Multi-Pass versus Standard rendering

[Tore]

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Edited April 10, 2018 by Tore

[Fuchur]

There are many things involved like soft shadows, which are distributed to the passes and so on.

If you do not use those neighter and really get rid of all the more advanced things, it might be the same, but I am really not sure about that.

 

Is it just out of interest, or is it for a specific reason you are asking about that?

 

Best regards

*Fuchur*

[Tore]

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Edited April 10, 2018 by Tore

[robcat2075]

The results for most rendering should be the same between the two. The same rendering engine is drawing the pixels.

A noticeable difference should be brought up as a possible bug. There are some effects that only work in Multi-pass.


The dig difference between regular and multipass is how the anti-aliasing is done.

Multi-pass subdivides each pixel (16 passes would be a 4 x 4 subdivision) and does a full render for each sub pixel then averages the sub pixels together.

 

However, a pixel that is among other pixels of similar color (like in a blue sky) needs no anti-aliasing and won't benefit from it. Multipass rendering wastes time subdividing it.

 

Regular render does "adaptive anti-aliasing." Every pixel is rendered, then the contrast between pairs of pixels is compared. If the contrast is above some threshold the pixels are subdivided and re-rendered on a 2x2 grid. The sub pixels are again compared for contrast and if any pairs are above the threshold they are subdivided again and rendered.

The result is that regular rendering has anti-aliasing detail equivalent to 16-pass rendering, but usually in far less time.

It is possible for an extremely detailed scene to take longer in regular rendering and I have noticed that objects with thin lines look better with multipass.

[robcat2075]

Well, mostly of pure interest, but also if they ARE the same core, it could be proposed to just have one render mode, with a row of options to turn off/on, say like: DOF - High Quality DOF - Motion Blur - High quality Motion Blur -...

 

The high quality DOF and motion blur can only work in multi-pass. AFAIK there is no way to skip sub pixels in the way that adaptive anti-aliasing does. Sub-sampling every pixel is how they work.

[Tore]

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Edited April 10, 2018 by Tore

[robcat2075]

For most purposes

 

Thanks for the explanation. So, as I understand it, if I have scene that is not using DOF or Motion Blur, I'll get faster rendering with at least the same (high) image quality, using standard instead of multi-pass rendering.

 

 

 

Yes. For most images, regular render will be faster and of the same quality.

Regular render generally takes about as long as a 4-pass render but has anti-aliasing that is (usually) as good as a 16 pass render.

For extreme anti-aliasing needs you can either render at a higher res with regular render and scale the image down or use a higher multipass setting.

[robcat2075]

Comparison of three different anti-alliasing results

 

(click to see full-res)

 

[robcat2075]

Here is a case where Multipass is potentially better

 

The materials for the wood grain and paint chips make many fine lines and edges that are challenging for anti-aliasing.

 

(click to see the full-res images)

 

Regular render took about 76 minutes and has many dark spots...

 

 

A 5 pass render took 19 minutes and gets a better appearance...

[robcat2075]

For this scene the global illumination/AO is the biggest cost of the render but in multi-pass that is distributed among the passes so more passes doesn't exactly equate to more render time

 

9 passes, 18 minutes

 

 

16 passes, 25 minutes

 

 

36 passes, 32 minutes