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I'm rendering my scene on 3 separate render layers with transparent background. All layers have strange tranparent outline (around hand and laptop on picture) which I tried to fix in compositor with no luck. [upd] when using eevee there is no such issue. What am I doing wrong?

[upd 2] I reproduced the error in a simpler scene. Check out this file and second image.

Strange outlines around hand and laptop

reproduced issue

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  • $\begingroup$ What file encoding are you saving to? $\endgroup$
    – troy_s
    May 19, 2020 at 0:48
  • $\begingroup$ are you using a Fresnel or Layer Weight nodes for these materials? $\endgroup$
    – moonboots
    May 19, 2020 at 5:58
  • $\begingroup$ @troy_s png rgba 16 bit $\endgroup$
    – Anton Bystrov
    May 19, 2020 at 15:06
  • $\begingroup$ @moonboots no, there are simple principled BSDF setups mostly $\endgroup$
    – Anton Bystrov
    May 19, 2020 at 15:07
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    $\begingroup$ I am assuming it’s not a holdout, correct? The problem is the garbage PNG format. Stop using it if you can. If you can’t, you’ll have to convert the alpha encoding. blender.stackexchange.com/a/46197/213 $\endgroup$
    – troy_s
    May 19, 2020 at 15:37

2 Answers 2

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(Please don't use links to site that require login or passwords...)

@Troy_S is correct on his comments I will elaborate a bit.

If you are adding an element with alpha channel over the hole of the same element on the background, created by another alpha channel, you will get those edges. The images are being added but the alpha channels are not combined.

enter image description here

Only the elements being overlaid should have alpha not the background. The idea is that you are adding an element over another using the alpha channel of the element on top, not one on top of a hole in the underlaying image. The proper operation is Alpha Over.

enter image description here

If you must "add" images on top of the alpha channel of another image (i don't know why you would), then all alphas should be combined.

Add the RGB image and the ALPHA channels separately and associate the alpha to the RGB information with an add alpha node.

enter image description here

Note however that adding is not the right concept to use when layering images. Alpha Over uses the alpha channel to control the mix of the top element. Add will not use alpha, it just adds the RGB values of one image on top of the values of another, and you might get into situations where many layers added together might look semi-transparent and bright instead of on top of each other.

enter image description here

To deal with more than two layers just use more alpha over nodes and add all of it on top of the background, like in this example:

enter image description here

For more info:

https://blender.stackexchange.com/a/49137/96912

and

Cycles alpha mask hole

and

https://blender.stackexchange.com/a/67371/213

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  • $\begingroup$ Alpha over works fine. The difference is in the calculation of the alpha. So alpha over, then set the alpha to the sum and done. You overcomplicated this. $\endgroup$
    – troy_s
    May 20, 2020 at 15:35
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Are you using the denoiser node in the compositor? I’ve had something similar happen using the denoiser and transparent backgrounds.

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  • $\begingroup$ No, I'm not. Toggling denoiser off from view layer panel also didn't solve the issue :( $\endgroup$
    – Anton Bystrov
    May 18, 2020 at 23:03

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