channel Swap Node description of Luminance.

Grain Filter

The Grain Filter parameter, when checked, instructs the Track node to use 3 x 3 averaging to remove noise from the image during the tracking operation. This does not remove noise from the source image itself, of course--it filters noise as part of the tracking process only.

Re-track Points Set by User

This checkbox enables you to specify whether the node should re-track any points you have set or adjusted manually. Unless you check the box before re-tracking, the node will retain the position data created by manually setting a point at a frame.

Minimum Source Processing

A checkmark in this box instructs Chalice to display only a small area around a point while processing the tracking data. The rest of the image will be set to black. This can speed the process considerably, as it frees Chalice from reprocessing the entire image at each frame.

Tracking Frame Range Parameters

The Tracking Frame Range parameters enable you to specify the range of frames to track and the tracking direction from the current frame (forward to the end of the sequence, or backward to the beginning). You can auto-track

  • the entire sequence,
  • a subset of frames, or
  • one frame at a time.

Track Actuator Buttons

Click any of these four buttons to start the tracking process. The node will then generate the track data based on which button you pressed and how you have set the other tracking parameters.

The plus and minus buttons track forward ( + ) or back ( - ) one frame. The arrow buttons track from the current frame either forward to the end of the range or backward to the start of the range.

Frame Range Controls

The Frame Range controls enable you to set the frame range to be tracked. This is especially useful in situations when most of the frames tracked well, but you need to re-track a subset of frames using revised parameter settings and/or hand-set key frame points.

Note:
Each time you actuate the track, every enabled track point in the specified frame range will be re-tracked. However, you can disable (temporarily "turn off") any points that do not need re-tracking by using the Enable/Disable buttons in the Points tab.

Points Parameter Tab

The Points tab enables you to create, review, adjust, and delete track points.

Add Point

Click the Add Point button to add a track point to the monitor. You can add multiple points, if you desire. Chalice will create an entry in the list area for every point added that will enable you to set its position, size and speed parameters.

Track Point List Parameters

Each time you click the Add Point button, Chalice creates an entry in the list area on the Points parameter tab. Each entry in the list includes the following parameters:

Enable / Disable Point Button

The Enable/Disable Point Button, when checked, instructs Chalice to display the tracking point in the monitor and include it in all operations. If the box is deselected, the point will disappear from the monitor and will be masked from operations.

Delete Point Button

Click this red "X" to delete the point from the Track Monitor and from the Track Points List Area.

X Position, Y Position

These parameters enable you to specify the x,y coordinate location for a tracking point. Alternatively, you may use the work monitor display to drag the point in to a new location. Chalice will update the changes in the X position and Y position parameters accordingly.

Size, Speed

Track points are represented by a rectangle with crosshairs in the center (the Size parameter), surrounded by a circle (the Speed parameter):

  • The Size parameter specifies the size of the rectangle, which defines the pixel area to be tracked and correlated. The Size rectangle should fit as closely as possible the feature to be tracked.
  • The Speed parameter specifies the radius of the circle in pixels. This circle represents the size of the area to be searched by the Track node to find a good correlation of the pixels in the rectangle ("Size") area.

You set both the Size and Speed parameters in the list area by keying in numerical values or by using the mouse to set the tuner to the desired value.

Error

This parameter displays the correlation error value for the point at the current frame. It indicates the extent to which the pixel area determined in the Size parameter correlates with the pixel area in the previous frame. The greater the error value, the less accurate the match.

STED Button

Click the STED button, available for each point tracked, to view the tracking data for that point in the Chalice Space-Time Editor.




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