<p>To ensure you get 'pure' transparency, you'll have to ensure the transparent world is used, instead of your (coloured) sky.</p>

<p>When using Blender Internal, make sure you have in the Render settings, under Shading panel Alpha set to Transparent</p>

<p><img src="https://i.sstatic.net/7E26B.png" alt="Transparency setting in BI"></p>

<p>When using Cycles renderer, make sure you have in the Render settings, under Film panel the Transparency box checked.</p>

<p><img src="https://i.sstatic.net/dRZIX.png" alt="Transparency setting in Cycles"></p>

<p>Without these settings your alpha will be pre-multiplied with the sky color, which will look like your example.</p>

CC BY-SA 3.0
2013-06-30T19:11:24.267Z
2013-06-30T19:11:24.267Z
jesterKing
<p>If I add a simple UV sphere that emits white light, and in the compositor add a Fast Gaussian blur effect, then when viewing on a transparent background there is a noticeable black halo around the glow.</p>

<p>Any ideas what the correct way to do this is?</p>

<p><strong>Update:</strong> It seems to have something to do with the horizon colour being picked up by the blur effect.  How do you prevent that from happening?  As a work around I can set the horizon colour to white, but I don't think that's the correct way to do it...</p>

<p>Here you can see with a red horizon colour, and changing the colour of the emitter to blue the effect is quite pronounced:</p>

<p><img src="https://i.sstatic.net/iBTvu.png" alt="enter image description here"></p>

CC BY-SA 3.0
2012-09-14T13:35:33.253Z
this is a good question. Did you try selecting only the light and not applying the blur to the whole layer?
Travis Crum
CC BY-SA 3.0
2012-09-14T22:30:05.580Z
@TravisDtfsuCrum Could you please show me how to do that? I'm trying to figure out the ID Mask nodes but not having much luck.
magritte
CC BY-SA 3.0
2012-09-17T12:55:42.860Z
go to the layer, do select all (ctrl or apple + a) then what I do is use the arrow key to press it once to the right then back to the left to make sure its still in the same spot but selects everything on the layer. Before you do the arrow key thing, make sure you have the move tool selected and not any selection tool. Once its selected try applying the blur. Let me know if this helps!
Travis Crum
<blender>
Blur effects on a transparent background leaves a black ghosting effect
magritte
# Blur effects on a transparent background leaves a black ghosting effect

(Asked by: username_0 on 2012-09-12)

If I add a simple UV sphere that emits white light, and in the compositor add a Fast Gaussian blur effect, then when viewing on a transparent background there is a noticeable black halo around the glow.

Any ideas what the correct way to do this is?

**Update:** It seems to have something to do with the horizon colour being picked up by the blur effect. How do you prevent that from happening? As a work around I can set the horizon colour to white, but I don't think that's the correct way to do it...

Here you can see with a red horizon colour, and changing the colour of the emitter to blue the effect is quite pronounced:

![enter image description here](https://i.sstatic.net/iBTvu.png)

**Tags:** blender

> **Comments:**
> **username_1**: this is a good question. Did you try selecting only the light and not applying the blur to the whole layer?  
> **username_0**: @TravisDtfsuCrum Could you please show me how to do that? I'm trying to figure out the ID Mask nodes but not having much luck.  
> **username_1**: go to the layer, do select all (ctrl or apple + a) then what I do is use the arrow key to press it once to the right then back to the left to make sure its still in the same spot but selects everything on the layer. Before you do the arrow key thing, make sure you have the move tool selected and not any selection tool. Once its selected try applying the blur. Let me know if this helps!  

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## 1 answers

### Answer by username_2 on 2013-06-30. Score: 1

To ensure you get 'pure' transparency, you'll have to ensure the transparent world is used, instead of your (coloured) sky.

When using Blender Internal, make sure you have in the Render settings, under Shading panel Alpha set to Transparent

![Transparency setting in BI](https://i.sstatic.net/7E26B.png)

When using Cycles renderer, make sure you have in the Render settings, under Film panel the Transparency box checked.

![Transparency setting in Cycles](https://i.sstatic.net/dRZIX.png)

Without these settings your alpha will be pre-multiplied with the sky color, which will look like your example.


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