# RotoMerge node¶

*This documentation is for version 1.0 of RotoMerge.*

## Description¶

Pixel-by-pixel merge operation between two inputs using and external alpha component for input A. All channels from input A arge merged with those from B, using RotoMask as the alpha component for input A: the alpha channel from A is thus merged onto the alpha channel from B using the RotoMask as the alpha value (“a” in the formulas). This may be useful, for example, to “paint” alpha values from A onto the alpha channel of B using a given operation with an external alpha mask (which may be opaque even where the alpha channel of A is zero).

A complete explanation of the Porter-Duff compositing operators can be found in “Compositing Digital Images”, by T. Porter and T. Duff (Proc. SIGGRAPH 1984) http://keithp.com/~keithp/porterduff/p253-porter.pdf

Note that if an input with only RGB components is connected to A or B, its alpha channel is considered to be transparent (zero) by default, and the “A” checkbox for the given input is automatically unchecked, unless it is set explicitely by the user. In fact, most of the time, RGB images without an alpha channel are only used as background images in the B input, and should be considered as transparent, since they should not occlude anything. That way, the alpha channel on output only contains the opacity of elements that are merged with this background. In some rare cases, though, one may want the RGB image to actually be opaque, and can check the “A” checkbox for the given input to do so.

### Operators¶

The following operators are available.

#### Porter-Duff compositing operators¶

- copy: A (a.k.a. src)
- over: A+B(1-a) (a.k.a. src-over)
- under: A(1-b)+B (a.k.a. dst-over)
- in: Ab (a.k.a. src-in)
- mask: Ba (a.k.a dst-in)
- out: A(1-b) (a.k.a. src-out)
- stencil: B(1-a) (a.k.a. dst-out)
- atop: Ab + B(1 - a) (a.k.a. src-atop)
- xor: A(1-b)+B(1-a)

#### Blend modes, see https://en.wikipedia.org/wiki/Blend_modes¶

##### Multiply and Screen¶

- multiply: AB, 0 if A < 0 and B < 0
- screen: A+B-AB if A or B <= 1, otherwise max(A, B)
- overlay: multiply if B < 0.5, screen if B > 0.5
- hard-light: multiply if A < 0.5, screen if A > 0.5
- soft-light: burn-in if A < 0.5, lighten if A > 0.5

##### Dodge and burn¶

- color-dodge: brighten B towards A
- color-burn: darken B towards A
- pinlight: if B >= 0.5 then max(A, 2*B - 1), min(A, B * 2.0 ) else
- difference: abs(A-B) (a.k.a. absminus)
- exclusion: A+B-2AB
- divide: A/B, 0 if A < 0 and B < 0

##### Simple arithmetic blend modes¶

- divide: A/B, 0 if A < 0 and B < 0
- plus: A+B (a.k.a. add)
- from: B-A (a.k.a. subtract)
- minus: A-B
- difference: abs(A-B) (a.k.a. absminus)
- min: min(A, B) (a.k.a. darken only)
- max: max(A, B) (a.k.a. lighten only)

##### Hue, saturation and luminosity¶

- hue: SetLum(SetSat(A, Sat(B)), Lum(B))
- saturation: SetLum(SetSat(B, Sat(A)), Lum(B))
- color: SetLum(A, Lum(B))
- luminosity: SetLum(B, Lum(A))

#### Other¶

- average: (A + B) / 2
- conjoint-over: A + B(1-a)/b, A if a > b
- disjoint-over: A+B(1-a)/b, A+B if a+b < 1
- freeze: 1-sqrt(1-A)/B
- geometric: 2AB/(A+B)
- grain-extract: B - A + 0.5
- grain-merge: B + A - 0.5
- hypot: sqrt(A*A+B*B)
- matte: Aa + B(1-a) (unpremultiplied over)
- reflect: A*A / (1 - B)

See also:

- “Digital Image Compositing” by Marc Levoy https://graphics.stanford.edu/courses/cs248-06/comp/comp.html
- “SVG Compositing Specification” https://www.w3.org/TR/SVGCompositing/
- “ISO 32000-1:2008: Portable Document Format (July 2008)”, Sec. 11.3 “Basic Compositing Operations” http://www.adobe.com/devnet/pdf/pdf_reference.html
- “Merge” by Martin Constable http://opticalenquiry.com/nuke/index.php?title=Merge
- “Merge Blend Modes” by Martin Constable http://opticalenquiry.com/nuke/index.php?title=Merge_Blend_Modes
- “Primacy of the B Feed” by Martin Constable http://opticalenquiry.com/nuke/index.php?title=Primacy_of_the_B_Feed
- grain-extract and grain-merge are described in http://docs.gimp.org/en/gimp-concepts-layer-modes.html

