View Full Version : mv2DToxik Pass alpha clipping blur in NUKE

11 November 2009, 08:16 PM
Ive done a 2d motion pass in MAYA 2009 for use in NUKE and seems to exporting OK except for the alpha.

see here (

in Maya, for the 2d blur pass, I did..
- a 32 bit .exr
- motion blur: full, by 1.0, open/close 0.5, quality 1,1,1,1

for the other render layers (beauty, spec, ao..etc) i did not enable motion blur in the render globals.

in NUKE, shuffle copied the vectors out to RGB channels and in vblur , using those channels as uv with forward. alpha set to rgba.alpha.

So why is the blur getting clipped by the alpha?


11 November 2009, 10:24 PM
if motion blur isn't enabled on your other layers, they won't line up together because of how maya time samples between frames. or something like that.

basically instead of enabling motion blur just go to render globals > mental ray > options > Force Motion Vector Computation and enable it. then just use the alpha channel from any other layer (beauty, matte, whatever) and copy that to your motion vectors in nuke.

11 November 2009, 10:35 PM
so does that mean all my render layers have to .exr too? all passes besides the 2dmblur pass are just 8bit Tiffs. I thought just the mblur layer needed an exr. Im kind of confused now...i thoughts the motion vectors i was passing in NUKE were from that exr node, but you are saying i need to pass the vector data from every layer i bring in? im not understanding..

11 November 2009, 10:57 PM
no not really

you still only need to put the mv2dToxik pass on one layer. but instead of enabling motion blur, just turn on "Force Motion Vector Computation" in your render globals (and make sure it's on for each layer). it means that all of your layers will go through that whole time sampling thing where you see the timeline shift slightly after you push the render button, but only the layer with the mv2dToxik pass will actually write anything to disk.

the reason you do this is so that your layers will all line up. if only one layer has motion blur, the frame will be slightly offset on that layer compared to your layers without motion blur.

so, turn off motion blur completely, turn on that checkbox in globals, and everything should be fine.

11 November 2009, 05:39 PM
hey thanks..think i'm getting the other layers dont need to be exr since not actually storing any motion data, just time shifting, right? i guess the blurred alpha needed in comp is coming from the toxik pass exr and somehow needs to get applied to all the layers in comp?
Im trying to avoid those hard lines/artifacting in the blur edges like in my image so i thought maybe wasnt doing some thing right with the alpha...

11 November 2009, 01:37 AM
Galakgorr: I did some tests to see how using 'force motionvectors' checked on compared with turning on full motionblur but leaving shutter open/close at 0.5 (which is what it is set at on blur pass layer). in both cases at rendertime, the frame in the timeline shifted 0.25 back and ahead of the current frame while doing calcs, but the rendered output was different. its like the force motionvectors on didnt use the 0.5 shutter open but maybe used 0 instead. so then those layers wont temporaly align with the blur layer.

Can you try to do same test to see if same result?

11 November 2009, 04:05 AM
Just out of curiosity, why wouldn't you enable motion blur for all participating layers?

11 November 2009, 05:09 AM
yea, im not sure..for now to be safe, im gonna enable it and leave it at same settings as blur pass...

as for the dark lines and nasty alpha, i forgot to copy out the alpha from the .exr to the mask channel before connecting to vector blur in Nuke. It was using the alpha from the comp for the blur...looking better at this point : )

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