Compositing Tools Overview
- Requirements
- Preparation
- The User Interface
- Still Image Mode
Once the initialization steps are completed, the full Compositing Tools interface becomes visible within the FLIP Fluids panel. This interface is divided into several sections, each providing tools for specific stages of the compositing workflow:
In the following sections, we will take a closer look at each part of the interface and explain its purpose and functionality.
The Compositing Tools are organized into multiple categories, each representing a specific part of the simulation or the scene. Here's a quick breakdown of what each term refers to:
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Fluid Surface
Refers to the final surface mesh of the fluid simulation — essentially, the visible water surface. This is the main simulation result.
Always available, as it is a core part of every simulation. -
Objects
These are scene objects that you manually add to the object list(i). Once added, they are treated as separate elements during compositing and can be individually faded or isolated in passes.
Becomes available once at least one object is added to the object list. -
Elements
A special type of object added to the same object list(i), but with a dedicated role in the compositing process.
Elements can be used to define foreground, background, reflection, or ground objects.
Becomes available once at least one element object is added to the list.
Their specific function is explained later in this documentation. -
Fluid Particles
Includes unmeshed fluid particles that can be rendered separately from the fluid surface.
Requires fluid particles to be enabled in the simulation settings. → More about Fluid Particles -
Foam and Spray
Grouped and rendered together in one pass. These particles generally appear on or above the fluid surface.
Requires Whitewater simulation to be enabled. → More about Whitewater Settings -
Bubbles and Dust
Also grouped into a single pass. These particles usually exist below or inside the fluid volume, giving them a distinct visual role.
Requires Whitewater simulation to be enabled. → More about Whitewater Settings
💡 In order to render individual passes, make sure the Activate Passes Rendering checkbox is enabled.
Not all passes are available by default — their visibility depends on which features are currently active in the FLIP Fluids Addon.
This section is primarily focused on the CameraScreen object, which acts as a projection surface for your background footage.
Additionally, it also covers the use of Blender's native background image settings.
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Show CameraScreen
Toggles the visibility of the CameraScreen object in the 3D viewport. -
Show Alignment Grid
Toggles an alignment grid overlay in the 3D viewport. This grid can be helpful for fine-tuning the positioning of objects relative to the footage.
More details on how to use this (i) -
Show Background Image
This checkbox controls Blender's native background image system. The image is automatically configured when importing footage via the CameraScreen setup (including the frame range). -
Background Opacity (Slider)
Adjusts the opacity of Blender's native background image. This is helpful for blending or previewing the footage behind your simulation. -
CameraScreen Button
This button is the same one used during the initial setup to create the CameraScreen object.
It remains visible for reference but cannot be used to add another CameraScreen — only one is allowed per scene, since only one footage source can be used at a time.
However, this button has a special function in Still Image Mode (i), which will be covered later in this documentation. -
Camera Distance Slider
This slider adjusts the distance between the CameraScreen object and the camera.
Although the object moves closer or farther away, it is automatically scaled to always fit perfectly within the camera’s view frame.
This feature allows you to influence how the background footage appears through transparent or refractive surfaces, without altering its visual size on screen.Distance = 50:
Distance = 25:
Distance = 10:
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Still Image Mode
Toggles the Still Image Mode for 2.5D-Animations.
A quick first video introduction can be seen in our Development Update Video: -
View Transform
This is a duplicate control of Blender's native View Transform setting, which affects how color data is interpreted and displayed during rendering.
The Compositing Tools automatically set this to Standard by default.
⚠️ It is important that the user manually chooses the appropriate view transform for their workflow.
Incorrect settings may lead to mismatched colors between the 3D elements and the background footage, especially when rendering passes for compositing.
The Objects List allows you to define which non-simulated (non-FLIP Fluids) objects should be included in the compositing workflow.
To add or remove objects, simply select them in the viewport and use the + or – buttons:
Objects added to this list are treated as isolated elements and will appear in the Objects render pass.
Once an object is part of the list, it will also influence the surrounding fluid fading:
💡 The red color indicates the fluid_surface. It´s not red by default. We have been using the Find Fluid Feature to enable red coloring to help with fading visualization:
You have full control over the fading. Settings can be found in the (Advanced) Fading Settings Section:
💡 The Objects pass becomes available once at least one object has been added to the list.
You can also add Elements to the list by clicking one of the dedicated element buttons. The selected object will automatically be added to the list and assigned its role:
Elements can be faded directly in the 3D viewport using their associated fader geometry.
Switching between the element and its fader is quick and intuitive — just click on the parent object in the list to toggle between both (dual function).
💡 The Elements pass becomes available once at least one object has been added to the list. When enabled each Element will be rendered as a seperated pass.
To seamlessly integrate 3D elements into real footage, the FF Elements_Passes material uses three object types:
FG (Foreground): Used to mask out fluid behind real objects. These are similar to roto masks and help simulate occlusion from elements in the footage.
BG (Background): Shadow catchers or caustic receivers. These objects remain invisible but display shadows or lighting effects from the simulation to blend it naturally into the scene.
REF (Reflection): Like BG elements, but with subtle reflections enabled to simulate reflective surfaces such as floors or water.
Ground Elements work like the other element types but are specifically designed to be placed beneath the water surface. They appear only in reflections and help enhance realism by mimicking a visible ground plane from the footage.
These objects do not produce a separate render pass — they are processed in the background and show up only through reflections. Like all elements, they support the Fader system to softly blend edges and avoid hard transitions.
