Matte Control Node

Keyer nodes are used to create an alpha channel on an image that does not have one. The Matte Control node is used to combine and manipulate the alpha channels embedded in images as well as masks created by masking tools.

Typically, you add this node to copy a color channel or alpha channel from the foreground input to the background input, or to combine alpha channels from the two images.

Matte Control Node Inputs

The Matte Control node includes four inputs in the Node Editor.

• Background: The orange background input accepts a 2D image that receives the foreground image alpha channel (or some other channel you want to copy to the background).
• — Foreground: The green foreground input accepts a 2D image that contains an alpha channel (or some other channel) you want to be applied to the background image.
• — Garbage Matte: The gray garbage matte input accepts a mask shape created by polylines, basic primitive shapes, paint strokes, or bitmaps masks. Connecting a mask to this input causes areas of the foreground/background combination that fall within the matte to be made transparent.
• — Solid Matte: The white solid matte input accepts a mask shape created by polylines, basic primitive shapes, paint strokes, or bitmaps masks. Connecting a mask to this input causes areas of the foreground/background combination that fall within the matte to be fully opaque.
• — Effect Mask: The optional blue input expects a mask shape created by polylines, basic primitive shapes, paint strokes, or bitmaps masks. Connecting a mask to this input limits the pixels where the matte control occurs. An effects mask is applied to the tool after the tool is processed.

Matte Control Node Setup

Below, the Matte Control node is set up to copy the foreground (green) input’s alpha channel into the background (orange) input. The output of the Matte Control is then an image with an alpha channel used as the foreground composite in the Merge node.

Matte Control Node Matte Tab

The Matte tab combines and modifies alpha or color channels from an image in the foreground input with the background image.

Combine
Use this menu to select which operation is applied. The default is set to None for no operation

• None: This causes the foreground image to be ignored.
• Combine Red: This combines the foreground red channel to the background alpha channel.
• Combine Green: This combines the foreground green channel to the background alpha channel.
• Combine Blue: This combines the foreground blue channel with the background alpha channel.
• Combine Alpha: This combines the foreground alpha channel with the background alpha channel. — Solid: This causes the background alpha channel to become completely opaque.
• Clear: This causes the background alpha channel to become completely transparent.

Combine Operation
Use this menu to select the method used to combine the foreground channel with the background.

• Copy: This copies the foreground source over the background alpha, overwriting any existing alpha in the background.
• Add: This adds the foreground source to the background alpha.
• Subtract: This subtracts the foreground source from the background alpha.
• Inverse Subtract: This subtracts the background alpha from the foreground source.
• Maximum: This compares the foreground source and the background alpha and takes the value from the pixel with the highest value.
• Minimum: This compares the foreground source and the background alpha and takes the value from the pixel with the lowest value.
• And: This performs a logical AND on the two values.
• Or: This performs a logical OR on the values.
• Merge Over: This merges the foreground source channel over the background alpha channel.
• Merge Under: This merges the foreground source channel under the background alpha channel.

Filter
Selects the Filter that is used when blurring the matte.

• Box Blur: This option applies a Box Blur effect to the whole image. This method is faster than the Gaussian blur but produces a lower-quality result.
• Bartlett: Bartlett applies a more subtle, anti-aliased blur filter.
• Multi-Box: Multi-Box uses a box filter layered in multiple passes to approximate a Gaussian shape. With a moderate number of passes (e.g., four), a high-quality blur can be obtained, often faster than the Gaussian filter and without any ringing.
• Gaussian: Gaussian applies a smooth, symmetrical blur filter, using a sophisticated constant-time Gaussian approximation algorithm. In extreme cases, this algorithm may exhibit ringing; see below for a discussion of this. This mode is the default filter method.

Blur
This blurs the edge of the matte using a standard, constant speed Gaussian blur. A value of zero results in a sharp, cutout-like hard edge. The higher the value, the more blur is applied to the matte.

Clipping Mode
This option determines how edges are handled when performing domain of definition rendering. This is profoundly important when blurring the matte, which may require samples from portions of the image outside the current domain.

• Frame: The default option is Frame, which automatically sets the node’s domain of definition to use the full frame of the image, effectively ignoring the current domain of definition. If the upstream DoD is smaller than the frame, the remaining area in the frame is treated as black/transparent.
• Domain: Setting this option to Domain respects the upstream domain of definition when applying the node’s effect. This can have adverse clipping effects in situations where the node employs a large filter.
• None: Setting this option to None does not perform any source image clipping at all. This means that any data required to process the node’s effect that is usually outside the upstream DoD is treated as black/transparent.

