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Image courtesy of Hossein Mirfattah

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The render speed in Maxwell, as in any other renderer, it highly dependent on certain parameters of the scene that can dramatically affect the speed of the process. The first we would advice advise to check are: 

Depth of Field: Maxwell calculates the correct depth of field in the image directly from the camera diaphragm aperture just like in a real camera. However, if you want to reduce the depth of filed effect and its cost on the rendering, increase the F-Stop value of your camera or use a Pinhole camera model, that produces no depth of field at all. 

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Geometry: Make sure your emitting surfaces are as low-polygons as possible. Usually a single sided polygon will suffice. 

Not enclosed: If possible, do not completely enclose your emitters inside a dielectric object. All light that is emitted through the dielectric object is considered caustic light and will take longer to render.

Not intersecting: Make sure your emitter is not intersecting other geometry. In most cases this will not be a problem but it may add to the render time and produce strange results in the lighting. Regular geometry can intersect other geometry.

Strength: Keep your emitters at real world values. If your render is turning out too dark, don´t raise the power to several thousands of watts as that will break the realism and can also introduce more noise in the render. Instead, you should control the exposure by using the camera exposure settings (f-Stop, Shutter speed, ISO).

MultiLight

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Nr of ML sliders: the more separate emitter sliders you have, the more RAM Maxwell needs as each light channel must be stored separately while rendering. If you do not necessarily need separate emitter control for all emitters you can "merge" them by applying the same emitter material to the geometries.

Color ML: This option will require more RAM than Intensity-only ML, because each RGB channel must be stored separately while rendering. Avoid this mode if you don't need to also change the color of the emitters while rendering.

Saturated/bright materials (very important)

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Do not use very white or fully saturated materials in your scene. For example, a completely white (255, 255, 255) material will take a very long time to clear and will also make the contrast in the image disappear. Also avoid completely saturated colors such as pure red (255, 0, 0). Pure white or pure saturated materials do not exist in real life so it is better to decrease the saturation a bit. For a white wall, around RGB 220, 220, 220, which is the albedo of white paper, is sufficient. See the page Realistic material reflectance for more info.

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There are a few important things to keep in mind to create efficient materials (including the satured/bright materials info above):

  • avoid more than one Layer set to additive mode, especially if the Layer weight is set to 100%. This means the material can eventually reflect back too much light and introduce noise in the render. If you want to create a plastic material, instead of having two BSDFs (one diffuse bottom layer and one specular top layer set to additive mode), it is recommended instead to use a single BSDF set to diffuse, and add a Coating component to it. Set the thickness of the coating high (eg. 5000nm) to avoid any coloration effects in the coating.
  • keep your materials simple: the vast majority of the most common material types need only two-three BSDFs in one Layer, or two Layers with one BSDF in each Layer. The more Layers you have the more material data Maxwell has to continuously calculate.

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