In order to preserve anonymity from time to time you may need to mask out faces using After Effects or Premiere, where the features in question are obscured by the effect of large pixels – in this case the Mosaic Effect.

This is relatively easy if you merely wish to disguise a stationary interviewee but it becomes more time consuming when the object is moving – for example, for hiding a car’s number plate. In this case, After Effects’ Motion Tracking system is very useful, whereby After Effects detects the path of the motion, and applies that to the Mosaic effect, so it follows the object.

A mask could be used to constrain the effect to the object’s shape, but perhaps the most efficient way is to apply the motion track to a Track Matte which in turn constrains the effect.

1. Adjustment layers  In this interview clip we will disguise the interviewee’s face. To apply an effect to part of an image – the face – you must use an Adjustment Layer and a mask. Above your interview clip create an Adjustment Layer. Then draw a mask on the Adjustment Layer around the part of the clip you wish to disguise.

2. Apply the effect  From the Effect menu select Stylize > Mosaic, and in the Effect Controls palette set the horizontal and vertical block size to 24 pixels each to disguise the subject. Select the Adjustment Layer and quickly hit the keys M and F in succession to reveal the mask feather settings. Set the feather to about 40 pixels.

3. Animating the mask  In a static interview the subject may barely move their head, and when they do the mask can be made to follow their movements by keyframing the Mask Path. Select the Adjustment Layer and press M, revealing the mask path property. Click the stopwatch to enable keyframing.

4. Follow the subject  Play the clip and when the subject makes significant moves out of the masked area, stop the clip. With the arrow tool, click on the inside of the masked area and move the mask to obscure the subject in the new position. Continue playing and tweaking the position where necessary.