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Hello, I am having this strange black artifact


Just to orient you I must say that the artifact is on the upper green area shown in the material channel.
It is a poligonal object with a transparent image based emiter assigned (the sparks)
Only in this viewpoint I get this black artifact at the end of the triangular wing (shown with yellow arrow)
In other viewpoints it is not shown.
The only thing I can think of is that in that point the thickness of the volume is very close to zero...
Any idea?

Thanks in advance

E