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As far as I know, in Cycles, when you use an Add Shader node, it simply adds shaders. I'm getting different effects of those I get with other software.

For instance, adding nothing to nothing should result into nothing, right?

Not quite so: enter image description here

It's making it really difficult to recreate the behavior of a standard real-time PMA shader.

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This all comes down to what the Transparent shader and the Add shader actually do. Think about it another way and the behaviour you are seeing makes more sense.

Instinctively, the Transparent shader is nothing, however, it is not. The shader defines what happens when a ray interacts with the surface - in the case of Transparent, the incoming ray hits the surface and the result is that the resultant ray leaves the surface travelling in the same direction and, if the surface is 100% white, is the same intensity as the incoming ray - ie, no change.

The Add shader takes multiple shaders and combines the result. So, adding two Transparent shaders we start with the incoming ray and each Transparent shader produces its own (unaffected) resultant ray. The Add shader combines these two rays and the result is a combined ray with twice the intensity of the original - ie, it actually amplifies the light.

If you look at your result you’ll see that this corresponds with what you see - where the environment light passes through a surface it becomes brighter, and the more surfaces it passes though the brighter ot becomes.

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  • $\begingroup$ ...and that's why I can force the color of the transparency down by 1/n where n is the number of transparent shaders added (yes, I tried that). Makes sense now, but also it's so counter-intuitive there's no sense in it. :) Perhaps there should be a Draw-over shader, for layered passes. $\endgroup$ – Fernando D'Andrea Nov 11 '17 at 22:07
  • $\begingroup$ Generally you ahould use the Mix shader since that makes more sense, intuitively - ie, with a factor of 0.5 you’ll get 50% of each shader - combined this is back to being 100% transparent. Generally, the Mix shader is more physically accurate and intuitive and the Add shader should only really be used for special cases. A couple of good cases for using Add would be to combine volumetric Scatter and Absorption, or for adding Transparent to Emission for, say, glowing gas/flame (emission by itself does not let light pass through). $\endgroup$ – Rich Sedman Nov 11 '17 at 22:23

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