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Old 11-08-2012, 09:08 AM   #1
Ruudy
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Premultiply or not

I have a question which confuse me for several years, it's really a PITA, I hope I can get some helps here. I've known all the basic and theory for alpha compositing, premult pic or unpremult pic, but still I can't get it quite well in Nuke.
1,In Nuke, the read node, when do I need to check the premultiplied option? Do I need to check it when I try to deal with a non-linear source?
2,In After Effects, there is two ways to interprelate pictures, Straight and Premultiply. In Nuke, how can I deal with Unpremultiplied Image and Premultiplied Image? Matte for the Unpremultiplied? Merge for the Premultiplied?
3, I use a Foreground sphere(it's moving), a background checkerboard plate to do a test. I rendered a masterlayer with motion blur, and then I rendered a sphere layer with motion blur and a BG layer and try to comp the last two together and try to get the same result as the first master layer, but I never get the exact same result, the edge for the sphere is always different! No matter I checked premultipied option in Read node or not, no matter how many Premult or Unpremult nodes I use, no matter I use Matte or Merge......
I am gonna cry...
Thanks for any help
 
Old 11-08-2012, 05:19 PM   #2
Ruudy
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In Nuke


In AE

Last edited by Ruudy : 11-08-2012 at 05:22 PM.
 
Old 11-08-2012, 05:28 PM   #3
Ruudy
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So here as you can see, I rendered out a master layer, a premultiplied FG, an Unpremultiplied FG and a BG(all have Motion Blur so that we can see the effect clearly). I use matte for the unpremultiplied image over the BG, merge for the premultiplied image over the BG in Nuke, but no matter how I change the setting for these two nodes, I will never get the result as same as the master layer. But in AE, I use straight interpolation for the unpremultiplied FG and use premultiplied interpolcation for the premultiplied FG, and both can give me almost the same look as my master layer.

How come is Nuke bad than AE? Do I miss anything in Nuke?
 
Old 11-08-2012, 09:00 PM   #4
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Actually I solve the problem by checking the premultiplied option off in read node and checking on the "Video Colorspace" option in Merge node, but I don't know why.
In my project I am testing using 8 bit picture iff. I want to know how to deal with 8 bit composition so that they can be merged properly? What's the difference between the "premultiplied" in read node and "Video Colorspace" in Merge(or Matte) node?
 
Old 11-15-2012, 02:55 PM   #5
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The premult has nothing to do with colorspace. Its for cutting out your elements from the background to layer it with other stuff. When you turn on the premult in the readnode, it premultiplies your image by your alpha, already in the readnode, so you have a transparent Background.

When you render something unpremultiplied out of your 3d application you get jagged outeredges like when you turn off the antialiasing. The information of the smoothness of the outeredges is here in your alpha. By premultiplying your unpremultiblied image by the alpha you get the nice filtered edges back.
 
Old 11-15-2012, 02:57 PM   #6
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The premult has nothing to do with colorspace. Its for cutting out your elements from the background to layer it with other stuff. When you turn on the premult in the readnode, it premultiplies your image by your alpha, already in the readnode, so you have a transparent Background.

When you render something unpremultiplied out of your 3d application you get jagged outeredges like when you turn off the anti-aliasing. The information of the smoothness of the outeredges is here in your alpha. By premultiplying your unpremultiblied image by the alpha you get the nice filtered edges back.
 
Old 12-19-2012, 06:09 AM   #7
Ruudy
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Quote:
Originally Posted by tomtomo
The premult has nothing to do with colorspace. Its for cutting out your elements from the background to layer it with other stuff. When you turn on the premult in the readnode, it premultiplies your image by your alpha, already in the readnode, so you have a transparent Background.

When you render something unpremultiplied out of your 3d application you get jagged outeredges like when you turn off the anti-aliasing. The information of the smoothness of the outeredges is here in your alpha. By premultiplying your unpremultiblied image by the alpha you get the nice filtered edges back.


sorry tomtomo I just notice your post.
If you check out the explanation of "Premultiplied" in Read Node and the "Video Colorspace" in Merge node in Nuke user guide, you will find that they are dealing with color space in a "premultiplied" way... I already get my 8 bit compositing worked by just click on the "Video Colorspace" in Merge node for every merge operation I have in my comp although I don't know exactly why...(by the way 32 bit input footage or sequence have no problem with the alpha, just put them into merge node with out any premult node or options checked)

Last edited by Ruudy : 12-19-2012 at 06:15 AM.
 
Old 12-19-2012, 06:09 AM   #8
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