How do you premult with white?

theta · **How do you premult with white? -** February 4th, 2009, 09:52 AM

I got a .tga sequence with alpha, rendered on a white background. When I premult it, I get a really bad white border around the edges, so I'm just trying to figure out how to premult the image sequence using the alpha, but with white.

Thanks

beaker · February 4th, 2009, 02:51 PM

This should solve it. Just change the constant to whatever color the BG is. For now on tell your 3d and photoshop guys that they should always make the BG black.

set cut_paste_input [stack 0]
version 5.1200
push $cut_paste_input
Dot {
name Dot1
selected true
xpos 345
ypos -141
}
set N28375e50 [stack 0]
Dot {
name Dot2
selected true
xpos 479
ypos -141
}
push $N28375e50
push $N28375e50
Invert {
channels rgba
name Invert1
selected true
xpos 227
ypos -145
}
Constant {
inputs 0
channels rgb
color {1 0 0 0}
name BG_COLOR
selected true
xpos 168
ypos -238
}
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy1
selected true
xpos 168
ypos -108
}
Premult {
name Premult2
selected true
xpos 168
ypos -69
}
Merge2 {
inputs 2
operation minus
name Merge1
selected true
xpos 311
ypos -69
}
Dot {
name Dot3
selected true
xpos 345
ypos -6
}
Copy {
inputs 2
from0 rgba.alpha
to0 rgba.alpha
name Copy2
selected true
xpos 445
ypos -16
}
Dot {
name Dot4
selected true
xpos 479
ypos 57
}

theta · February 4th, 2009, 03:14 PM

Hey thanks Beaker, that worked really well. It's one button in After Effects, but I never understood the process behind it. Thanks again.

beaker · February 4th, 2009, 03:15 PM

I'm making it a gizmo and going to post it on highend3d

kakubei · 10:08 AM

Hello, I'm new to Nuke, coming over from Shake and am very much interested in this solution. Could you explain how the magic works?

I have a bunch or renders that HAVE to have white backgrounds because of Final Gather render in Maya and need to avoid the white edges at comp time.

Thanks a lot.

PS
I also noticed a couple of things:

1. if you do any color correction to the image you will get some nasty outlines
2. how do you comp it? I've attached a Merge (Over) node to the last node you have (Copy2) but the background is not showing, even thought the alpha looks ok.

forkazoo · September 3rd, 2009, 01:35 PM

It's not very magic.

If you look at the provided node tree, and go through it step by step it should be pretty clear.

It takes the background color with the opposite of the image's alpha, and does a premult. This leaves the image with the background color wherever the original image was transparent. Black where the original image was opaque.

Then, it subtracts the matted background from the original image.

I'm not exactly sure why you feel you HAVE to have white backgrounds coming out of Maya. I'm not hyper experienced with Final Gather, but if you uncheck "primary visibility" for an object, it can still cast light for Final Gather, but you can render your main object against the standard black background.

1 - Read up on color correction and premultiplication. No real magic there, either. It's the same in any app.
2 - I didn't have any problem just using an ordinary merge/Over. Not sure what issue you were having.

kakubei · September 4th, 2009, 08:29 AM

Thanks for the reply forkazoo,

Yes, I can read the nodes, but I was hoping for an explanation of why it worked, which you seemed to have offered. I know about premult but wasn't sure the best way to tackle this problem.

Regarding white on Final Gather and Maya:

1. Read up on Final Gather and how the background color of your camera (or anything in the scene) affects it. I have to have a white background for the camera so that I get the effect I want with Final Gather, the downside of course is the white background.

Anything else I might use in the scene to have that effect will not work if I turn off primary visibility.

Diogo Girondi · September 4th, 2009, 01:17 PM

Read up on Final Gathering and you will see that you can hide that background in the final render while still getting the FG solution

kakubei · September 5th, 2009, 03:54 AM

Quote:

Originally Posted by Diogo Girondi

Read up on Final Gathering and you will see that you can hide that background in the final render while still getting the FG solution

He, he, you got me there.

But honestly, I've tried every which way to get rid of the white background of the camera environment and have not found a way which still lets Final Gather read the colour and act upon it. So I'm open to suggestions.

Diogo Girondi · September 5th, 2009, 07:05 AM

I don't know in Maya but if you're using the environment stack with a environment shader you should have a option in there to set the background intensity. But you can also set that by using the nodes of the shader tree and visibility settings of a object.

kakubei · September 5th, 2009, 07:25 AM

No, what I've found that works best is setting the Camera Environment (which just means the camera background color) to white. There is no separate control for the visibility of this attribute (like you find in objects under the Render attributes), so turning this off negates the FG benefit, turning it on will give you a white background. It's not a shader...

Diogo Girondi · September 5th, 2009, 07:50 AM

Why not use a constant connected to to your environment shader set in a pass?

kakubei · September 7th, 2009, 09:04 AM

Not sure I understand you. I can't find anything like a constant in HyperShade, but have to admit I'm not great with textures.

Maybe we're talking about 2 different things, I'm not using and environment shader at all, just the camera environment attribute (see attached picture).