About Portal Area Light

Hi kchow,

That might be because of the Volumetrics you are using.

It always depends on your Scene and Lighting and your Hardware, how much the performance gain actually is, but it's a gain, that is basically 'for free'.

With most speed-ups, the quality will suffer (a bit or much), so it becomes a balancing act.

It all depends on what you're working on. If you're just making a still image, you hit render once and you're done, you're like well 4 seconds, big deal. But then if you're working on an animation of a flyby of a room on a house, and you have to render 1,000 frames, you are saving 4 seconds per frame, that's 66.6 minutes you are saving, so over an hour of render time. And animations can have thousands of frames. And then if you have not one but various portals throughout your scene, say you have 4 rooms and you are saving 4 seconds in every room you render, now that's 16 seconds of render time you're saving. And if you have to do a 1,000 frames animation and you're saving 16 seconds per frame, that time saver shoots up significantly. 

Hm, that surprises me too, that without Portal, it Renders faster...well, Portals are more for quality, than for speed it seems. And that actually makes sense. In an enclosed scene, with light coming through a relatively small window, for instance, rays, shot from the Camera have only a small chance to 'find the light source' (window). A Portal 'tells' Cycles, where to 'look for the light' and that means more Samples will find the Light Source and contribute to the Lighting. It doesn't necessarily mean that Blender uses less Samples (although it might for some Pixels, when using 'Adaptive Sampling', now enabled, when using the Noise Threshold Threshold)...Then there is also Path Guiding, which helps Cycles to clean up the Noise faster on certain Scenes, but that only works on CPU at the moment.

Like you know by now, there is a lot to it; there are no simple rules, what to use when.

Hi Keswick,

Awesome you took the time to dive into this!

From what I've experienced there's about a 100 settings you can optimize for faster renders, if not more. Each has it's own benefit and drawback and usually combining multiple of them works best, as well as making changes that fit your scene e.g. path optimization for a scene with caustics. 

Anyways here's the funny part though; on better hardware setups the difference, especially when doing only 1 or 2 optimizations, can actually be 0 or even a net negative (longer renders). I don't know why, or how, that happens and I expect this is something that would require both some of the big brains at Blender AS WELL AS people over at the GPU manufacturer. 

I've done multiple videos on render performance and re-rendering the same scene 100's of times, with the exact same settings, still has resulted in HUGELY varied results. A 1 min 30 sec render could range anywhere from 1 min 25 sec to 1 min 35 sec, with 0 changes to the settings. My idea is that Blender and your GPU play in synchrony and that this balance of communication between the software and the hardware is what drives the final speed of the render. This communication varies depending on other stuff your PC is doing, even when personally not doing anything. Background tasks, CPU usage, GPU usage for things etc or even the time between renders (waiting longer seemed to give my GPU some breathing room and made the renders faster?!). Again 100's of factors.

I believe that adding these small optimizations in the long run is a positive, as understanding they exist and applying them is useful for when you really need it. In all other scenario's it might add a few seconds of rendering, but that 'is what it is'.

Cheers and happy rendering!