View Full Version : Vector Blur Wonky?

07-28-2006, 08:46 AM
say, i've been playing with the Discovery Edition and i've been rendering old scenes out. i've noticed that Vector Blur is not working *at all*. can anyone confirm this on any scene. i've tested the exact same scene in 8.5 and it works just fine.

07-28-2006, 10:21 AM
If there are any subpatched objects in the scene, it seems to not work properly.

07-28-2006, 11:04 AM
you're right. good troubleshooting.

that seems to suck, eh? i wonder if it has something to do with the new camera stuff or the new displacement features.....

07-28-2006, 11:43 AM

to get the best results from motion vector blur, you need to make sure you use it correctly, as it comes with many technical limitations. Let's define exactly what motion vector blur is:

Motion vector blur is not a true 3D blur. It is an image-processing cheat, which uses a combination of motion information generated by LightWave in the form of vectors (which are straight lines) for every rendered pixel in a shot. That information is then taken by the image processing plug-in, and blurred along the length of the vector with the parameters that you provide in the plug-in's property panel. This blur process is a 2D process. As some polyons are not visible to the camera during rendering, their color and shading information are not available to either the motion vectors or to the blur process, and artifacts then occur.

If you understand the limitations, you can use the blur to the best advantage (and there are many advantages, such as speed), and avoid the pitfalls. Here are some tips:

First, whenever you have two objects crossing paths, you will see some artifacts in the area where the objects overlap. This is a known limitation of the algorithm, and is the same in any application that offers motion vector blur, including ReelSmart Motion Blur. The issue has to deal with motion vector generation where areas of objects are obscured by other objects, as that color information is not present when the blur is calculated. Essentially, the algorithm is forced to guess in that condition.

The solution to the above is to render objects in separate passes, and composite them together for the final result. This is common practice in production companies, but individuals tend to view this as a chore (which it is, but it does give you more control).

Second, you should always render against a black background color, and then comp the background in later. If you render with a background image, or color other than black, the final result will not look as good, again due to the motion vector generation and blur process.

Third, motion vector is a "straight line" blur, not a curved blur. There will be some situations where the blur is not appropriate to use. These situations should be quite clear when you make some initial test renderings. The solution is to switch to the traditional LightWave motion blur, or build an object that has some blur mapped in, such as a propeller blade.

Fourth, there is one particular case which motion vector blur can never solve, and that is best described by a simple example which we call the "fan in a cage," such as a desktop fan that has a wire cage that surrounds the fan blades. As motion vectors are linear, and as geometry is constanly being obscured by the motion, this case will never solve correctly, and should be handled by one of the other means described above.

And, last but not least, remember that motion vector blur is intended to be viewed in motion! What may be objectionable in a still image may not ever be noticeable in motion.

Hope this helps!

Jay Roth

07-28-2006, 01:59 PM
Don't get me wrong, I fully understand how vector blur works and that it is not a true motion blur. For most of my motion blurs, if I need it to look nice but wish to save time, I normally combine both vector blur as well as LWs brute force motionblur. The problem is that in 9, vector blur seems to have become unable to calculate the vector motions of subpatched objects. I reported this error at one point before the final release of 9, and have simply assumed that the problem was not high enough a priority to get attention yet. I have delt with it by freezing all of my geometery in a final scene.

07-28-2006, 11:06 PM
I have delt with it by freezing all of my geometery in a final scene.

i most likely will be unable to do that for most of my scenes, since subpatched objects means deformed objects. i cant freeze deforming objects. hope they get this fixed. if there's nothing wrong with vector blur, and the problem lies in the internal hooks (which i they they are because of the new subpatching features) then there's no way to do any third party workaround.

07-28-2006, 11:15 PM
and i think i'm right: if you use Photoshop buffer export, you will see that turning on Motion X and Motion Y buffers will give you a valid result when there are no subpatch objects. but with subpatched objects (and even with Catmull-Clark), the buffer is black, as if the object didnt exist there. so even other plugins are affected, not just vector blur!

chime up people!

07-29-2006, 05:29 AM
Yeah it seems something was forgotten, as this seems to be the 2nd bug/issue around subpatch objects and rendering in LW9.

Rendering to fields and vector blur does not like subpatch objects in LW9.
**** i just needed to use VBlur today.... Bugger!

07-29-2006, 09:23 AM
They disabled motion vectors somewhere around OB10 during the beta, they probably couldn't get it to work in time, or thought it was not high priority enough so left it like this for now.


- Fixed a bug by disabling motion vector generation for adaptive meshes. Those two features are incompatible currently.

10-18-2006, 02:12 AM
Can someone from NewTek give some indication as to when this might be fixed?

08-28-2008, 05:58 PM
As a newbie, can anyone explain to me how to do this blur technique for a black airplane propeller? Do I need to model and render the propeller as a separate object from the rest of the airplane and then apply the filter? Any help is much appreciated.

08-28-2008, 10:28 PM
for certain it is more advantageous to model the prop separately because it will be animated separately from the aircraft. you may, as you already have said, render out the props separately with higher motion blur. regardless of whether or not you use Vector Blur, render it against black.

you may use other techniques such as modelling the prop with wider blades so you can get good blur results with lower motion blur settings. but really depends on your scene (i.e. if the props start and stop).

08-28-2008, 11:27 PM
You can also render a front view image of your propellor, take that into Photoshop and use radial blur on it, including the alpha channel. Map this onto a disc, using the alpha channel in it's transparency, and use in place of the modeled propellor.

Rendering them separately with higher quality motion blur is a good solution too. 2D motion blur like Vector Blur may be the fastest rendering, but it will also look the worst for things like propellors.

08-29-2008, 10:57 AM
I created the prop in PaintShop Pro v8 and saved the image with an alpha channel, but I can't get LW8 to properly use the alpha channel. I followed the LW help instructions about alpha, but can't get any transparency. Suggestions? Thanks.

08-29-2008, 08:46 PM
It doesn't work in LW like you might think, if you have the alpha in your rgb image you need to go into the Image Editor, clone the image and set it to 'alpha only'. Then use this in a transparency texture - but you'll need to invert it, so it's white where you want transparency... which is the opposite of what you'd expect from an alpha...

08-29-2008, 10:26 PM
Thanks! I think I have it working.. finally. So I need the RGB image in the color setting, layer inverted, and the cloned alpha in the transparency setting, not inverted. It looks right anyway. Not exactly intuitive. Thanks again.

08-31-2008, 03:34 AM
No prob - but you need to invert the alpha/b&w/transparency image, not the rgb...