View Full Version : Limiations of RPF motion blur / How big studios do motion blur

08 August 2008, 09:36 PM
I've got an extremely time-consuming rendering done in RPF format, and I planned on applying motion blur in post using Combustion beacuse an already ridiculous render time would be 4-16 times longer if I did motion blur in 3D.

The problem is, for the first time, I'm noticing some serious limitations on RPF motion blur that have never really been such a problem before.

The main problem is that the blur doesn't seem to be adapated to the speed of the object. In other words, an object that's moving, say, 50 pixels across one frame gets about the same blur as an object that's only moved 5 or 10. The problem with this is that really fast moving objects need to become streaks in order to look naturally (especially since I'm working at 24 fps, not 30 or higher). Instead of streaks, however, what I get is an object that appears to jump across the frames beacuse each of its locations across the animation aren't being "connected" by a longer blur.

Meanwhile, objects moving slowly are being blurred too much!

What I get as a result is an animation that's the opposite of what I need-- too blurry overall, except for the really fast objects, which are too sharp.

Since this is unacceptable for any animation in which all objects aren't moving at consistent and similar speeds, what are my options? Obviously rendering true motion blur is off the table, since I've already got a render time that spans in the days and would literally become weeks if not months if I had to multiply it by a healthy number of motion blur passes.

I know the bigger studios and agencies have a lot more rendering muscle than I do, but even in thier case, a multiplier is a multipler-- x8 or x16 the render time is still a massive overhead.

Do the bigger guys actually render 3D motion blur or is there simply a better post effect solution that I haven't found yet? I've used both Combustion and the ReelSmart plug-ins and have yet to find anything that handles the kind of situation I've described here.

08 August 2008, 05:29 PM
It sounds like your render engine isn't generating proper velocity channel data. You could always try doing a velocity Render Element pass (make sure to set a valid Maximum Velocity in the settings) and use this in a Gbuffer Builder to replace the existing velocity channel info. I do all my motion blur using 3d motion blur with brazil2. Most post effects don't support rotational motion blur properly, which is what I usually have.


08 August 2008, 01:59 PM
options include:

true 3d motion blur

mental ray has a newer post render shader that does HDR image blur to your render much faster than true 3d blur.

the above two solutions, however, "bake" the blur in your primary render so if you do too much or not enough, it probably requires a re-render from the 3d app.

render a velocity pass and feed to a compositor for thing like Revision's Smartblur plugin, Combustion's G-Buffer builder or Toxik's native blur tool. these are examples of things that can read a velocity pass and dial up or down moton blur in realtime.

RPF velocity for combustion is another option but a bit older.

its important to consider them for each job because while true 3D blur takes the longest and might require re-renders, it typically looks the best because its most accurate. trade offs in any solution.


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08 August 2008, 01:59 PM
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