Contract/Expand
This shrinks or grows the matte similar to an Erode Dilate node. Contracting the matte reveals more of the foreground input, while expanding the matte reveals more of the background input. Values above 0.0 expand the matte, and values below 0.0 contract it.

Gamma
This raises or lowers the values of the matte in the semitransparent areas. Higher values cause the gray areas to become more opaque, and lower values cause the gray areas to become more transparent. Completely black or white regions of the matte remain unaffected.

Threshold
Any value below the lower threshold becomes black or transparent in the matte. Any value above the upper threshold becomes white or opaque in the matte. All values within the range maintain their relative transparency values.

Restore Fringe
This restores the edge of the matte around the keyed subject. Often when keying, the edge of the subject where you have hair is clipped out. Restore Fringe brings back that edge while keeping the matte solid.

Invert Matte
When this checkbox is selected, the alpha channel of the image is inverted, causing all transparent areas to be opaque and all opaque areas to be transparent.

Solid Matte
Solid mattes are mask nodes or images connected to the solid matte input on the node. The solid matte is applied directly to the alpha channel of the image. Generally, solid mattes are used to hold out areas you want to remain opaque, such as someone with blue eyes against a blue screen.

Enabling Invert inverts the solid matte before it is combined with the source alpha.

Garbage Matte
Garbage mattes are mask nodes or images connected to the garbage matte input on the node. The garbage matte is applied directly to the alpha channel of the image. Generally, garbage mattes are used to remove unwanted elements that cannot be keyed, such as microphones and booms. They are also used to fill in areas that contain the color being keyed but that you wish to maintain.

Garbage mattes of different modes cannot be mixed within a single tool. A Matte Control node is often used after a Keyer node to add a garbage matte with the opposite effect of the matte applied to the keyer.

Enabling Invert inverts the garbage matte before it is combined with the source alpha.

Post-Multiply
Image Selecting this option multiplies the color channels of the image against the alpha channel it creates for the image. This option is usually enabled and is on by default.

Deselect this checkbox and the image can no longer be considered premultiplied for purposes of merging it with other images. Use the Subtractive option of the Merge node instead of the Additive option.

For more information on these Merge node settings, see Composite Nodes.

Matte Control Node Spill Tab

The Spill tab handles spill suppression in the Matte Control. Spill suppression is a form of color correction that attempts to remove the screen color from the fringe of the matte.

Spill is the transmission of the screen color through the semitransparent areas of the alpha channel. In the case of blue- or green-screen keying, this usually causes the color of the background to become apparent in the edges of the foreground subject.

Spill Color This menu selects the color used as the base for all spill suppression techniques.

Spill Suppression When this slider is set to 0, no spill suppression is applied to the image. Increasing the slider increases the strength of the spill method.

Spill Method
This selects the strength of the algorithm used to apply spill suppression to the image.

• None: None is selected when no spill suppression is required.
• Rare: This removes very little of the spill color and is the lightest of all methods.
• Medium: This works best for green screens.
• Well Done: This works best for blue screens.
• Burnt: This works best for blue screen. Use this mode only for very troublesome shots.

Fringe Gamma
This control can be used to adjust the brightness of the fringe or halo that surrounds the keyed image.

Fringe Size
This expands and contracts the size of the fringe or halo surrounding the keyed image.

Fringe Shape
Fringe Shape presses the fringe toward the external edge of the image or pulls it toward the inner edge of the fringe. Its effect is most noticeable while the Fringe Size value is large.

Cyan/Red, Magenta/Green, and Yellow/Blue
Use these three controls to color correct the fringe of the image.

This is useful for correcting semitransparent pixels that still contain color from the original background to match the new background.

Matte Control Node Settings Tab

The Settings tab in the Inspector is also duplicated in other Matte nodes. These common controls are described in detail HERE.

About the Author

Justin Robinson is a Certified DaVinci Resolve, Fusion & Fairlight instructor who is known for simplifying concepts and techniques for anyone looking to learn any aspect of the video post-production workflow. Justin is the founder of JayAreTV, a training and premade asset website offering affordable and accessible video post-production education. You can follow Justin on Twitter at @JayAreTV YouTube at JayAreTV or Facebook at MrJayAreTV