Why rendering in brighter scene is faster than darker one?

James

I have a scene where's light comes from one side shining on a character.

It seems, that when rendering from one side where the scene looks brighter, is faster than the darker side.

Why?

superlativecg

Same reason night time pictures need longer exposure.

Paintbox

Yup, it's like a camera catching photons. Less photons means longer rendering for all the rays to make a meaningful picture.

Also if there are many corners and crevices, there are more bounces taking away some speed as well. That's why a lone figure in a hdri renders superfast, but in a room at night takes (a lot) longer.

Padone

Of course you can also give up convergence, that doesn't always work fine anyway, and use a fixed number of samples plus the denoiser. That's how to work fast.

James

what does convegence do ?

charles

Iray is rays, coming out of the camera and trying to ~randomly bounce off objects seeking a light source. So reverse from real light. When a path from camera, bounces that eventually connect with a light source, the path is reversed back doing exposure on the bounced objects and applying the shader instructions. rays that don't connect with a light source are ignored and basically wasted processes. So the more light the more chances these bounces connect with a light source and hense more photo process take place per itteration and hense faster rendering.

This is why HDRI scenes are extremely fast in opposed to indoor lighting with point and spotlights.

So even though Iray can produce amazing image quality it's pretty inefficient to other rendering mechanics.