The Import Images as Elements button becomes available after enabling Still Image Mode. It opens a file dialog that allows you to select multiple image files, which are then each added as separate elements to the object list.
This feature is primarily intended for 2.5D animation workflows. A dedicated section in this documentation covers it in more detail.
The Fading Section is the core of the compositing toolkit. It allows you to control how the fluid surface, whitewater particles, or even unmeshed fluid particles are faded into the scene.
⚠️ To use the Fading Section, a simulation must already exist, as it generates the required fluid_surface object. Once available, you must click the Apply All Materials button. This assigns a special compositing shader developed specifically for the Fading tools. This shader is tightly connected to the Fading settings and enables seamless integration of fluid surfaces and particles into your scene. Without this > step, the fading features will not function correctly.
Multiple fading methods are available and can be combined for more advanced control. The following sections will break down each option in detail.
Once all requirements are met and the fluid_surface object has its material assigned, you'll initially see the fluid without any fading. At this point, no fade effects are applied — it's simply the raw fluid surface.
💡 In the example shown, the Remove Mesh Near Boundary option is enabled to hide fluid near the domain edges, leaving only the top surface visible for > compositing. Read more here: https://github.com/rlguy/Blender-FLIP-Fluids/wiki/Domain-Surface-Settings#meshing-against-boundary
To start fading the fluid, enable the Fader checkbox. At first, the fluid surface may disappear — this is expected.
By default, the fader object is positioned in the corner of the domain box. To reveal the fluid again, simply move the fader over the water surface. You'll then see the fading effect in action, allowing the fluid to blend seamlessly into your footage.
Depending on the camera angle and lighting, it can sometimes be difficult to see the fluid surface while adjusting the fader. To solve this, the Find Fluid button temporarily colors the fluid bright red and removes transparency, making it much easier to identify and fine-tune the fade area.
This is especially useful during setup and is used throughout this guide to keep the fading process clearly visible.
If the main fader is moved close to the edges of the domain box, you might get a clean fade toward the center — but sharp, visible edges toward the domain borders.
To fix this, enable the Domain checkbox. This adds an additional fade effect from all domain box sides inward, softening the fluid near the outer edges.
This option works seamlessly alongside all other fading methods and helps prevent hard transitions in compositing.
Another powerful option is Speed Fading. When enabling the Speed checkbox, the fluid surface becomes visible only where the fluid is in motion. Areas where the fluid has come to rest are faded out automatically.
This creates a dynamic fade effect based on fluid velocity and can be combined with any other fading methods. It uses the Speed attribute, which is automatically enabled during the simulation’s initialization process. More about attributes: https://github.com/rlguy/Blender-FLIP-Fluids/wiki/Domain-Attributes-and-Data-Settings
Object Fading is automatically enabled when you add any object to the object list (except elements) — no checkbox required.
As soon as an object is added, it acts as a fader. In the example shown, all other fading methods are disabled to clearly demonstrate this effect. Of course, Object Fading can also be combined with all other fade types for more complex results.
When Whitewater is enabled (foam, bubbles, spray), the fading settings for the fluid surface are automatically applied to the particles — no extra setup needed.
To avoid rendering issues like black dots from transparency, particle fading is handled through scale reduction: particles shrink to zero based on the same fading logic as the fluid.
⚠️ Important: If Whitewater wasn’t enabled during the initial material setup, you must click Apply All Materials again. This assigns the correct compositing shader > to all Whitewater particle objects. The process is fully automatic, but this step is required anytime new features are added after the first material > assignment.
The Velocity and Invert buttons provide advanced control for special cases — for example, in a river simulation where the fluid is constantly in motion. In such cases, Speed Fading is ineffective, as there’s no still water to fade out.
By enabling Velocity, you can fade areas based on acceleration and deceleration instead of just movement. The Invert button is useful if the fade direction appears reversed — for instance, depending on the river’s flow direction.
Details on how to use these features will be covered in a dedicated tutorial.
Yes — there are customization options for all fading types. You’ll find them by expanding the Advanced Fader Settings section.
Each fade method (Object, Fader, Speed, Domain) has its own Color Ramp, pre-configured with handles that can be adjusted to fine-tune the fade effect and enhance the final result.
At the bottom of this section, you'll also find the Particle Fading Controls — a set of sliders specifically for whitewater particles. These sliders control values inside internal modifier or geometry node networks to further refine particle behavior.
At the bottom of the fading section, you'll find the Fade Footage row with two sliders: Footage and Normal.
The Footage slider controls whether fading transitions into transparency (value = 0) or directly into the footage (value = 1). Setting this to 1 lets you preview the composited result directly in the viewport, with fluid fading into the background plate.
This works best when Remove Mesh Near Boundaries is disabled, allowing the fluid mesh to fully enclose the scene. That way, objects viewed from the front are properly covered and visually "disappear" beneath the water surface via the projected footage.
The Normal slider adjusts the influence of surface normals on the fade and can be used for fine-tuning visual blending.
At the very bottom of the panel, you'll find the Comfort Area — a collection of helpful shortcuts for quick access to essential functions already found elsewhere in the UI:
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Select Domain Quickly selects the simulation domain.
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Resolution / Bake Settings Direct access to key simulation settings.
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Set Visibility Options Applies visibility presets to help keep the scene clean and organized.
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Apply All Materials Reassigns the correct compositing shaders to all relevant objects — useful after enabling new features like Whitewater.
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Refresh / Fix All A general-purpose reset button. If something appears broken or out of sync, this will usually fix it.
At the bottom, you'll also see Launch Render Passes — this button remains disabled until Passes Rendering is enabled.
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