motion blur question

[posterus]

hello all,
i was playing around with motion blur this afternoon and noticed this. i am curious why v-ray's motion blur will add the blur effect ahead of where the object actually is. take a look at the comparison image below. both hammers were rendered on the same frame (frame 7) one has mblur enabled and the other does not. the red arrow indicates the direrection the hammer is going. how can there be that much blurring going on ahead of where the hammer actuallt is?
movie: http://www.vrayelite.com/misc/mblur.html

[Dynedain]

Because it is blurring with the location of the hammer in the following frame.

[posterus]

ah makes sense
thanks

[joconnell]

Right so ted here's why:

Vray tries to replicate how a camera works which is by opening the shutter and closing it again every frame - what it's doing is correct, it's capturing the objects movement from where it was at the start of the frame (lets say frame 1 for arguments sake) and where it ends up (lets say frame 2).

When vray calculates the blur, it does it using sub frame sampling to determine how the object has travelled between frames 1 and 2 and thus catches all of the positions in between. How many positions in between is determined by the motion blur geometry samples. The default of 2 isn't great, it'll sample the object at its starting position frame 1, and it's end position frame 2. This is a linear blur between the two positions and if the object rotates or arcs in any way, it'll make a boxy looking blur. If you set the geometry samples to 3, it'll add another sample and sample the objects position at 3 places. Frame 1, frame 1.5 (cut down the middle) and then frame 2. If you set it to 4 samples, you'll get better arcs again since it'll get the object at frame 1, frame 1.33, frame 1.66 and frame 2.

Now on to your problem. What vray is doing is totally correct, but what we're used to seeing is the scanline vector motion blur. As you can see, the vray blur moves the object off it's position and rotation when you turn on blur into a place between where it starts and finishes. The scanline doesn't, it simply makes a rough motion vector of how the object moves and smudges it along the direction of the vector without changing its position or rotation. Lets take for example and object that moves from the left of screen to the right of screen in 1 frame. If you render this in vray, you'll correctly get the object positioned between the left and right with motion blur. In the scanline, it'll render the object in the position it started from in frame 1, and then smear the object in the direction of where it's headed - most importantly the centre of the blur is not halfway between where the object started and finished, but right where the object started from. What this gives the effect of, is that the object had actually started further to the left than at frame 1 and is moving to the right. Here's a diagram:



On the top line you have a set of 3 frames rendered with no blur in the scanline - vray would look the same. On the second line you have the scanline vector smudge. If you notice how it's working, it renders the object in place, then smudges across it based on the direction it was moving between the current frame and the previous frame. In this case frame one is wrong technically since it's not seeing how the object is moving between frames one and two. Likewise frame 3 is wrong since the object is getting some blur - its come to a total halt at frame 3. On the bottom frames its vray blur which in this case is getting that the object is moving between the left and middle and sampling it as it moves - it's correct in this case. Again the second image shows it moving between the middle and right positions so vray gets it correct here too sampling again as it moves. On the last image, the sphere has come to a halt so it correctly gets no blur.

Now that's all very well and good but what it we dont want technical accuracy - where would we need vrays blur to behave like the scanline blur? Loads of places. There could be times that you need to render in the scanline and combine this with a vray render - there might be an effect that only renders in the scanline or is quicker to render in the scanline such as transparency mapped hair like hairFX or even afterburn (I know it works - its just an example). You might want to render a series of matte passes in the scanline for speed reasons.

Thankfully vray gives us a control to determine where it's taking as the point to base its blur around - we can see that the scanline blurs on the frame whereas vray blurs between frames. This control is called interval centre in the vray camera dialog in the render settings. If you set that value to 0, it centres the blur around the start of the frame. 0.5 (default) is based halfway between frames and is the most correct, and 1 will centre the blur around the end of the frame (probably least useful). so if you want to get closer to the scanline, set your interval centre to 0.

Here's an image set to inteval 0:



Not totally the same but you can see in the middle frame its weighted to the objects position like the scanline.

This also helps if you're integrating things into live action footage - If you've tracked an object and its position is locked at each frame, using the vray motion blur will actually put it 0.5 frames out of sync with the footage and it'll slip all over.

[cpnichols]

If you use the VrayCamera that is what the shutter offset is all about. 180 degrees is a 1/2 frame motion blur, and then you can either offset the shutter to blur at frame start, frame end or center on frame, depending on hwo you offset it.

[jonnybefree]

great explanation!!! thanks man.

[joconnell]

If you use the VrayCamera that is what the shutter offset is all about. 180 degrees is a 1/2 frame motion blur, and then you can either offset the shutter to blur at frame start, frame end or center on frame, depending on hwo you offset it.

Absolutely - a shutter is just a rotating disc that opens a hole to allow light in for a specific time - If you ever talk to a camera operator and ask about the shutter angle it'll tell you how long the motion blur duration is - the shutter disc rotates 360 degrees each frame and the shutter angle is how long the shutter is open for. If the shutter angle is open for 180 degrees, that's half of the frames time as chris mentioned, a shutter angle of 90 degrees would be a quarter frame and so on. You pretty much divide 360 degrees by the shutter angle to find your motion blur duration with a cinema camera.

[posterus]

much thanks for that explanation. it's really helpful.
-joe