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This is what I have so far:

enter image description here

The rendered image has a transparent background, but the glare filter creates an output with a black background, so after merging the two togheter, the background turns black but I need it to be transparent, what can I do?

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    $\begingroup$ I don't think that the Glare node is specifically involved in this kind of issue. It should use the source image's alpha channel already. Seems instead that you unchecked the "Use Alpha" box in the Composite node. Try to re-enable it and let us know if the problem persists. $\endgroup$
    – Carlo
    Jul 14 '16 at 0:05
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    $\begingroup$ @tacofisher you are not using the alpha at all, connect the renderlayer node's alpha output(first node) to the composite node's alpha input(last node). $\endgroup$
    – user2816
    Jul 14 '16 at 0:10
  • $\begingroup$ Ok, thanks both of you for your help, now the background is transparent but I have another problem, the glare remains within the chains and the yellow beam, but doesnt "smear" to the rest of the picture (the black part of the picture) $\endgroup$
    – tacofisher
    Jul 14 '16 at 0:45
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    $\begingroup$ Use alpha over instead of color mix. And then read this post: blender.stackexchange.com/a/44137/1853. The glow over the transparent cannot be displayed in the render viewer. Your image is fine if you comoposite it over a different background. $\endgroup$
    – user1853
    Jul 14 '16 at 0:46
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    $\begingroup$ As long as you do the compositing in blender you should be fine. Read the answer. $\endgroup$
    – user1853
    Jul 14 '16 at 1:25
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Use Alpha Over to composite the two images:

enter image description here

To have transparency enable "Use Alpha"

NOTE that if you try to display the image on the render viewer using Color+Alpha (RGBA) you won't see the correct results. To view the glow information you should use Color only (RGB)

enter image description here

enter image description here

enter image description here The glow information is there, you just can't see it, enter image description here

The viewer is broken and cannot represent associated alpha correctly. Pixels that are emissive and transparent at the same time will not show (for example when rendering fire).

If you composite an image (or a solid color) behind it you'll see the correct results though.

enter image description here

If you need to export the image with transparency use EXR, do not use PNGs, then just bring the EXR sequence and composite using alpha over.

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  • $\begingroup$ realted: blender.stackexchange.com/questions/41574/… $\endgroup$
    – user1853
    Jul 14 '16 at 1:33
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    $\begingroup$ Cegaton I wish that I could upvote this more than once. Its one of Blender's age old mysteries. $\endgroup$
    – 3pointedit
    Jul 22 '16 at 0:27
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    $\begingroup$ I second @3pointedit wish. I want to upvote this 10 times. And give you an award for that "the viewer is broken" gif. $\endgroup$
    – Gez
    Feb 24 '19 at 16:33
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Blender 2.92 and above - Viewer is fixed :)

Wrong interpretation of emmissive light on alpha in viewport is fixed now.
The proper way of appearance is to be on Alpha invisible, but still there.

enter image description here

If you check separately R, G, B channels, emissive light (like glare) is there. So when composited over nontransparent pixels emission is there.

enter image description here

Also when saved this "gloving" on transparent BG as OpenEXR file, can be composited same way.

enter image description here

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    $\begingroup$ The "Screen" node is actually a "MixRGB" node (select Screen from the dropdown). $\endgroup$
    – Logic1
    Nov 7 '20 at 16:17
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    $\begingroup$ Doesn't work in Blender 2.91.2 while actually rendering - saves files without glare on transparency, but only F12 shows it. $\endgroup$
    – Angel
    Jan 28 '21 at 16:17
  • $\begingroup$ OK I did edit from 2.83 to 2.92 ... $\endgroup$
    – vklidu
    Apr 29 '21 at 21:54
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You can separate RGBA, then recombine with the RGB parts contributing to the A instead of the original image. Use a mix color node or something to combine the RGB channels then feed into the alpha of the combine RGBA node. Use a material index mask to splice in anything you want to keep pure(ie the metal lightsaber part of this image)

https://i.gyazo.com/f1c0b0fd6ca821aeda8efe26ca5e8181.png

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    $\begingroup$ Since it was not explained clearly, that "Lighten" node is actually called a "Mix" node. $\endgroup$
    – Logic1
    Jul 4 '18 at 23:48

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