[LesliePere]

Hello,

I am outputting the alpha channel along with the rendered image. The image is mostly white/light. When the alpha channel is specified, the image is being rendered against a black background.

Naturally the issue is that when I try to composite the render + alpha in PS, you can see the black fringed edges.

Is there a way to specify to render the image against a different color other then black?

Thanks!

[tom]

You may find this useful:
http://think.maxwellrender.com/maxwell_ ... n-110.html

[LesliePere]

That's the method I've been using (with the manual labor option), but unfortunately that cannot be used with non-opaque alphas.

I guess I should've mentioned that I am compositing ice over another image. In order to see portions of the image behind the ice, I need the grayscale alpha. When I create the alpha with the "Opaque" swithc OFF, you cannot use the matting commands in PS since it will remove grayscale values from the alpha channel that I'd need.

[mverta]

Just curious on two points:

1) Have you tried using an actual compositing app, like After Effects?

2) You are aware that the image seen through the ice won't display correct refraction/reflection effects as if it were in render?


_Mike

[tom]

Interesting. It's highly essential unmultiplying a pre-multipled image before doing the compositing. Trying to keep the pre-multipled color and removing alpha directly means you're removing an opaque alpha indeed, which means the render channel is either oversprayed or will already be composited over same background. Could you please post an example?

[LesliePere]

Thanks for the replies guys!

Mike:
No, I have not tried AE, because I'd like the final result to be big for a greeting card and wasn't sure if AE can handle such a big size. Yes, I am also aware that reflections/refractions won't be correct, but it's only for my entertainment purposes so I don't care about that.

So here is the render channel:


This is the alpha output with "Opaque" OFF:


This is the straight composite in PS. See the black edges around the P (most visible on the right side) and around the snow field:


Executing the black matte command in PS will give you some rather undesirable result:


And so does the Defringe command (1px):


The scene is setup to render with Maxwell's sky, which is being masked out by the alpha when it renders. So the current workaround is to render out the scene first with the grayscale alpha, then render again but this time the opaque alpha only. Finally, I can composite first the opaque alpha and matte out the black edge pixels, then use the grayscale alpha to use the transparency for the ice. Phew...! I'm already sweating.

If there would be a way to render out the opaque alpha and the grayscale alpha as separate files but at the same time, it would save an extra rendering, I'd think.

The result using the opaque + grayscale alpha gives correct result:

[Mihai]

I wonder which one of these is more 'correct'. If you do a straight render without checking alpha so the sky is visible in the render, what does the P look like then?

[LesliePere]

Well, the sky is a separate image. I didn't put it into the scene because I don't know what the final image will be. This way I could just render out the scene and put whatever BG I want later.

[Mihai]

Yes, I ment the Physical Sky from the render. It would be interesting to see how much of the P is visible with the sky visible in the background.

[tom]

Executing the black matte command in PS will give you some rather undesirable result:

Technically speaking, above is the only correct way of compositing a pre-multiplied image using transparent alpha. It might not give you the expected (although, I see a pretty correct image) but don't let the expectations confuse you. Also as Mihai said, you should set the sky photo as a background image using Image Based Environment system in Maxwell to validate how would it come out.