Rendering times vastly different


I have a very simple scene - a room with a few doors, a window and a shoe (blend attached), in total ~28k faces, and it takes sixteen hours to render. The problem being that I also have a single object (ashtray, attached) with a similar number of faces, made from glass, which means refraction, caustics, high transmission/volume path tracing, and an incomparably higher number of modifiers which rendered in two. Everything but face count is completely different and all I know says that the room scene should render in a matter of minutes, not days!

Can someone explain what is causing the room which is nothing but diffuse and glossy to need a day to render?

EDIT: I forgot to disable partial rendering (cropped around the shoe) when uploading, it’s not meant for the shoe alone, it’s the whole scene.


The render is slow because:
You’re rendering at a quite high resolution.

You’re rendering in “Progressive Refine” mode, which is slower and uses significantly more memory than tile rendering.

Cycles is not practical for scenes like your hallway.
Cycles is an unoptimized path tracer, meaning it fires “rays” from the camera and lets them bounce until they hit a light source. In your case the light sources are either in another room or hidden behind a complex venetian blind. Therefore the chance of a ray actually finding a light source is very low and as a result convergence takes forever. So, you can either make it easier for Cycles to find the light sources or just use another render engine.

BTW, from my understanding the polygon count has very little effect on a path tracer’s render time, so there is no such logic in place as “few polygons = must render quickly” or “same polycount = should render in similar amount of time”.

I’d like to ask why this is considered high resolution. It is just slightly over an A4 format at full print resolution. To put it differently, what would a “normal” resolution be?

Well, I benchmarked a scene with varying amounts of faces and it turns out you’re right. I don’t know why the last time I checked (and the last time I had a large scene) Blender froze completely. Also, did a really simple scene right after this one and it seems that there is next to no problem with noise once there’s ample light (at least here there was only one) - I had the render set to a thousand samples just for kicks but I could have stopped at about fifty. There was barely any change.