## Inputs¶

Input | Description | Optional |
---|---|---|

B | The main input. This input is passed through when the merge node is disabled. | Yes |

A | The image sequence to merge with input B. | Yes |

Mask | Yes | |

A2 | Yes |

## Controls¶

Parameter / script name | Type | Default | Function |
---|---|---|---|

Operation / `operation` |
Choice | over | The operation used to merge the input A and B images.
The operator formula is applied to each component: A and B represent the input component (Red, Green, Blue, or Alpha) of each input, and a and b represent the Alpha component of each input.
If Alpha masking is checked, the output alpha is computed using a different formula (a+b - a*b).
Alpha masking is always enabled for HSL modes (hue, saturation, color, luminosity).
atop: Ab + B(1 - a) (a.k.a. src-atop)average: (A + B) / 2color: SetLum(A, Lum(B))color-burn: darken B towards Acolor-dodge: brighten B towards Aconjoint-over: A + B(1-a)/b, A if a > bcopy: A (a.k.a. src)difference: abs(A-B) (a.k.a. absminus)disjoint-over: A+B(1-a)/b, A+B if a+b < 1divide: A/B, 0 if A < 0 and B < 0exclusion: A+B-2ABfreeze: 1-sqrt(1-A)/Bfrom: B-A (a.k.a. subtract)geometric: 2AB/(A+B)grain-extract: B - A + 0.5grain-merge: B + A - 0.5hard-light: multiply if A < 0.5, screen if A > 0.5hue: SetLum(SetSat(A, Sat(B)), Lum(B))hypot: sqrt(A*A+B*B)in: Ab (a.k.a. src-in)luminosity: SetLum(B, Lum(A))mask: Ba (a.k.a dst-in)matte: Aa + B(1-a) (unpremultiplied over)max: max(A, B) (a.k.a. lighten only)min: min(A, B) (a.k.a. darken only)minus: A-Bmultiply: AB, 0 if A < 0 and B < 0out: A(1-b) (a.k.a. src-out)over: A+B(1-a) (a.k.a. src-over)overlay: multiply if B < 0.5, screen if B > 0.5pinlight: if B >= 0.5 then max(A, 2*B - 1), min(A, B * 2.0 ) elseplus: A+B (a.k.a. add)reflect: A*A / (1 - B)saturation: SetLum(SetSat(B, Sat(A)), Lum(B))screen: A+B-AB if A or B <= 1, otherwise max(A, B)soft-light: burn-in if A < 0.5, lighten if A > 0.5stencil: B(1-a) (a.k.a. dst-out)under: A(1-b)+B (a.k.a. dst-over)xor: A(1-b)+B(1-a) |

Bounding Box / `bbox` |
Choice | Union | What to use to produce the output image’s bounding box.
Union (union): Union of all connected inputs.Intersection (intersection): Intersection of all connected inputs.A (a): Bounding box of input A.B (b): Bounding box of input B. |

Alpha masking / `screenAlpha` |
Boolean | Off | When enabled, the input images are unchanged where the other image has 0 alpha, and the output alpha is set to a+b - a*b. When disabled the alpha channel is processed as any other channel. Option is disabled for operations where it does not apply or makes no difference. |

R / `AChannelsR` |
Boolean | On | Use red component from A input(s). |

G / `AChannelsG` |
Boolean | On | Use green component from A input(s). |

B / `AChannelsB` |
Boolean | On | Use blue component from A input(s). |

A / `AChannelsA` |
Boolean | On | Use alpha component from A input(s). |

R / `BChannelsR` |
Boolean | On | Use red component from B input. |

G / `BChannelsG` |
Boolean | On | Use green component from B input. |

B / `BChannelsB` |
Boolean | On | Use blue component from B input. |

A / `BChannelsA` |
Boolean | On | Use alpha component from B input. |

R / `OutputChannelsR` |
Boolean | On | Write red component to output. |

G / `OutputChannelsG` |
Boolean | On | Write green component to output. |

B / `OutputChannelsB` |
Boolean | On | Write blue component to output. |

A / `OutputChannelsA` |
Boolean | On | Write alpha component to output. |

Invert Mask / `maskInvert` |
Boolean | Off | When checked, the effect is fully applied where the mask is 0. |

Mix / `mix` |
Double | 1 | Mix factor between the original and the transformed image. |