Corona Renderer for 3ds Max – How to Render Faster (Part 2)

Excessive Brightness Slows Down Rendering

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Having more intense light sources does not always result in faster rendering and less noise. To clarify this issue, I conducted a small experiment.

On the left side, the teapot is illuminated by direct light, while on the right side, it is lit by indirect light.

In the direct lighting experiment, the scene is set inside a closed cylinder with three light sources positioned above and a teapot placed underneath. In the indirect lighting experiment, the scene is within an open cylinder surrounded by a closed box, with three light sources outside and lower than the cylinder (the indirect light reflects off the ceiling of the box). All settings are the same, and the diffuse level of all materials in the scene is set to 0.5.

The render time for 20 passes decreases as the light intensity decreases

In the experiment, in both cases of direct and indirect lighting, when the light source changed from very low to medium intensity, the time to render 20 passes did not change. However, when the light source was increased to a very high level, the render time doubled, resulting in a loss of detail. I attempted to recover the image using Photoshop, but this introduced significant noise and caused the image to become blurry.

Thus, we can conclude that excessively high light intensity significantly reduces render speed. On the other hand, very low light intensity only slightly increases render speed but leads to high noise levels after post-processing. Using a light source with just enough intensity, or slightly below average, yields the fastest results with the least noise and makes post-processing easier. In other words, it’s better to render a slightly dark image and then brighten it rather than rendering an overly bright image and trying to reduce the brightness later.

The Fewer Indirect Light Sources, the Faster the Render

Instead of illuminating the object directly, indirect light is diffused through another medium, such as a mirror, curtain, or reflections off walls, ceilings, floors, and other objects, before reaching the target object that needs to be illuminated.

Instead of placing a cylinder in a cavity, position a hidden rectangle on the outside; this will significantly speed up the rendering process and yield much better results.

Indirect light sources are not a design flaw; however, they can introduce significant noise and slow down the rendering process considerably. Therefore, whenever possible, minimize the use of indirect light sources by employing direct lighting techniques that still achieve results similar to those of indirect lighting.

The simpler the lighting scenario, the faster the rendering.

Rendering speed is greatly affected by the lighting setup. A complex lighting scenario occurs when:

There are many light sources, for example, 15 or more.
The light sources have complex shapes, such as a teapot assigned with Corona Light material.
Multiple indirect light sources affect the scene through “bright” materials like mirrors or white walls.
Light sources interact with each other, such as two lights placed in sequence with one light set to occlude other lights.

The number of light sources often depends on the design. However, we can group several lights close together into a single light source to simplify the lighting scenario while still achieving similar results. For example, replace four closely placed round downlights with one square light, consolidate five spheres in one wall light into a single sphere, turn off Emit Light for complex glowing objects like lightbulb filaments, and replace that light source with a sphere light to reduce indirect light sources while avoiding overlapping lights.

Adaptive Light Solve option in Corona Renderer 6

In addition, there is one point to note, which is the use of the New Light Solver option in the Render Setup > Performance tab. This is a technology that was being tested in Corona 3 and earlier, and it has to be enabled manually. In Corona 4, it is enabled by default and is called the Adaptive Light Solver. When this option is checked, Corona will determine which light sources are important and primary, allocating more processing power to them, resulting in an image with less noise.

Not using Light Mix will speed up rendering.

Although it is extremely convenient and easy to use when showing different lighting scenarios to clients, as well as adjusting the lighting directly during rendering, Light Mix does reduce rendering speed, even with ample RAM.

Moreover, in files with many lights, it occupies a significant amount of memory and may cause crashes if the hardware is not sufficient. Therefore, if you are only rendering one final view, try to set the lights correctly in the scene; the file will render faster.

Use Portal Light when rendering interiors with windows.

An important note: this content is only applicable for Corona Renderer 5 and below, as starting from Corona Renderer 6, a new feature called Adaptive Environment Sampler has been added to replace Portal Light.

In almost all cases, Portal Light will significantly increase rendering speed when rendering interiors or scenes inside a house with light coming from a small window. It can be said that Portal Light is a necessary element if you want to optimize your render file for the fastest performance.

Creating a Portal Light is very simple; draw a flat plane with no thickness, the orientation and mesh density do not matter (of course, the fewer the better), then find and assign the CoronaPortalMtl material to the plane you just created. Portal Light works best when the plane is perfectly fitted into the window opening at the exact outer edge of the window frame. It can be placed outside the window with a slight size adjustment, but it must absolutely not be placed inside the window, as this could even turn the Portal Light into one of the causes of noise.

How to Properly Position Portal Light. Improper placement can even cause errors and make Corona render slower

Portal Light should be added to all openings (i.e., areas where ambient light enters), not just in the current camera view but in all openings within the scene. However, Portal Light does not always equate to faster rendering. It will only help speed up rendering when done correctly; if used incorrectly, the rendering speed will not increase significantly, and in some cases, it may even slow down the rendering process and create unwanted lighting errors.

Written by Dang Tien Dung

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