Nuke Merge Node apears screen!!!

[artistdedigital]

Hi Everyone,

I have a little problem with nuke while using merge node.

Recently I was doing a comp in Nuke where the director wants me to comp a shot of a car dashboard. They shoot the dashboard in a moving car where the shoot requires to crash the car on the side of the road and the speedometer needle moves from above 100kmh to straight 0kmh. I track the shoot and stabilize it. Did a little roto to separate the needle. But when I merge the needle over the BG plate it behaves really wired. Like a screen effect and the needle appears under the speedometer numbers!!! What am I doing wrong???

After a bit of internet search I have found that I have to do something with the shuffle node, but not sure what and why??? Any help would be highly appreciated.

Another post I found with the same problem (http://forums.creativecow.net/thread/255/15), but I didn't quite get the last part of it

Can someone please help me into this?????

Thanks heaps...
Deep

[mreithoffer]

make sure that your needle FG element has the roto as Alpha, then premult and merge over the BG

[jeremybrown]

make sure that your needle FG element has the roto as Alpha, then premult and merge over the BG

Yes. Basically, you've got to make sure that your alpha channel for the needle a) exists before the over and b) that the needle rgb is being premultiplied by the alpha likewise, before the over.

You can do this all in the roto node:

rgb -> roto (alpha) premult rgb -> merge
^
background


or if say the roto was done separately or if you like to have your operations split up in to more nodes, you can set the roto node to output alpha or shuffle in the alpha from another source and use a premult node before the merge.

rgb -> copy -> premult -> merge
^ ^
alpha background

[artistdedigital]

wow.....it works now...thanks heaps guys...i guess i understand more now...cheers...