I have noticed that I get a considerable increase in noise when using a HDRi image to light to my scene.
The scene has a box with a volumetric material encompassing the entire view area, and it is fire fly free when lighting with point lamps, but incredibly noisy when lighting with HDRi.
Is there any node trick, or setting in Blender, that I can tweak that might resolve this? I have tried with an without multiple importance samples, and various volumetric settings (though I may have missed the magic one!).
Multiple Importance Sampling and setting the size to be equal to the longest edge of your HDRI are about all you can do to keep things accurate. You could try to do the HDR in reflections only and “fake” the colors coming from it with point lights near your object, but this completely defeats the purpose of using HDRI in the first place.
Basically just make sure you’re using Branched Path and turn up the samples for the HDRI in the world settings.
Or you could use a dirty trick by using a .jpg (or .png) instead the actual HDR image. Obviously you would lost some contrast and depth of the lightning you would get from the true HDR, but it’s relatively easy to tweak it using RGB curve to get pretty similar results - just with a LOT less noise.
