Merge (Plus) givin alpha results larger than 1.0

Software Natron
1

Hi everyone!
I were trying to compose the diffuse passes (color, direct and indirect) from a render, and I ran into a problem: composing together direct and indirect diffuse througt “Merge (Plus)” was triggering some coloring problems later when placing said diffuse pass over a background.

I actully spotted the problem: because of Plus operation my image (fully alpha=1 before) has Alpha=2 in at least some areas now. I figured out some alternative solutions (fully replace alpha channel with a Shuffle node or checking “Alpha masking” in Plus node) so my question is not about how to solve it (or it is if you know a better way).

I was wondering: is this correct? I understand that RGB= emission and alpha=occlusion, and following that a value of 2 in alpha will un-uncclude? In which cases this could be helpful?

I come from Nuke and I’m kinda newbie in postproduction, but the case is, in Nuke (although the alpha is beyond 1 after Plus operation too) when I place the image in a background it works well without need of checking the “Alpha masking” option, so… is something I’m missing?

I’m sorry if it is a dumb question or if I’ve made a mess trying to explain it, as I said i’m kinda newbie and started using Natron some days ago.

layout_alphaValue2
layout_alphaValue21915×1026 157 KB

AlphaMasking
AlphaMasking2560×1440 5.02 MB

noAlphaMasking
noAlphaMasking2560×1440 4.04 MB

2

Plus does exactly what you expect, it pluses the colour values from one picture on top of another. There are only a few very special occasions where plus makes sense. When you want to combine alphas you should usually use max to add alphas or in case you want to subtract alphas you invert one and then multiply. In these cases you will not create superblacks or superbrights (values below 0 or over 1)
If you want to add an alpha to an image you either use shuffle or you manually set the alpha of the picture to 0 (black) by i.e. doing a CC on the alpha or what ever way you like, and then you add the new alpha (ideally not by using add but max iven though it in this case the values are the same)

hope this was usefull

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