this post was submitted on 02 Feb 2024
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Ray tracing speed primary depends on the number of pixels, not the complexity of the scene.
The complexity of your scene makes a huge difference. If your scene has fewer things for light to bounce off of, doing the ray tracing is much faster
(Source: I do blender renders with cycles)
So I'm not exactly sure how Blender implements this. There can be a few details that can make a huge difference. Just for starters, is Blender rendering 100% ray tracing here, or is it a hybrid model with a rasterizer. Rasterizers tend to scale with the number of objects, while ray tracing scales with the number of pixels. A hybrid will be, obviously, something in between.
Then there is how it calculates collisions. There is a way to very quickly detect collisions of AABB boxes (basically rectangles that surround your more complicated object), but it takes a little effort to implement this and get the data structures right. You can actually do Good Enough sometimes by matching every ray to every AABB, and then you do more complex collision checking against what's left, but there's a certain scale where that breaks down.
Blender is generally very well done from what little I know of it, but I'm not sure how it handles all these tradeoffs.
So, as far as I know the cycles engine does ray tracing until it hits a noise threshold, then does ai denoising for the final cleanup. You can see where the more visually complex parts of your render are, because it will take a lot longer to render to a less noisy state. I don't know specifics of how it works under the hood, but given how complex parts of your image take longer to tender to an acceptable threshold than simpler parts it seems obvious to me that render time scales with complexity.