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I modeled a house and wanted to render the rooms from the inside with the approximate sun lighting through the window. However cycles is incredibly slow in doing that. What should I change to increase my render speed? I am okay with sacrificing quality for it as it is not yet my final render and I am still planning the room layout.

The room is physically correct with ceiling and floor (cube with multiple small holes in it). I'm rendering in perspective mode using Cycles.

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  • $\begingroup$ How is your room set up? Is it completely physically realistic (with a ceiling?), or is it open-roof? Light sources and samples contribute to slowing down the render, but there's not necessarily a safe way to cut down on them without decreasing the quality of your scene. In addition, are you doing an orthographic top-down floorplan view, or are you doing a variety of perspective views? $\endgroup$ – Kyle Willey May 30 '13 at 16:13
  • $\begingroup$ changed my answer to include that question. For the orthographic floorplan view I am using freestyle. Decreasing quality is not a big problem. $\endgroup$ – miceterminator May 30 '13 at 16:18
  • $\begingroup$ Can you add a screenshot to the question? $\endgroup$ – CharlesL May 30 '13 at 16:42
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It sounds like you are using a sun lamp for it, that really slows it down. So the only thing I can add to Gwenn answer is: for interior renders, use planes with an emission shader in every window hole, and disable the "camera" option in the "ray visibility" panel in the object properties so you don't see it when rendering. You can combine that with a sun lamp if you want hard edge shadows coming from the sun.

I use this method a lot and it really speeds things up.

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    $\begingroup$ This. Lighting setup has a huge effect on performance. If you are relying on bounced light from the outside to give you good illumination of a room, then it will be slow and noisy. Adding emitters (lamps or mesh) inside the room is the best way to cut down on the noise and the rendering time. Combine this with a decreased bounce setting, the render time should improve a lot. $\endgroup$ – Mike Pan May 30 '13 at 20:46
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    $\begingroup$ Emitters in windows are a manual replacement for light portals. Other relevant tricks from the Cycles documentation are smooth light falloff and filter glossy options. wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/… $\endgroup$ – brecht May 30 '13 at 21:21
  • $\begingroup$ Would it somehow be possible to du a preliminary render to get the approximate lighting and than use the walls as diffuse emission sources themselves with the light that fell on them during the preliminary render? (All of the Materials used are diffuse) This should probably be a new question $\endgroup$ – miceterminator May 31 '13 at 7:18
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There are a number of things you can do to reduce the render time in Cycles. Here are a few that I frequently use:

  • Reduce resolution: If you can get away with a smaller output image, or if you're just doing a test render, set the 'Percentage scale for screen resolution' to something low, like 25% or 50%.
  • Decrease the number of faces: On small items that don't need an excruciating detail, instead of using a subsurface division modifier, set the shading to 'smooth' and consider using a decimate modifier to reduce the number of surfaces that light has to reflect off of.
  • Switch to GPU rendering: In the User Preferences view, go to the System menu. Under 'Compute Device', select CUDA. Then, when you go to render, in the Device dropdown, you'll have the option 'GPU Compute'. (This only works if your computer has a GPU with a CUDA compute rating of 2.0 or higher.)
  • Decrease the number of light bounces: If you don't have highly refractive materials, like glass or gemstones, it can pay to reduce the number of light bounces Cycles computes. In the 'Render' menu of the Properties panel, scroll down to 'Light Paths.' If you set the minimum number of bounces to 0 and the max to a low number (like 4 or 5), you should greatly improve your render time without sacrificing too much realism.
  • Change tile size: In the Properties panel > Render menu, scroll down to 'Performance'. If you're using GPU rendering (see above), set the tile size to 128x128 or so. If you're using CPU, set it to 16x16.
  • Simplify: In the Properties panel > Scene menu, scroll down and check the Simplify option. This sets global limits on mesh detail size and soft shadow calculation that can really improve rendering time.
  • Use 'Multiple Importance Sample': For each of your light sources, scroll down to 'Settings' and select 'Multiple Importance Sample'. This greatly speeds up the rendering time for glossy and diffuse surfaces, and allows you to use fewer light samples to achieve the same quality render.
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    $\begingroup$ this is a very generic answer that is very well explained on blenderguru. My question was specific to interior as most of the scene is lit by indirect light, I do not understand why you edited the question. $\endgroup$ – miceterminator May 30 '13 at 21:15
  • $\begingroup$ Tests have shown that the larger the tile size, the faster the rendering: size 128 - 1m 37sec; size 512 - 51 sec. Almost twice as fast. Scene setup: one sun, simple room model, GPU rendering, 4Gb video RAM. $\endgroup$ – Ruslan L. Feb 21 '17 at 10:49
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One thing that may speed it up faster than anything else is lowering the bounces. Since the image is interior, you'll have a lot of light bounces greatly increasing render times. To fix this, try setting the min bounce to 3 and max bounce to 4. You won't loose too much realism and if your iterating and rendering a lot, it helps to speed things up.

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