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I am using png images and the Images as Planes add-on to create background trees for a scene. This works fine until I try to combine this with Volume Scatter for a mist effect. The alpha portion of the images lightens, showing the rectangular outline of the image against the sky (hdri). I'm using Cycles Render. I have the density set to .02. If I set it to .005 the effect is not too noticeable, but the mist effect is minimal. Edit: I may have figured out a solution. In the side box of the Add Images as Planes add image window there is an option called Premultiplied, when I chose that, the effect seems to go away.

trees with volume scatter at .02 density

The outline shows up against the sky with the Volume scatter set at .02. It appears that where the two alpha images overlap the effect cancels out. Maybe a work around would be to add a large alpha plane over the whole scene. No volume scatter The trees with no volume scatter.

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    $\begingroup$ Could you please edit your question and provide some screenshots of the described problem, and material and fog settings? $\endgroup$ Mar 25, 2017 at 1:42
  • $\begingroup$ I added two images, I apparently don't have enough reputation to add more. Note that the effect can better be described as lightening not darkening as I first stated. $\endgroup$ Mar 25, 2017 at 2:38
  • $\begingroup$ You should post that as an answer bellow instead. $\endgroup$ Mar 25, 2017 at 5:01
  • $\begingroup$ It still isn't perfect, I'm trying to figure out why many of the images still show a line on top of the image. $\endgroup$ Mar 25, 2017 at 18:49
  • $\begingroup$ The line above the image seems to be a problem with the png I made in Gimp. Thanks Duarte, for cluing me in on how this board works. $\endgroup$ Mar 25, 2017 at 21:50

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I deleted my earlier "answers" as they turned out to be mostly useless. The answer seems to be to use Branched Path Tracing in the cycles render settings. This eliminates the effect of the alpha transparency outline of the images as planes showing up when using volume scatter. Blender HD has an excellent tutorial on all the obscure settings in cycles render, explaining with great examples what each one does. https://www.youtube.com/watch?v=NhOdnOow6DY&t=394s

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