I have been watching this video on Youtube (https://www.youtube.com/watch?v=sAVxxuT9v_g) and tried to recreate what the instructor was on about the ‘Multiple Importance’, and this is my test file,
20230129_cycles_multiple_importance_0002.blend (897.7 KB)
However, when I turned off the Multiple Importance for all lights, including the area light in front:
and then turned them on again:
I just cannot see the differences. Can someone elaborate more on the use of this flag please?
In video, he is using mesh lights with IES profiles when you are using only spots.
His point is that he noticed a difference for mesh lights.
His room is closed. So background is not a source of lighting for his scene.
The term " Multiple Importance Sampling" is repeated at several different places in UI.
It is present in World properties, Light Properties and for Shading.
The manual gives a good explanation for each case.
For an HDRI map used by World shader, it makes an importance map to use brighter pixels more as light sources than darker ones. It is preventing fireflies and noise due to conflicts from multiple light sources by creating a hierarchy.
Basically, for lights, it is the same thing. Option consists in enabling/disabling their contribution to indirect light sampling.
Manual is precising that use case is for area lights and reflections.
It is used for surface shading to control emission of materials, the opposite way of lights by enabling/disabling their influence on direct light sampling.
In 3.5, a new light tree technique will be used. For surface shading, setting will be replaced by choice to considering front side, back side of mesh, both or none for emission sampling.
Thank you so much for replying. I still have problem understanding what you have written here. I did see that he used different mesh lighting but I thought it’s just a light source, like spot lights etc… As you mentioned, I found ‘Multiple Importance’ in Properties for World, Lights, and Materials,
I am slowly understand the problems with indirect light contributions and the bouncing of light rays. The funny thing is I found this video (https://www.youtube.com/watch?v=svWEnvcbYwk) and apparently this has been used in Blender since version 2.68.5, ages ago, 2013.
I never noticed in later versions until 3.5 Alpha. What has been happening in between, do you know?
Mesh lights are light sources. But in those case, light is emitted from several triangles or pixels from texture.
One mesh light is corresponding to multiple light sources. When a spot is one light source from a point.
Yes. I am referring to presence of Multiple Importance setting in material properties for emission from surface or for volume shaders.
Indeed, the impact of setting was more obvious when Cycles was less efficient.
Cycles has been rewritten for Blender 3.0, with new sampling methods and ways to denoise render.
So, effect of Multiple Importance sampling is less obvious ; when you don’t know that is related to reduction of noise.
In blender 3.4, when path guiding was introduced, developers hesitated to remove the setting.
It looks like new light tree and path guiding are not mature enough, yet, to remove it, in 3.5.
But that is possible that Multiple Importance sampling settings will be completely removed in future releases.
My simple explanation is probably actually wrong, but here is how I think of it.
Raytracing shoots light ray out of the camera towards the scene and they bounce around randomly until some of them eventually hit a light source. Many never hit a light source. So, sometimes it’s beneficial to shoot a light ray directly from the camera to the light source. Multiple importance sampling tells the render engine “this is a light source”.
This is why light portals are useful. The entire hdri environment is a light source, but if your camera is in a room with a few small windows, it’s going to take a lot of samples before rays randomly pass through the window and hit the HDRI. Light portals over the window make rays shoot straight from the camera to the window, resulting in more accurate lighting and less noise. Same with mesh emission. Same with very very tiny light sources.
Point light on the left, emission material on the right with surface multiple importance disabled:
