View Full Version : Chain saw motion blur

02-03-2010, 03:10 AM
I'm having some trouble rendering a proper motion blur effects on a chainsaw rig. (In fact that's a tentacle with blades spinning along) Basicly I used Proton's tank track trick (with morph) to animate the blades but even with fractional frames activated the blades seem to be too slow. I can't use more than 150 / 200 % motion blur because then the tentacle moving around looks too blured. Does anyone has any idea? I tried already to render tentacle and blade separately (using the surface opacity set to constant value =0 to turn off one of the material) but the compositing does'nt look right (black from the alpha showing where the blade intersect the tentacle.)

thank you for your propositions!

02-03-2010, 03:34 AM
Hard to say from what you've said, the blades 'seem' to be too slow? All I can suggest is to speed them up!

What kind of motion blur are you using?

Fractional frames don't affect the render at all btw.

02-03-2010, 05:43 AM
Ok, imagine the speed of real chainsw blades to be let say, 2 full rotations by second. the trick I use is to clone the blades along a spline (shape of the tentacle) with the original blade position just on the spline's first point (rail clone) then in another layer clone the same number of blade BUT offset the original blade by the distance of one blade (basicly, the motion of one blade forward). The result might look the same BUT the order of the blades is shifted. Create a new morph with the background morph fonction and you get a "1 blade forward" motion throught morph mixer in layout.
in morph mixer just create an eveloppe to repeat the morph going from 0% to 100%. Search for Proton's video tutorial, It's probably clearer than my explanations.
Anyway, the point is that you can't go fast enough compare to the real thing to get the proper look with only 25 frames / sec. and a 1 blade forward motion instead of a 360 for the whole chain, about 70-80 blades twice by second. (hence the test with fractional keyframe allowing me to animate the morph below the one frame limitation.)
Yes when I read my post I start having a headake too...

02-03-2010, 03:12 PM
Ok I've used that trick (it's actually older than Proton's video, but he's not laying claim to the idea anyway). I don't think moving one tooth per-frame is fast enough, you could try moving the morph to 200%, but it might look funny when they change direction.

Another thing to try is scale the tentacle animation to double the keyframes, so it moves 1/2 speed, and render every-other frame. Then you can turn up the amount of motion blur without over-blurring the tentacles.

02-04-2010, 01:09 AM
So, I tried to push the morph up to 200% but the blade change size and pop big when reaching 200%. Same problem, just a little less noticeable, with smaller values.
I tried the photoreal blur but It blast my render time to unbearable levels. Actualy my best reesult so far is to use the classic MB with 20 passes and an AA of 8. The effect I get is more like a miltiplication of the blades. in After Effects I use Smart Motion blur to blend the blades together and It looks usable.
I still have to try your trick with doubling the keyframes and rendering every two frames. (I like the idea!). I remember for a bird, I created clones of the wings and offset them from the originals with a 50% dissolve ... It gave a nice effect with a bit of MB. I could try that also.

Thank you for your help Toby!
BTW, how did you get the mention "dismember" with your name? Pretty cool!
And thank you for "advertising" the Utube vid... ;-)

02-04-2010, 01:46 AM
Glad to (try to) help! They shouldn't change size, there must be some scaling in the morph. Well good luck with the other ideas.

Can't honestly say I 'earned' the title Dismember, you can change them under 'User CP' near the top of the page.