When you open this UI element RealFlow will start to voxelize and render the current frame. There is a difference between → "Maxwell Previews" and "Maxwell Interactive":

"Maxwell Preview". RealFlow renders the entire simulation frame by frame. The result is stored as an image sequence and can be viewed in the → "Movie Player".
"Maxwell Interactive". This mode is also called "FIRE". RealFlow creates a temporary render of the current frame that cannot be saved. When you change → viewport perspective, lighting, and materials the render will be updated in realtime.

To activate "Maxwell Interactive" please click on Display shelf > FIRE (see icon above)
By default, all scene elements have the same colour. If you want to change the standard colour go to the node's Node Params > Node > Colour.
Materials can be applied under Selected node > Node Params > Maxwell Render > Material (see screenshot below).
All viewport actions and material changes will be applied to the render instantly.

Maxwell Render is configured globally under

File (OS X: RealFlow) > Preferences > Preview > Maxwell Options

But in most cases you will most probably define individual settings for every project:

Display shelf > Maxwell Scene Preferences
The parameters here and in the preview preferences are identical, so they are only explained once.

Here are the most important options:

“Global intensity” is a multiplier for the scene's light intensity.
With “Use Physical Sky” you have access to the “Light” slider, where the current time of day is adjusted. The scene's colour scheme will change according to the current “Hour”.
If the preview is blurry increase “Image Quality” to 100%.
“Max Sampling Level” is used to reduce noise. The higher the sampling level, the longer the render takes.
For interactive renders, the “Draft” mode is enough; for the → preview render set “Engine” to “Production”.
As long as you are in interactive (“FIRE”) mode, the new settings can be used with the “Apply” button. Once you are satisfied, click on “OK”.

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