Compositing node: Alpha Over does not work

[Luukezor]

Hey everyone,

i got a little problem while working with the compositing nodes.
At the moment I am trying to create a sci-fi sun or something like that.
I wanted to use the Alpha over node to apply some athmosphere behind the actual sphere.
For that I followed the tutorial of Blender Guru to create a reallistic earth.
I have done exactly the same node combination and tried to fix it my way but I cant solve it.
In the picture I attached to my question you can see what I did.
I can either turn the factor all the way up, then only the 2nd image port shows up or turn it down, then the other one shows.

I would be very thankful for any tip or advise

-Luuke

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[J_the_Ninja]

I can either turn the factor all the way up, then only the 2nd image port shows up or turn it down, then the other one shows.

Short answer: enable alpha channel and re-render.

Long anwser: The reason it works in the tutorial (I’m assuming, haven’t watched it) is because that one is rendered with an alpha channel. Alpha over blends based on either the alpha channel of the second input, or the factor input (or both). Since your image doesn’t have an alpha channel, and you didn’t connect anything to fac, alpha over isn’t able to blend selectively, every pixel gets the same factor, in this case, 0.857.