|Copy Background takes everything that is visible behind a layer in the preview and copies in into that layer.|
Copy Background is most commonly used to apply an effect to a particular shape or area instead of the entire screen. It has only one option, Fill.
Fill determines how much of the layer's original fill will be visible in the layer. The default value of 0% will completely override the layer's original fill with that of the background. A value of 100% will show only the layer's original fill.
Layers with Copy Background applied display whatever is behind them faithfully, regardless of time or position, or changes to any other layer. Even when effects are added to the layer itself, this is the case, as can be seen below.
Figure 1: Moving Circle with Copy Background and Mosaic
Adding Mosaic to Faces
As seen in Figure 1 above, it is possible to use Copy Background to block out a person's face or any other sensitive information on screen.
Let's take the above background image, and use Mosaic on only the woman's face. After adding an image layer to your timeline, tap +, then tap Shape, then choose a circle. We can resize it to the shape of the woman's face by dragging on the handles around the circle when we have the circle layer selected. We can then drag the circle over her face to cover it.
At this point, with the circle layer highlighted, tap Effects, Add Effect, then Copy Background. The fill of the circle will be replaced with that of the background. Next, tap Effects, Add Effect, and then Mosaic. Immediately we can see that the mosaic has covered the woman's face. We can adjust the mosaic settings and move the circle, and the circle layer's fill will update automatically.