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I know several questions have been asked regarding a problem with glow effects not showing up with transparent backgrounds, but I have a slightly different issue. For me, the glow effect does show up, but it gets cut off at the borderline of the alpha background.

With background:

Image with backgr

Without background:

Image without backgr

My node setup:

Image os node setup

Anyone know of a simple fix?

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  • $\begingroup$ Where do you add the background? I don't see it in the 'node setup picture' maybe you could post the .blend file? $\endgroup$
    – Pythogen
    Commented Dec 31, 2015 at 8:29
  • $\begingroup$ pasteboard.co isn't very reliable at loading images ... can't see those images you posted on my end. $\endgroup$
    – hawkenfox
    Commented Dec 31, 2015 at 17:12
  • $\begingroup$ @Pythogen The background is an actual object. It's just a plane with a gray material. When I rendered the image with the transparent background, I just hid the backdrop object from view and from the render. Hawkenfox, sorry about that. My friend got me hooked on it cause it's so easy but I know, it's unreliable. I tried uploading the pictures using the question editor but it said the files were too big. Maybe try again? Usually it works after a second try. $\endgroup$
    – Jace Cotton
    Commented Dec 31, 2015 at 20:56
  • $\begingroup$ This question has a bounty and cannot be voted as duplicate: blender.stackexchange.com/questions/34002/… Pls see the answer in this question. $\endgroup$
    – Jaroslav Jerryno Novotny
    Commented Jan 2, 2016 at 20:49
  • $\begingroup$ @Jerryno Is that really the only option? In that case it would be simple with GIMP using the "color to alpha" option, but the problem is mixing the RGB channels together after removing a black background renders a translucent image which isn't going to work in my case because I have more than just the glow effect. $\endgroup$
    – Jace Cotton
    Commented Jan 2, 2016 at 21:02

1 Answer 1

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You can't do that with a PNG. What you need there is to make some transparent pixels luminescent, and that's only possible with an associated (sometimes called "premultiplied") alpha channel. PNG uses unassociated alpha. That means that the alpha over operation will multiply the alpha on the foreground plate as part of the operation, destroying the information that sits on the transparent pixels.

The only way to solve this is to use a format that allows associated alpha channel.

Keep in mind that programs like GIMP (and Photoshop used to, but I'm not sure if it still does) internally convert images to unassociated alpha, making compositing that kind of images imposible. The same happens with graphics for the web if you're going to use PNGs.

Blender's compositor and other compositing programs will allow you to do that, but if you need to use it in one of the programs mentioned above, you'll have to export the glow plate as a separate file and add it over the composite.

Here is a simple example that demonstrates how easy this is to accomplish correctly:

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    $\begingroup$ related info: wiki.blender.org/index.php/User:Sobotka/So_WTF_is_Alpha and: wiki.blender.org/index.php/User:Sobotka/… $\endgroup$
    – user1853
    Commented Jan 3, 2016 at 4:42
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    $\begingroup$ Eek! I should probably finish that first entry now I suppose. Here is MasterZap destroying everyone in a CG forum on this very issue. forums.cgsociety.org/archive/index.php?t-278718.html Zap also came up with the most eloquent description of the two alpha formats; the alpha format determines which of the two compositing operations should be used. Unassociated alpha is (FG.a * FG.RGB) + ((1-FG.a)*BG.RGB) where associated alpha's operation is FG.RGB+((1-FG.a)*BG.RGB). Only in the latter case is RGB with zero alpha a pure addition. $\endgroup$
    – troy_s
    Commented Jan 3, 2016 at 19:43

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