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i was trying to make a simple setup in compositor, when rendered the image is perfect:

enter image description here

But after saving it, some white noise appears near the transparent part of the image:

enter image description here

here is the simple node setup, in theory, the image should be exaclty the same, right?

enter image description here

Output settings:

enter image description here

Here is the blend file

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  • $\begingroup$ Perhaps your image isn't saving the alpha correctly, make sure to connect that alpha value from your image in the compositor to the alpha from the composite. Also make sure that in your output properties it's saving with RGBA. $\endgroup$
    – Nascent Space
    Commented Feb 10, 2021 at 3:57
  • $\begingroup$ I updated the post with this settings, and the image still with this white noise $\endgroup$
    – RodrigoGama
    Commented Feb 10, 2021 at 5:17
  • $\begingroup$ I'm pretty sure it's not noise, the gradient looks too clean. By the way what is that image that's in the holes? It looks very stretched. $\endgroup$
    – Nascent Space
    Commented Feb 10, 2021 at 5:21
  • $\begingroup$ Alpha issue ... Can you share your file? $\endgroup$
    – vklidu
    Commented Feb 10, 2021 at 6:47
  • $\begingroup$ i Added the blend file link in the post, it's just a default blend file with this node setup $\endgroup$
    – RodrigoGama
    Commented Feb 10, 2021 at 15:24

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Hmm .... blender uses your PNG's alpha channel in a very strange way on output. Even PNG has black&white alpha it is displayed as partially transparent ... almost without transparency. So I would expect your question should be - Why PNG's alpha channel is ignored in my render?

Anyway...

What you can see as "white noise" around alpha edge looks the same as what I can get with Straight alpha

enter image description here

When switched to Premultiplied alpha it looks like your render ... the residue of brighter pixels is something that I can see on your original image too so I guess it is not an issue here.

enter image description here

But your original PNG is already set with straight and looks normal ... switch to premultiply in image properties make things even whorse ... probably wait for some better answer :) I'm confused now ...

Anyway ... reassigning Alpha channel with Set Alpha node solved issue for me. Also when I append rendered image back into blender (that is what you did, I guess, right?).

enter image description here

If you want to clear edge you can try Dilate/Erode node for Alpha channel ...

enter image description here

If you need this pixel cut from mask in final use InPaint node after Set Alpha.

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    $\begingroup$ Thank you! it works a lot better now $\endgroup$
    – RodrigoGama
    Commented Feb 14, 2021 at 16:44
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    $\begingroup$ Inpainting / dilating / eroding is a sure sign that the alpha is being mishandled, as it is here. There are other much more well informed posts on this website to read, and I’d encourage folks who are having issues to read them. $\endgroup$
    – troy_s
    Commented May 22, 2021 at 19:34
  • $\begingroup$ @troy_s Since I know your opinion about PNGs (as garbage) :) I didn't think there can be something that can make this issue better. Anyway - this specific PNG is something that I haven't seen before :) $\endgroup$
    – vklidu
    Commented May 23, 2021 at 19:31

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