Tutorial Four-Point Tracking. Four-Point Tracking

Transcription

1 Four-Point Tracking This tutorial shows you an alternative way to track a shot and remove the tracking reference points by tracking only once. Usually, you need to track twice, once to track the shot and then a second time to track the shapes to remove the reference points. In this tutorial, you ll reuse the tracking information and apply it to shapes you create using a Matte effect. You will also see how to use a DVE to offset and resize the tracker layer image, which can be a lot easier than trying to resize it within the tracker. 1 of 16

2 Capturing Media and Setting Up the Shot 1. Create a new sequence with the following settings: Video Format: PAL D1 4/3 Resolution: Full/Uncompressed 2. Import the following media files using the capture settings shown here: Eye.mov Screen.mov If you downloaded the 4-PT_Tracking_Small version of the media, the Screen.mov clip has been compressed and may require additional work to create a clean composite. 3. Drag the Screen clip to the timeline. 4. Create a graphics container clip. 5. Drag the Eye clip from the browser to the Effects Tree. 6. Add a Composite effect (Tree Effects folder) to the tree and connect the nodes as shown here: The Eye clip is composited over the Screen clip. 2 of 16

3 Tracking the Dots on the Screen 1. Add a Tracker effect (Tree Effects folder) to the tree and connect the nodes as shown here: 2. Go to the beginning of the timeline and open the Tracker property editor. 3. Select In 2 ref from the edit list to view the reference clip. 4. Position the four trackers over the four dots on the computer screen. 3 of 16

4 5. Zoom in the viewer and resize each of the four target areas, so that they are a little bit bigger than the dots. Make sure that you have more black than green in the target area. This will make it much easier to track. 6. For each reference, R1-R4, set the If Below Threshold parameter to Stop. This will stop the tracker when the confidence is less than the threshold value. 4 of 16

5 7. Select the Update option to update the viewer as the reference points are tracked. 8. Click the Track Forwards button to begin tracking. 9. When the tracking is finished, deselect the View option in the Tracker Effect property editor. 10.Close the Tracker property editor, and then go to the beginning of your sequence. 5 of 16

6 Resizing the Eye Layer You can see that the eye layer is too small for the computer screen. Instead of going back into the tracker and resizing the layer manually, you can use a DVE. This lets you resize the layer and view the results at the same time. 1. Add a DVE to the tree and connect the nodes as shown here: 2. Open the DVE property editor. 3. On the Options property page, deselect the Output Frame Size option. If you leave this option on, the image will remain cropped by the border of the tracker even if you scale it up. 6 of 16

7 4. On the DVE property page, translate and scale the image up until it is a little larger than the computer screen. Using a DVE to scale and position the image is much easier than aligning it from within the tracker. This method lets you view the result as you scale and position the image. Tip: If you zoom the viewer out, it will be easier to scale your DVE. 7 of 16

8 Keying Out the Green Screen Now you need to key out the green of the screen, so that the eye clip appears to be on the computer screen. 1. Add a Blue-Green Keyer to the tree and connect the nodes as shown here. 2. Open the Blue-Green Keyer property editor. 3. On the Key property page, click Pick Key Color and then with the eyedropper cursor, click the green portion of the screen. The Eye clip now appears to be on the computer screen. 8 of 16

9 4. Select the Output Matte option to view the matte. Tip: You can apply some pre-blur to remove some of the noise in the image. 5. Switch to the Matte property page, click Pick FG, and then click on a gray area in the viewer to force the foreground to be opaque. 6. Click Pick BG and then click on a dark gray area of the computer screen to force the background to become transparent. The computer screen should be transparent, except for the reference points, and the rest of the image should be opaque. Tip: You can shrink the matte to fine-tune the edges of the key. 9 of 16

10 7. Turn off the Output Matte option. You can now see that the Eye clip fits nicely within the boundaries of the computer screen. Notice, however, that the points used as reference for tracking are still visible. You will remove these points in the next section. 10 of 16

11 Removing the Tracking Reference Points Instead of using the shape tool in the Blue-Green Keyer to remove the dots on the screeen and then using another tracker to track the shapes, you will re-use the existing tracker information in combination with a Matte effect. 1. Make sure you re at the beginning of the timeline. 2. Add a Matte effect (Keyer folder) to the tree and connect the nodes as shown here. 3. View the Matte effect. 4. Open the Matte property editor. 11 of 16

12 5. On the Shapes property page, select the Ellipse tool and draw a shape around each of the dots. Make sure the Fill Inside option is selected and set to 0 for each shape. This sets the alpha value of the inside of the ellipse to 0, which makes it transparent. 6. Deselect the View option and close the Matte property editor. 7. Copy the Tracker node and connect the new tracker node to the tree as shown here: 12 of 16

13 8. Open the Tracker property editor, switch to the Quality property page, and make sure that only the Alpha option is selected in the Channels box. You don t need to process the RGB channels. Only the alpha channel is necessary. 9. Select Inp 2 ref from the edit list and Ctrl-click each of the trackers. Ctrl-clicking the trackers matches up the layer and the reference. 10.Select Output from the edit list and click the Viewer Alpha 25% icon in the viewer toolbar. 11. View the Tracker effect. 12.Scrub through the effect to make sure that the shapes covering the dots move with the image. 13.Deselect the View option and close the Tracker property editor. 14.Click the Viewer Alpha 25% icon again to reset the viewer. 13 of 16

14 Combining the Alphas Now all you have to do is combine the alpha channels of the Blue-Green Keyer and the Tracker using a Matte Composite effect. 1. Add a Matte Composite effect (Tree Effects folder) to the tree and connect the nodes as shown here. 2. Open the Matte Composite property editor. 14 of 16

15 3. On the General property page, set the Channel for Input 2 to Alpha. This looks at the alpha channels of both the Blue-Green Keyer and the Tracker effect. 4. Set the Operator to Min for Input 2. This takes the minimum value for each alpha to create a clean matte. The dots no longer appear on the computer screen. 15 of 16

16 5. View the Matte Composite. 6. Click the Viewer Alpha Full icon in the Viewer toolbar to view the alpha channel. 7. Change the Operation for Input 2 to see how the various options affect your alpha channel. You can see that the Min option gives you the result you want. Operation: Min Operation: Max 8. Press Shift+4 to reset the viewer. 9. Deselect the View option and close the Matte Composite property editor. 10.Scrub through the effect to see the result. 11. Go back to the top timeline and process the effect in frames. 12. When the processing is complete, play back your sequence Avid Technology, Inc. All rights reserved. Avid and the Avid DS logo are registered trademarks of Avid Technology, Inc. All other trademarks contained herein are the property of their respective owners. 16 of 16