Slow scene render fast... why?

fred9803

I worked on 2 scenes this morning and this happened.

First scene, brightly lit, 2 small emissive objects, a few small reflective surfaces, no transparency maps apart from eyelashes/hair etc.. Two characters. Took 36 minutes to render.

Second scene, much lower lighting, multiple emissive objects, large reflective surface (floor), trans maps on several large cloth objects (lace), same two characters, render resolution and Iray setting as the first one. I thought here we go this will take forever. Took 12 minutes to render out. (RTX 2080)

There must be something else at play other than the usual suspects that affect Iray render times. Wish I knew what it is.

I'm not complaining, but have no idea or explanation why this would happen. A few times before I seem to have hit some sort of sweet-spot with scenes that "should" render real slow but didn't. Has this happened to anyone else? Why is it so?

 

 

Cenobite

What i do now is use non linear point lights and adjust the lumenosity according to the area you need lit if you are doing still frame pictures you could also parent the lights to characters for animations i figure, this gives better light then most lighting use in scenes, the headlamp ruins things so you need to light up the scene without using emissive shaders because i find emissive shaders drag performance down somewhat and leave firefly on the textures blurring out the details, Point lights, non linear lights & spot lights are better to use just take more time to position correctly. I find these lights render the scene quicker with more detail.

 

I tried a few different light sets out, using different shaders to light up areas or place certain lit objects to illuminate area but the standard lights the DAZ3D editor provide when place right with the correct settings are usually the best option to highlight parts of characters & scenes.

In the shot below i used a linear pointlight on the woman standing with the clip board, without out the point light the character was too dark and the head lamp was way too bright on the surrounding environment washing out all the other textures.

There are lots of emissive lights from the coloured light bars in this scene which speckel out the render and cause the render to take longer, particularly when using Ray Tracing as you can see from the reflections in the window surfaces in the render below, they reflect to all other glass sufaces and glossy surfaces that hold a certain reflection value which reflect all other angles to all props in scene causing some slower render times because of all the reflection, the more reflective surfaces the more light bounces around the scene taking longer to render.

Richard Haseltine

If the emissives were the only lights in the first scene then, no matter how bright they were, they still had to get bounced light into the shadowed areas for the image to converge- and if most surfaces were quite mat that would have taken time. If the second scene had a wide scatter of emissives, however dim, and if there were lots of chnaces for the light to reflect without major attenuation then it would have been much quicker to achieve and even scattering of hits and so converge.

fred9803

Thanks Richard, that makes sense. 

Cenobite

Heres a render i just did, it removed the head lamp even tho i had it set on then it only took 20 minutes to half render when it should of been baking longer, the graphics are grainy poor quality even tho the area is well lit, HDRI backgrounds also seem to vanish after adjusting some props and need constant reseting as skydomes which is another issue thats become annoying. The render is uncomplete yet it finishs the render time why? when previous renders with much less took 2 hours to fully render.

Lighting is a huge issue in this editor, one second the light is fine the next it ruins the complete picture, i find the head lamp is set right the first time you load something into scene after you move around to position to get the shot you want all lighting goes haywire & you're then spending hours trying to get the light right all over the scene.