Motion Blur GI & Matts

[Adam H. Stewart]

I am working on a scene that comprises flippng through the pages of a book. It has all the bells and whistles - Motion Blur and GI. I am using Vray lights. What I need to do is matt out area's of the pages to comp footage in post. Pretty straight forward so far right ?
The way I am getting my matt for the pages is pretty simple, I apply a B&W image to the page and re-render with all the ligths turned off, the White is self illuminated. The motion blur settings remain the same for both passes.
The problem is my motion blur does not match between my beauty render and my matt render. The B&W material has less motion blur for some reason. Does lighting affect motion blur should I leave my GI turned on? How will I get my matts if I cannot apply a B&W material should I created objects the same size as my matted are's and use object ID to extra the matt with Combustion?
I am pretty stumped on this one.

Thanks in Advance for your help

[Morbid Angel]

Adam, I've been doing same thing for all my mattes and never had this problem. Motion blur is computed based on the object, not based on its brightness (luma). There for I can only say that your motion blur settings need to be double checked.
Again maybe you have done this, but I cant see how it wouldnt work. We've been doing this for a long time and it worked for us.

P.S. I assume your moblur is 3d?

[Adam H. Stewart]

Yes

Yes I am using 3D motion blur in Vray - I will do a test making sure nothing has changed - it seemed very very strange to me that the matts where different - I can't understand it. It was like they where all a couple of Pixels off - the light difference should't affect things - this is the way I always do my matts to but I rarely use Motion Blur with GI.

Thanks for your help I'll do a settings run down.

[Morbid Angel]

ahhh i see so your difference is in few pixels?.. this is not a problem of 3d then, check your premultiplicaion of alphas in cumbustion.