Controls / Point In 1–4
Exposes user-editable point controls.
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Utility / ExpressionsAdvanced
CustomTool
Expression-driven channel processing
Provides programmable per-channel image processing through expressions, user controls, LUTs and intermediate variables.
CustomTool
DaVinci Resolve · Fusion
CustomTool is Fusion’s general-purpose expression node for building calculations that do not justify a dedicated compiled tool.
Overview
What CustomTool does
The node exposes user Numbers, Points and LUT controls, intermediate Setup variables and expressions for Red, Green, Blue, Alpha and auxiliary channels. This makes it suitable for procedural image math, custom channel remapping and prototype effects.
Intermediate expressions help break a complicated formula into readable stages. User controls can then expose only the parameters an artist needs to adjust.
CustomTool is powerful but easy to make opaque. Naming exposed controls and documenting the math is part of building a maintainable Fusion comp.
Mental model
Think of it as a small programmable image processor embedded inside the node graph.
Common uses
Custom channel math
Procedural color transforms
Expression-based masks
Prototype effects
Reusable artist controls around image equations
Flow anatomy
Inputs & outputs
The port structure shows how this node fits into a Fusion graph.
Image
The image whose channels and coordinates are available to expressions.
CustomTool
Fusion node
Image
The processed image after the node operation.
Inspector
Important parameters
Start with these controls before reaching for more complicated workarounds.
Controls / Number In 1–8
Exposes scalar controls for expressions.
Controls / LUT In 1–4
Exposes editable lookup curves.
Point In 1–4
Provides four custom point inputs that can be referenced by CustomTool expressions.
Number In 1–8
Sets the Number In 1–8 threshold.
LUT In 1–4
Provides four custom LUT inputs that can be referenced by CustomTool expressions.
Setup 1–8
Defines reusable setup expressions and variables.
Intermediate 1–8
Stores intermediate calculations to simplify final channel formulas.
Red
Defines the Red output expression.
Green
Defines the Green output expression.
Blue
Defines the Blue output expression.
Alpha
Defines the Alpha output expression.
Aux Channel Expressions
Calculates additional technical channels when needed.
Random Seed
Controls deterministic random behavior inside supported expressions.
Reseed
Generates a new random sequence.
Show Number 1–8
Shows Number 1–8 in the Viewer for setup or diagnostics.
Viewer guides do not normally change the rendered image unless the control explicitly says otherwise.
Name for Number 1–8
Renames the custom Number controls so their purpose is clear in the Inspector.
Show Point 1–4
Shows Point 1–4 in the Viewer for setup or diagnostics.
Viewer guides do not normally change the rendered image unless the control explicitly says otherwise.
Name for Point 1–4
Renames the custom Point controls so their purpose is clear in the Inspector.
Show LUT 1–4
Shows LUT 1–4 in the Viewer for setup or diagnostics.
Viewer guides do not normally change the rendered image unless the control explicitly says otherwise.
Name for LUT 1–4
Renames the custom LUT controls to match the curves they are being used for.
Practical setup
A reliable workflow
- 1
Define the user-facing controls first.
- 2
Build complex math in Setup and Intermediate slots.
- 3
Write the final RGBA expressions using those variables.
- 4
Check alpha independently from RGB.
- 5
Rename exposed controls before handing the comp to another artist.
Typical node chain
Image + User Controls→
Expressions→
Custom RGBA Output
Examples
Where this node fits in real work
Custom RGB remap
Prototype a channel transform without writing a Fuse.
MediaInCustomToolMediaOut
Artist-facing macro logic
Use Numbers and LUTs as clean controls over expression math.
MediaInCustomTool with exposed controlsMerge
Troubleshooting
Common mistakes
Putting every operation into one unreadable expression.
Forgetting that alpha may need different logic from RGB.
Leaving user controls with generic names in a shared comp.
Production notes
Practical tips
Use Intermediate slots as named conceptual stages.
Keep formulas deterministic unless random behavior is intentional.
Promote only the controls artists actually need.
Related nodes