Corona Renderer for 3ds Max – How to Render Faster (Part 1)

Don’t set the Albedo intensity too high.

Table of Contents

In Corona, Albedo can be understood as the “brightness” of a material. It is the combined result of various material components, including diffuse (color), reflective (reflection), refractive (refraction), and translucent (transparency). For instance, white paint is “brighter” than gray paint. Glossy gray paint is “brighter” than matte gray paint, and stainless steel is “brighter” than wood, and so on. Note that the emission setting in self-illumination, used for light-emitting materials, does not contribute to Albedo intensity.

You can quickly check Albedo by adding the CShading_Albedo channel in Render Setup > Render Elements. When rendering, select the CShading_Albedo channel to observe: the whiter the area, the higher the Albedo. Areas highlighted in red indicate an excessively high Albedo, which will slow down the rendering process. The more red areas, the longer the render time, and vice versa.

An example of incorrect Albedo settings causing longer render times. Source: forum.corona-renderer.com

So what is a reasonable Albedo? According to the developers, a material’s Albedo should ideally be around 70%, which equates to 0.7 / 1 or an RGB value of 180.

Each component’s intensity can be calculated using the following formula:

level (0-1) * color (0-255)

If the level (ranging from 0 to 1) is x and the color value (ranging from 0 to 255) is y, we will have:

(x/1) * (y/255) < 0.7

For example, with a white material, to achieve an Albedo of 0.7, we can:

Set the diffuse color to pure white and the diffuse level to 0.7 / 1.
Set the diffuse color to approximately 180 / 255 (~ 0.7 / 1) and the diffuse level to 1.

Materials with excessively high Albedo values, especially those with absolute Albedo (e.g., a completely white material), will yield poor results (such as overexposed images with loss of detail) and can also cause longer render times.

Note that lowering Albedo will result in a darker render, so we will need to adjust the intensity of the lights or increase the Exposure parameter. However, this approach will be quicker and yield better results than trying to fix an overexposed scene due to the material settings.

Be Careful When Using Refraction & Caustics

When looking at the Refraction section in the material settings panel, we will see an option labeled Thin (no refraction). Glass materials in Corona are divided into two types based on whether this Thin (no refraction) option is checked or not.

The difference between Thin Glass and regular glass with Caustics enabled.

If the Thin (no refraction) option is checked, we will have thin glass material suitable for objects like window glass, light bulb shells, soap bubbles, and other similarly thin objects. This type of glass will not exhibit refraction effects, meaning the Caustics option (light gathering effects) cannot be used. We will see the object appearing completely transparent with only reflection shadows. While this is not physically accurate, this type of glass renders significantly faster and is recommended for most transparent materials in renderings.

If the Thin (no refraction) option is not checked, we will have glass suitable for objects like bottles, glass furniture, colored glass objects, liquids, and other substantial objects with significant thickness. This type of glass will include all the physical effects of regular glass, and particularly when the Caustics option is enabled, it will produce very realistic light gathering effects. The downside is that it renders very slowly, especially with Caustics, which can increase rendering times by 2-3 times. It is best to avoid using it unless in special cases, such as close-up renders or when rendering pools or aquariums.

Use Reflective Mirror Materials Sparingly

As we know, the brighter a scene is due to materials, the longer the noise reduction takes. Using many reflective mirror materials will require the machine to perform additional calculations, thereby reducing rendering performance.

When creating materials with high reflectivity, such as metals or mirrors, we mainly use the reflection channel, so set the diffuse channel to 0 by lowering the Level to 0 or setting the color to absolute black (RGB0).

Use Emissive Materials Properly

This is a common mistake when using Corona, where users fail to distinguish between the two types of emissive materials: Corona Light and Self Illumination, and when to use them. This is one of the causes of fireflies noise—an undesirable type of noise that does not disappear even after thousands of render passes.

The Corona Light Material is essentially a light source, just like Corona Light, used to illuminate the scene. The Self Illumination material is essentially an option in standard Corona materials, used for objects that glow in the dark but do not illuminate the surrounding environment. This distinction is very important and should be noted.

At the same render time, the top section uses Corona Light as the light source, while the bottom section uses Self Illumination as the light source.

Confusing these two types of emissive materials can create a lot of noise because their nature as light sources is completely different, and they are calculated differently. Corona Light is used to illuminate the scene, while Self Illumination is meant to emit light on its own. You can see a significant drop in rays/s performance as soon as you mistakenly use these two types of materials interchangeably.

Incorrectly setting up lighting can significantly increase noise in the render.

However, there is a case where you should use Self Illumination to illuminate the scene, specifically when using dim light over a very large area (for example, a large hot air balloon in the middle of a square). In this case, using Corona Light will significantly increase render time. If you just want a small spot of light that does not illuminate the surrounding scene, use CoronaLightMtl and turn off Emit Light. This will create a light area that, while not physically accurate, will render much faster than Self Illumination. I often use this feature for very small objects, such as the filaments of light bulbs or candle flames.

Use Corona Bitmap Instead of 3ds Max Bitmap

Corona supports both types of maps: Corona Bitmap and the default 3ds Max Bitmap. Both maps yield similar results, but there are several advantages to using Corona Bitmap. In addition to providing more filtering options and offering more information in the settings, using Corona Bitmap will typically speed up rendering slightly. Furthermore, using Corona Bitmap for the Bump and Displacement slots will enhance render quality.

Convert Bitmap to Corona Bitmap in Corona Converter.

Converting Bitmap to Corona Bitmap can be done using the Corona Converter (right-click on the Viewport, select Corona Convert, and then choose Bitmap > Corona Bitmap). Unfortunately, this conversion does not occur automatically when you drag and drop a Bitmap into the Slate Material Editor. Therefore, I recommend saving this conversion for the end.

An example of a file set up with materials correctly: with three light sources, a resolution of 1200×900, and a noise level of 3%, rendering on a Ryzen 1700 takes only 6 minutes and 18 seconds.

The above content covers the entire first part about material optimization. For simple scenes like the ones above, when the lighting scenario is not too complex, optimizing the materials alone can significantly increase rendering speed. Wishing you success, and see you in the next parts!

Written by Dang Tien Dung

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