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I'm rendering with composition, my goal is to isolate an object, its shadow, its reflection and the background in order to be able to adjust the background image/color, and the intensity of the shadow and the reflection independently.

The final result with Cycles ("Render Result --> Composite") is as on the left side:

enter image description here

When looking at the alpha channel (bottom) I see it's 0 for the reflection area, which seems incorrect. If the image is saved and reloaded (right side) pixels with alpha = 0 are gone.

The compositing network which may not be optimized as I'm learning how it works:

enter image description here

I don't understand how the composited image can have an alpha layer not matching what is displayed, and I don't know how to save an image which has the correct reflection area.

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Since that you've set Alpha to 0, why are you asking that image has become transparent?

The alpha in PNG is Straight, in Blender - Premultiplied so that you have to set up some alpha to get saved in PNGs, for example like this:

enter image description here

Then you load the image, you will notice some difference:

enter image description here

But it will be equal if you convert alpha:

enter image description here

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  • $\begingroup$ They appear transparent, because alpha is 0. Png doesn't keep color information in this case in purpose of file compression though $\endgroup$
    – Crantisz
    May 5 at 16:01

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