Simulation - Previews

Previews are an essential part of RealFlow's simulation pipeline, because they help you to evaluate your simulation at the scene's target frame rate. During a simulation, RealFlow creates and stores huge amounts of data. With complex Hybrido scenes, the amount of data can exceed several hundreds of Gigabytes easily. Even with fast SSD drives it is impossible to load, draw, and play back this information in realtime. Therefore, previews are created to get an impression of the fluid's and bodies' dynamics and motion. Previews are also used for comparisons, for example when you have to create different versions of a simulation with varying parameters.

We can differentiate between two types: OpenGL and Maxwell Previews,

OpenGL Previews

An OpenGL preview can be seen as a screenshot from the currently active viewport. This means that the preview contains the entire visible information, including the grid, the background gradient, information about nodes and particle numbers, and the applied shading modes. If the scene contains a camera it is possible to create the preview from the camera's point of view. To look through a scene's camera, press the 5 key. If there are more cameras simply toggle through them with the 5 key. By default you can see a frame around the viewport when you look through a camera, indicating the field of view. To disable this frame, go to

View > Camera Safe Frame

The recoding of the preview happens frame by frame and its size depends on the parameters under “Preferences - Preview” or – if available – the camera's “Width” and “Height” settings. To trigger this process, click on

Playback > OpenGL Preview

Alternatively, you can also click on the “OpenGL Preview” icon of the “Display” shelf, or use the small button in the lower right corner of RealFlow's GUI. By default, the software writes out a series of PNG images and stores them in the projects “preview” folder. If you want to change the location where the files are stored please open:

Export > Export Central > Preview

To edit the target folder, double-click on the path of the “Video preview frames (.png)” entry and enter a new location.

Once the preview is finished, RealFlow will assemble the files and open them as a video in the “Movie Player”. There, you have several controls to playback your preview or save it as a video. It is also possible to add customized..??

Maxwell Previews

This preview type is completely different from OpenGL, because here the scene's elements are really rendered. You can apply materials, change the lighting situation, hide elements from the render engine, and create a final render with production quality.

The first step is the simulation, of course. Once you have finished this part you can start to apply materials. For this purpose, every RealFlow node provides its own “Maxwell Render” panel under “Node Params”. There, you will also find several node-specific settings. RealWave surfaces, for example, can be tiled to create the impression of a nearly endless ocean. The number of splash or spray particles can be boosted with the “MultiPoint” option to get realistic results.

After this step you can do a first fast interactive preview. When we say fast we actually mean realtime. With the “Display” shelf's “FIRE” button you are able to render the scene while you are rotating, panning, or zooming in the viewport. The image in the preview window is constantly updated and you get immediate feedback. Even if you change a node's material, these changes will be recognized and the render will start again automatically.

If you are not happy with the current lighting then open the “Maxwell Scene Preferences”, also available from the “Display” shelf. In the appearing dialogue you can make all necessary adjustments: change the time of the day, use HDR/EXR images to illuminate the scene, define parameters like image quality, motion blur, or scale. When you hit “Apply”, the window remains opened and your changes will be sent to the render engine. This way it is easy to change the scene's environment on the fly. When you are ready confirm with “OK”.

A full description of the “Maxwell Scene Preferences” can be found here.

To trigger the final render process, click on the “Display” shelf's “Maxwell Preview” button or start it under

Playback > Maxwell Preview

A really nice feature with Maxwell previews is the possibility rendering the preview via RealFlow's Command Line version. This means that the render process will be done outside the GUI and you do not have visual feedback in RealFlow. But, the finished images can be opened from the project's “preview” folder. Under “Export Central” you can also specify if you want to store the scene as a Maxwell Scene file (MXS). When a Maxwell preview is rendered using the Command Line version RealFlow will first check if the scene contains a camera (Maxwell previews – not the FIRE option – always need a camera). If the camera is missing a warning appears and the render process will not start.


If you want to render the preview inside RealFlow's GUI then this is possible as well, of course, but this method is slower than the “Command Line”. In the GUI you can monitor the render process. The final images will be assembled and sent to the “Movie Player” for playback.

All this is possible inside RealFlow and without having to export data to your 3D application.

If the rendered images do not match the adjusted size (e.g. 1280 x 720) please go to "Maxwell Scene Preferences" in the "Display" shelf, and change "Image Quality" to 100%.

Hide Objects from Maxwell Render

Many simulations require different helper objects, for example to enclose a fluid, build a barrier for rigid bodies, and so on. These elements are normally not intended for rendering and therefore it is necessary to exclude them from the Maxwell engine. This can be done with a few mouse-clicks:

(Multi-)select the object(s) you want to hide in the final render and click on

Display shelf > Hidden from Maxwell Render

To make it visible again for Maxwell, use the “Render with Maxwell” button in the same shelf.


Another possibility is located under

Selected node > Node Params > Maxwell Render > Renderable

By default, this option is set to “Same as display”. This means that the node will not be rendered when it is invisible in the viewport, but you can also change it to “No” to hide it.