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In my situation, I want to render many narrow spotlight all overlapping in mist.

enter image description here

You can see in this image how all the spotlights overlap. The rendering I'm trying to produce is a simulation, and therefore I want to keep the geometry mostly as it is.

I've tried a few approaches:

Use spotlights into a volume

The issue here is that the minimum angle of a spotlight in Blender is 1 degree, which is still too large for me! (My spotlight should be 0.5 degrees)

Blender Render / Volume emit cylinders + Scattering volume

enter image description here The main issue with this approach is that in areas of very strong overlap, some bits are even darker than background. I.e. where there's lots of light crossing over, the result is shown as dark.

Perhaps the 'emit' object are also casting a shadow.

Cycles Render / Volume emit cylinders

image-3

Get a better result (the areas of most overlapping aren't the darkest). But still getting some black parts.

For Blender Cycles, I also tried disabling Ray Visibility settings on each beam object for

  • Transmission
  • Volume Scatter
  • Shadow

but the result is largely the same

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  • $\begingroup$ Not a real answer, but did you try to apply a boolean to all your objects to avoid intersecting geometry? $\endgroup$ – Pyros Jul 25 '17 at 8:14
  • $\begingroup$ Hi Pyos - no sorry i haven't tried yet as it's non-trivial (needs a script to union them all in pairs). And I have the same presumption that it would have an undesired effect (areas of high overlap wouldn't be differentiated in terms of output intensity). $\endgroup$ – Elliot Woods Jul 25 '17 at 8:48
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    $\begingroup$ For the Cycles volume emitters, have you increased the number of transparancy bounces to a suitably high value? Could be that you're maximg out on bounces. $\endgroup$ – Rich Sedman Jul 25 '17 at 13:20
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    $\begingroup$ An idea for the spotlights into a volume - you can get spotlights to less than 1 degree in Cycles by manipulating the intensity of the lamp based on the Normal - similar to this answer blender.stackexchange.com/a/80007/29586 but imagine a small circular image instead of the test grid. Taking it further you could use a Dot product node of the Normal with itself to calculate the distance from the centre and to generate a circular spotlamp of any size - instead of using an image. Hipe this helps - can write as an answer but not for quite some time as limited internet connection at moment. $\endgroup$ – Rich Sedman Jul 28 '17 at 9:27
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    $\begingroup$ @Rich - that's fantastic! I've been trying to create video projectors in other physically based renderers without success. (I'm still Blender beginner, but feeling the investment of time paying off) $\endgroup$ – Elliot Woods Jul 29 '17 at 5:50
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For Cycles volume emitters you need to ensure that the number of bounces in the Light Paths panel is set sufficiently high - otherwise the rays cannot travel through all of the surfaces.

light paths

The Transparency Max and Min values determine how many transparent surfaces rays can pass through. Increasing both values will allow rays to pass through more transparent surfaces unaffected.

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  • $\begingroup$ Specifically Transparency bounces helped, could you mention that in your answer? Thank you $\endgroup$ – Elliot Woods Jul 28 '17 at 4:45

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