Existential question : How to get accurate masks and use it right

[spyzor]

Hello everyone,

I have an existential question and I don't know why I don't seem to find a good answer.
I didn't post this in the problem forum because I don't think it's a vray bug.


Here is the issue I have :

When I render an object I can't have a perfect mask for it. The only accurate mask method I found is to render the object and save the image as RGBA.

I made an image to show it all (please check it at full size)


For getting the result I saved the render in Exr.
The first cube is saved with RBGA directly in exr like that.


The other are composited in Photoshop, by experience I would have a similar result in After Effect.
So I have the RGB and the Alpha in a separate file and added the alpha as a mask for one and used the multimatte for the other.

You can see that the mask is never perfect as the first result.
I know that you can play with the mask in Photoshop and After Effect to make it work a little bit better but it's never perfect.
Another solution I hear often also is to render a bigger image the make it smaller so I won't have that extra edge.


So I have 2 questions :
Am I missing something to use the masks the right way ?
Why does the render don't produce a perfect mask that you can use directly ?



Please, get me a perfect solution

[Moriah]

You have to save as an unpremultiplied alpha (or save normal EXR and import as unpremult), do your corrections in unpremult and in the end do a premult node or whatever the equivalent is in AE. You shouldn't have pixel border problems this way.

[spyzor]

Ok but that only work with the alpha and After Effect if I understand correctly.
If I use a multimatte or Photoshop I don't find the unpremult option. I found a tuto to get it right in Photoshop : http://www.digitalartform.com/archiv...iting_pre.html

But I'm still wondering why can't we have it straight and pure from vray.
What is the purpose to get a mask premultiplied if the goal is to be able to select a part of an image ?

[spyzor]

Could maybe someone from Chaosgroup give his opinion about that and how it's intended to work ?

[joconnell]

Photoshop has terrible alpha handling to start with.

The idea of a mask being premultiplied is that how transparent the edge of an object is, is part of the information that you want to have. If you had nothing but blocky, unpremultiplied edges for your masks then the soft edges of your objects would get too much colour correction.

[spyzor]

That's exactly what I wanted John... I really thought Photoshop wasn't making a difference.

I just tested on After Effect and it is perfect .
The solution is just use another software...

Thanks !

[joconnell]

Maybe try krita.org (free) or the new beta of affinity designer for windows and see if they'll behave any better?

[spyzor]

I just downloaded Affinity