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I'm trying to add a glowing sun effect to my scene for an animation in "Blender Render". I put the sun in its own render layer, and the scene in its own. In the compositor I had the sun render layer go through its nodes, and then combine with the regular scene at a mix node. When I plug it in to the output, I get either the sun with its effects, or the normal scene blurred when I change the active scene.

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    $\begingroup$ Is Transparent checked in Render tab > Shading rollout > Alpha ? Also do not use Mix node set to Screen in this case, overlay with Alpha Over $\endgroup$
    – Mr Zak
    Nov 10, 2016 at 18:44
  • $\begingroup$ @MrZak That fixed it, but how can I have my background in effect as well? $\endgroup$
    – Keyk123
    Nov 10, 2016 at 19:57
  • $\begingroup$ I'm not sure what do you ask.. You will have your background in place when 2 conditions are considered: transparency of the foreground image; and one image is overlayed on top of another one. Transparency in BI is achieved as stated above, overlaying is already done in your nodes. It will lighten both your images (hence I suggest looking into Alpha Over or MixRGB set to Mix mode) but the effect will be there. $\endgroup$
    – Mr Zak
    Nov 10, 2016 at 22:32
  • $\begingroup$ @MrZak When I follow your above instructions, the background is transparent, instead of the world background I have set up. Is there a way to use the Alpha Over node and still have my environmental background show up? $\endgroup$
    – Keyk123
    Nov 11, 2016 at 1:44
  • $\begingroup$ Well I see I was mostly considering Cycles and not Internal when writing about overlayin environment, it's just simple there. In Internal it's not AlphaOver who makes background transparent, it's absence of environment information there. For overlaying scene above the background see here, but it's not about overlaying world background. You could render scene with transparent background in one scene, and sky - in another without transparency (not sure how to do that without workarounds in BI) $\endgroup$
    – Mr Zak
    Nov 11, 2016 at 14:05

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Nodes inputs have this behaviour: the upper input is considered as the "background", the lower is overimposed as much as the fac slider goes toward 1.0, with the choosen blending mode (in your case: screen).

In your nodes setup the first mix (screen) node has the fac set to 0.0, meaning that the lower input (blur 5 and 5) is excluded, you only have the "35 and 35" working.

Iin the second mix (screen) node you should switch the two inputs.

In Blender generally we use a different way to manage the layers than Photoshop or others image editors: you could render the "background" scene with transparent sky, set the sun scene as background (upper input of a mix RGB node, set to "mix"), normal scene on the lower input, use the alpha value of the normal scene as FACtor, so that the only part of sun scene is visible only on the transparent portions of the normal scene.

The image in the fac input (alpha of the normal scene) means: black = show upper input only, white = show lower input completely mixed with upper according to the choosen blending mode, greyscale inbetween.

To inspect the images of the various outputs of the nodes: Ctrl click them more than one time: they will show their various values in a circular mode.

If you understand this general behaviour you will be able then to let the sunlight "bleed" on the normal scene.

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  • $\begingroup$ I appreciate the answer, but am still unsure of how to get it to work. Could you walk me through it step by step? I'm new to the compositor. $\endgroup$
    – Keyk123
    Nov 10, 2016 at 21:00

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