A matte in Nodespell is a grayscale image or alpha channel that says where something applies: white is fully on, black is off, gray is partial. Nodespell builds mattes with Keyer, Roto and AI cutouts, refines them with Erode/Dilate, and uses them to cut out images, mix two images, or limit any effect through its Mask input.
Alpha, straight and premultiplied
Section titled “Alpha, straight and premultiplied”Every image node in Nodespell carries RGBA. The alpha channel is the image’s own matte. Two conventions matter when you combine layers:
- Straight (unpremultiplied) alpha keeps full colour values in RGB, with alpha stored separately.
- Premultiplied alpha stores RGB already multiplied by alpha, which is what Over-style merges expect.
Merge and Composite premultiply each foreground for you, and Custom merge expressions see straight alpha, so most graphs need no manual step. When you do need to switch, use Premultiply (multiplies RGB by alpha) and Unpremultiply (divides RGB by alpha). To move a matte between images, Channel Copy writes any source channel, or its luminance, into the target’s R, G, B or alpha, much like Nuke’s Copy or Shuffle.
Pull a key with Keyer
Section titled “Pull a key with Keyer”Keyer writes a new matte from the image itself.
- Add Keyer after your plate and connect the image.
- Choose a Mode (table below). For Chroma Distance, set Screen Color to the backing colour.
- Set Output to Matte View to see the matte as black and white.
- Raise Clip Black until the background is solid black, and lower Clip White until the subject is solid white. Softness widens the transition between them.
- Turn on Invert if the matte is the wrong way round.
- Set Output back to Write Alpha or Premultiplied RGB.
| Mode | Matte is based on | Use for |
|---|---|---|
| Luminance (default) | Brightness of each pixel | Bright elements on dark (fire, sparks, sky) |
| Chroma Distance | Distance from Screen Color | Any solid-colour backing |
| Green Screen, Blue Screen, Red Screen | How much that channel exceeds the other two; the backing goes to black | Coloured screens |
| Saturation | Colour saturation | Separating colourful subjects from neutral backgrounds |
| Control | Default | What it does |
|---|---|---|
| Clip Black | 0.08 | Matte values at or below this become 0 |
| Clip White | 0.35 | Matte values at or above this become 1 |
| Softness | 0.02 | Extends the ramp between the clip points (0–0.5) |
| Combine Alpha | Replace | Replace discards the input alpha; Intersect multiplies the new matte with it; Union combines their coverage |
| Output | Write Alpha | Write Alpha keeps RGB and writes alpha; Matte View shows the matte; Premultiplied RGB multiplies RGB by the new alpha |
| Mix | 1 | Blends between the original alpha and the new matte |
Keyer does not suppress spill. Reduce green or blue fringing afterwards with Hue Correct or a grade limited by the matte.
Grow or shrink a matte with Erode/Dilate
Section titled “Grow or shrink a matte with Erode/Dilate”Erode/Dilate shrinks or grows edges, like Nuke’s Erode filter.
| Control | Options | Notes |
|---|---|---|
| Operation | Dilate (default), Erode | Dilate grows, Erode shrinks |
| Channel | Alpha (default), Luminance, RGBA | Which values the operation follows |
| Radius | 0–16, default 2 | Measured in whole pixels |
| Mix | 0–1 | Blend with the original |
Erode a key by one or two pixels to remove a dark fringe, or dilate a garbage matte so it safely covers the subject.
Draw shapes with Roto
Section titled “Draw shapes with Roto”Roto draws Bezier and straight-line shapes as a matte. It is available inside compositors only.
- Select the Roto node and draw in the compositor viewer with the Pen, Rectangle or Ellipse tools. With the Pen, click to place points and drag while clicking to draw a curve; click the first point or press
Enterto close the shape, orEscto cancel. - Drag a point to move it, double-click a point to switch it between Smooth and Sharp, and double-click an edge to add a point. Hold
Shiftwhile drawing a rectangle or ellipse for a square or circle. - In the shapes panel, set each shape’s Combine to Add, Subtract or Intersect, rename, lock, hide or reorder shapes, and set Edge softness Inside and Outside, for the whole shape or selected points.
- Output is Alpha Matte (default) or RGBA.
- To animate a shape in a video Compositor, move the playhead and select Add key. Editing a shape on any frame adds or updates a key there.
Cut out or mix through a matte
Section titled “Cut out or mix through a matte”Two nodes apply a matte directly. Both work in the Graph and inside compositors.
| Node | Inputs | What it does |
|---|---|---|
| Apply Mask | Image, Mask | White keeps the image, black makes it transparent, gray is partial. Mask source is Brightness (default) or Alpha. Existing transparency is kept unless Replace transparency is on. Strength fades the effect. Output size matches Image; the mask is stretched to fit. |
| Mask Mix | B (base), A (masked), Mask | White shows A, black shows B, gray crossfades. Mix at 0 returns B. Colours are alpha-weighted to avoid fringes. Output size matches B. |
Light Wrap helps a keyed element sit in its plate: it bleeds light from the image on its background input into the foreground’s edges, with Size, Intensity, Background Blur and a Blend of Screen, Add, Over or Soft Light.
Limit any effect with its Mask input
Section titled “Limit any effect with its Mask input”Most image effect nodes, such as Color Correct, Blur, Grade and Denoise, have an optional Mask input and a compact Mask control group in Parameters.
| Control | Default | What it does |
|---|---|---|
| Mask Source | Alpha | Which channel of the mask image drives the effect: Alpha, Luminance, Red, Green or Blue |
| Invert Mask | Off | Applies the effect outside the mask instead |
| Mask Mix | 1 | Blends between the unmasked and masked result |
Many of the same nodes have Channels, which chooses the RGBA channels the node writes. Unselected channels pass through unchanged, so you can, for example, blur only alpha.
Get a matte automatically
Section titled “Get a matte automatically”For an automatic cutout of a still image, use the Silhouette model node, which removes the background and returns a transparent Cutout at the original size. Its Edge adjustments let you expand or contract the edge, feather it, and reduce colour fringe. Silhouette is a model node, so it runs as a task on Nodespell rather than in your browser; see Remove background. The Cutout’s alpha can feed any Mask input, Apply Mask or Mask Mix.
Frequently asked questions
Section titled “Frequently asked questions”Is there an Alpha Matte node?
Section titled “Is there an Alpha Matte node?”No separate node is named Alpha Matte. Use Apply Mask to cut out an image with a matte, Mask Mix to choose between two images, or Roto’s Alpha Matte output for a drawn matte.
Why is my keyed edge dark or fringed?
Section titled “Why is my keyed edge dark or fringed?”The edge may be premultiplied twice or carrying background colour. Check that only one Premultiply happens before Merge, erode the matte by a pixel with Erode/Dilate, and reduce spill with a hue-limited correction.
Can I key a video?
Section titled “Can I key a video?”Yes. Keyer works inside a video Compositor on every frame. Outside a compositor, Graph image nodes process still images.
Which channel should a mask image use?
Section titled “Which channel should a mask image use?”Use Alpha for cutouts and Roto output, and Luminance for grayscale mattes such as depth maps or painted masks.
Related
Section titled “Related”- Compositing: Merge modes and premultiplication.
- Color correction: spill and hue-limited fixes.
- Depth and defocus: depth maps as mattes.
- Filters and effects: which effects accept masks.
- Remove background: automatic cutouts.