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If I press the "include alpha-button" in the Mix-Color-Node, I get white borders. Otherwise i've got a perfect alpha but no blue background.

white-border problem

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  • $\begingroup$ pasteall.org/blend/27169 $\endgroup$ – user2642 Mar 1 '14 at 18:04
  • $\begingroup$ Yes but i've concretize my question, I hope. $\endgroup$ – user2649 Mar 2 '14 at 15:27
  • $\begingroup$ @user2649 If your original question was asking about the same thing (but was unclear), please edit the original instead of asking a new one. $\endgroup$ – gandalf3 Mar 2 '14 at 19:01
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Use the Alpha Over Node instead of the mix node. Color -> Alpha Over

enter image description here

This will also work if you use an image instead of a RGB node.

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  • $\begingroup$ and with an image? $\endgroup$ – user2642 Mar 1 '14 at 18:13
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    $\begingroup$ what image? In your question you mention nothing of an image. $\endgroup$ – Vader Mar 1 '14 at 18:16
  • $\begingroup$ If I use an image instead of a RGB-Node. $\endgroup$ – user2642 Mar 1 '14 at 18:16
  • $\begingroup$ yes it will still work $\endgroup$ – Vader Mar 1 '14 at 18:20
  • $\begingroup$ and when you now press only alpha-button. You will see a white outline $\endgroup$ – user2642 Mar 1 '14 at 18:22
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You do not need a mix node, Use an alpha-over node. Although blender shows you a border around the cube, it is actually doing what you want. Use the following node setup. You do not need a scale node per se, I only have it because the background image is larger than the cube render

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If you were to export a render with the shown node setup and import it into another compositing application you would see that this is the correct method. The alpha is retained and the composite shows the default backdrop from when you rendered it out of blender, but you can easily change that since the alpha is retained.

enter image description here

The alpha channel as viewed in another 3d app.

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The node setup

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If you lower the Width in the Film Panel under the render tab from 1.0 to 0.01 the border will dissolve.

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  • $\begingroup$ This is not the right way to solve this problem. $\endgroup$ – Vader Mar 2 '14 at 20:25
  • $\begingroup$ The width is used for smoother antialiasing. The higher the pixel filter value (width), the smoother the aliasing. Values below 1 results in sharper renderings. $\endgroup$ – Christoph Werner Nov 29 '19 at 10:32

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