Eevee + Bloom + Alpha (How I did it)

Hi people,

This is the first time I’ coming here to contribute. English is not my first language. So, before hand, forgive me about any language mistake.

I figured out how to get a good alpha when rendering bloom in Eevee. This is a technique I use in Photoshop to cutout fire and some others transparent images over a black background.
RGB images are made of the combination of three colors combined by the screen blend mode.
So… I render a transparent image to get the alpha with the shape of my objects in the scene.
Separate the color channels and blend them using the Add Blend Mode and use them as a mask to add theirs information to the rendered alpha channel.
I used the Red channel information as factor in a Color Mix node to mix black whith pure White.
Then I use the Green channel information as factor to mix the previous node using the Add Blend Mode whit pure white.
Next I use the Blue channel information as factor to mix the result with it using the Add Blend Mode again.
This will combine the channels in a grayscale image showing exactly and only where there is some color on the image, as a alpha channel would do.
At last, I just added a last node to color Add this result with the original alpha and connect it to Alpha Input on the Composite Output node.
Depending on the scene some adjustments can be necessary. Do it using contrast or RGB levels.

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Eevee - Bloom plus Alpha.blend (1.2 MB)