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I try to render some shapes with EEVEE bloom effect and transparent background. I understood that the rendered file output had to be OpenEXR instead of PNG format in order to keep the effect and the transparency.

Nevertheless, when opening the OpenEXR files into the VSE, and adding a background clip, there are always some black contour artifacts around the rendered shapes.

How could I get rid of that and have it done properly from the beginning ? Blender version used is 2.90.

Note : I tried to tick the use alpha box in the compositor. It works better but the colors are changed.

enter image description here

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    $\begingroup$ Try compositing directly in the compositor using an Alpha Over node, not in the VSE. $\endgroup$ – susu Nov 30 '20 at 16:23
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At first I thought it has something to do with the blend mode of the material (settings panel: Opaque, Alpha Clip, Alpha Hashed, Alpha Blend). But, that seems not to be the matter.

The reason of this artifact lies probably within the AlphaOver blend mode in the VSE. You can find a very nice explanation with CGCoockie (Working with Alpha Edges for Compositing in Blender).

A possible work-around is the following:

  1. Duplicate the strip with the 'blooming'shape.
  2. Set the Alpha attribute in the Source panel to "Straight".
  3. Increase the scale of the duplicated strip a little untill the black artifact is covered.

Work-around

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  • $\begingroup$ As susu mentioned above, transparency works well in the compositing window. So I don't understand why it should be different in the VSE (it sounds like a bug to me). $\endgroup$ – Maartis Dec 3 '20 at 10:58
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As susu mentionned, another workaround is to use the compositing window with an alpha over node : enter image description here

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