Fill Behind

From 3.8BlurCompositingOtherRaster

Fills the area behind the layer with a modified copy of the layer, with optional adjustments such as blur, saturation, tint, and brightness, providing a convenient way to quickly adapt a photo or video for use in a project with a different aspect ratio or orientation.

Properties
Strength

The strength of the blur used to blur the background copy of the layer. Increasing this also zooms in the background copy slightly to avoid a transparent fringe forming around the edges of the screen where the blur ends.

Range: 0 to 2; Default: 0.3
Feather

The amount of feathering around the edges of the foreground (original layer) as a proportion of the blur Strength (this is ignored if Strength is zero).

Range: 0 to 1; Default: 0.05
Saturation

Controls the amount of color in the background copy of the layer. Negative values remove color, with -100% resulting in a grayscale background. Positive values boost the colorfulness any existing colors in the background. Use zero for the original color.

Range: -100% to +100%; Default: +0%
Brightness

Controls the brightness of the background copy of the layer. Use zero for the original brightness. Negative values make background darker, and positive values make it lighter.

Range: -100% to +100%; Default: +0%
Gamma

Apples a gamma correction to the luminance of the background copy of the layer.

Range: 0.01 to 9.99; Default: 1
BG Alpha

The opacity of the foreground (original) copy of the layer.

Range: 0% to 100%; Default: 100%
FG Alpha

The opacity of the background copy of the layer.

Range: 0% to 100%; Default: 100%
Tint Color

A tint color to be applied to the background copy of the layer, based on the setting of Tint. Only the hue and saturation of the tint color are used; the original luminance is preserved.

Default:  
Tint

The amount of Tint Color to apply to the background copy of the layer.

Range: 0% to 100%; Default: 0%
Edge Color

A color to apply around the edges of the foreground (original layer), based on the setting of Edge Strength. This is a soft border, the thickness of which is determined by the blur Strength, so this does nothing when Strength is zero.

Default:  
Edge Strength

The opacity of the soft edge drawn around the foreground (original layer) using Edge Color. If this is zero, no edge is drawn. At 100% the innermost pixels of the edge are fully opaque, while the outer ones are transparent. The edge thickness is based on the blur Strength, so when Strength is zero, this is ignored.

Range: 0% to 100%; Default: 0%
Scan Line Strength

The opacity of gaps or dark areas between optional CRT-style scanlines rendered on top of the background copy of the layer. Set this to zero to disable scanline simulation.

Range: 0% to 100%; Default: 0%
Scan Line Count

The number of scanlines to render, when Scan Line Strength isn't zero.

Range: 100 to 2160; Default: 720
FG Offset

Shifts the position of the foreground (original layer) independently of the background.

Default: 0, 0
FG Scale

Controls the size of the foreground (original layer) independently of the background.

Range: 0.01 to 100; Default: 1
BG Offset

Shifts the position of the background copy of the layer independently of the foreground.

Default: 0, 0
BG Scale

Controls the size of the background copy of the layer independently of the foreground.

Range: 0.01 to 100; Default: 1

Usage

This effect works best with photo and video layers. Apply Fill Behind, then adjust the parameters as desired. If you wish to use Feather, or apply an edge around the layer using Edge Strength, be sure Strength is high enough, as the feather and edge options are based on the Strength setting.

In most cases, it is not necessary to apply any keyframe animation to the properties of this effect. However, the FG Offset and FG Scale properties can be animated to create transitions, and for static photos, applying subtle animation to the BG Scale and FG Scale properties can make the project feel more dynamic.

This effect's properties are versatile enough to achieve a wide variety of background looks. Be sure to try the various presets, which can be used on their own or serve as inspiration for your own background styles.

Use Cases

The main use-case for this effect is to provide an interesting background in place of the normal black bars that appear when you use a video or photo that is a different aspect ratio that the project. For example, if you want to use a vertical video in a square project, you can apply this effect to provide an interesting background fill on the sides that are not covered by the video.