Change in lights: took render time from 2 hours to 14 hours

lamoid_5f20d3e469

Has anybody here experienced something like this? 

I was using 3-point lighting with large light panels -- spotlights given rectangle geometry with height and width == 230 cm, photometric mode, render emitter off. The scene was indoors in a long rectangular room. For some reason that I have not figured out, the light in the room suddenly dropped off to nearly black some distance behind the rear fill-in light. Weird. Never seen that before. Just for expediencies sake, I replaced the rectangles with point radiators with luminous flux adjusted to illuminate the scene to similar brightness as it was by the panels. That solved the intensity dropoff problem,

The render time of the scene, however, increased from two hours with the rectangular lights to 14 hours with the point radiators. I am using iray with an nvidia 2070. Both scenes were rendered by the GPU. The CPU and CPU fall-back options were both disabled. GPU monitoring programs showed that both renders were in fact in the GPU. Also Windows 10 Education v 1903 build 18362.778, CPU intel core i9-990K @ 3.6GHz (8-core 16 threads) 32GB RAM. Daz Studio 4.12.1.118, nvdia geforce driver V 27.21.14.5656 (nvidia 456,55) DCH dated 24 Sept 2020.

Why would a change from panel lights to point lights cause a 7-fold increase in render time of the same exact scene?

Richard Haseltine

Because with a point source instead of planes more areas are relying on indirect lighting (light paths that bounce off other surfaces to get there, instead of coming direct from the source) and given the much more limited starting points for paths it is less likely that a given path will end up bouncing to a given pixel - either alone will slow the render down, combined I'm not surprised at the change.

lamoid_5f20d3e469

I knew you would understand what was going on here, Richard.That makes sense. Thank you!

BTW, I figured out why my first image was abruptly very dark in the back. Even though I set the light panel to not render -- render emitter off, it still blocks light as if it were a visible object in the scene. Telling studio to not render the emitter does not make it completely transparent.