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I want a semi transparent particle that adds to the color behind it. Stack enough on them on top of each other it will be brighter and brighter until it is pure white. It is of course volumetrics I really want.

Example image:

I have tried this but it only decreases transparency. Not much surprise there. Is it at all possible?

Failed image and node setup

My result with the Add shader. Light isn't added (then it would be white). It is only the transparency that goes away.

Add shader result

My node setup with the Add shader. I have also tried it with reversing the colors of the two shaders I add. Similar result.

Add shader node setup

Start of a solution

Thanks to Gandalf3, I have begun to solve this with only a shader. This is the shader.

Nodes

This is a test render. I need more geometry, tweek the alphas and some more, but basicly, it is what I want.

Render

I had to increase bounces in light path settings. This is experimental values but work for now. If not changed, the central part of the flames would be black.

Light_paths_settings

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This setup will add the transparent shader to itself when seen by the camera. To avoid excess fireflies, an emission shader is used for non-camera rays.

enter image description here
I used two transparent shaders in this case, but if you don't need to control them separately then only one is needed.

Result with three overlapping objects:

enter image description here

Note that with many overlapping objects you may need to increase the max number of Transparent bounces in Render settings > Light Paths to avoid getting black spots where there are more overlapping transparent surfaces than the specified number of bounces.

Update

With the new transparent depth output of the light path node, you can avoid getting black when you run out of transparent bounces. This setup will replace the transparent shaders with an emission shader when the max number of bounces is reached:

enter image description here

This way you can use lower numbers of transparent bounces and not have annoying black spots.


Original answer:

enter image description here

This will make the emission shader become brighter the longer a ray travels through the object:

enter image description here

However, this only works with a single object, so it doesn't look too good with many overlapping particles.

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I think the only way to make a material add to what's behind it is to use compositing. To separate the added material from the rest of the scene, your going to have to use RenderLayers.

  1. Move the add object to a separate layer (M).
  2. In the RenderLayer tab select the layer that the add object is on: enter image description here
  3. Select all layers (`) and hit render.
  4. Open the compositor and Select your two render layers: enter image description here
  5. Add a mix node and set the Blend Type to Add: enter image description here

Final Result: enter image description here

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And here is another way how to make additive material. Possibly it will be helpful for someone.

Image

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  • $\begingroup$ Neat space ship. $\endgroup$ – David Aug 11 '15 at 17:41
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Actually, I continued fiddling with the Add shader and came up with this. I am not really sure why this works better than the others. It may not be true additive blending but it looks good.

fireball

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  • $\begingroup$ You should post this as an edit instead of an answer. $\endgroup$ – Daniel Pendergast Dec 6 '13 at 20:49
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    $\begingroup$ @DantheMan from what I know it is absolutly ok to answer your own questions. $\endgroup$ – stacker Dec 7 '13 at 19:54
  • $\begingroup$ Ah, sorry. Didn't notice that it was an answer. $\endgroup$ – Daniel Pendergast Dec 7 '13 at 22:26
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I did additive material like this:

SIMPLEST SETUP: Image

WITHOUT EMISSION: Image

WITH EMISSION: Image

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