motion blur interval center error?

[Morbid Angel]

Hey everyone. We have been trying to figure this out for a while now and in the process got different results. We are trying to understand the function of vray's interval center option. While comparing to different renders and different software in each one we got different result so we did the very basic thing. We took a sphere and moved it along the screen in 3 frames.

then i did same setup in lightwave and hit render here is what we got:


the most obviously correct seems multipass and vray with interval center of 0.0
Mental ray and lightwave have no options to allow shifting of the objects.
My question is can some one explain this thing to me...
The reason for this being that we have first came accross this in production where our rendered models were slindinf against the live footage, but in viewport they were fine, so we started looking into this. In my opinion of 0.0 interval center seems correct then why vray default value is 0.5?
But later after talking to few people I came to a different conclusion that how can a motion blur be calculated if the object yet hasnt been on the next frame there fore in the real world where camera doesnt know where the object will go at frame 3 it will only capture blur from frame 1-2, so in this conclusion the lightwave's blur is correct and everything else isnt..
now im totaly confused haha

Thanks.

[egz]

You are correct, the value of 0.0 matches the viewport most correctly. That's the value I use almost all the time. I don't know why the default values are set the way they are. The duration default of 1.0 is physically impossible in film cameras for instance.

BTW: I made a few small tools on vray.info to calculate duration when matching real footage:

http://vray.info/topics/t0113.asp
http://vray.info/topics/t0114.asp

[Morbid Angel]

yeah thanks egz, we have used them at one point...

[Morbid Angel]

Im interested to see what Vlado can say about this

[esi]

Hey,

Egz, it would seem that the LW sample is the correct one. If you think about how a camera works, and you view frame two and the ball is moving from the middle of one frame to the other, so if you view frame two you will see half of the movement therefor the motion blur will be to the left of frame in the above example. It should not be in the middle. That would denote that the camera can see where it is going, before it gets there?

The other issue is, if the ball is traveling from frame one edge of frame one to the edge of frame 1 and so one and so on so we would see the motion blur as shown above as correct. So it really comes down to how the motion of the object (ball) is viewed by the software.

I would really like to see Vlado explain this to us? It is a serious issue for us here at Enigma. We had some composing issues with regards to this problem, and would like to have it resolved.

The simple question is. Why is the software seeing where the object is going when it does not know where it is going in the first place?


DFX Supervisor
Enigma Studios Inc.

[egz]

A camera doesn't need to know anything, it just records what it sees while the shutter is open. The object in this example follows a set path from frame 1 to frame 3, and even though there might be keyframes or specific positions on whole frames the object does have a different position in space for each fraction of a frame, or subframe. What the VRay camera does is to just record this as a regular camera - it doesn't render at frame 2, it renders a time period from frame 1.75 to 2.25 (with duration 0.5, interval center 0.0).

Just like in a regular camera the duration controls the shutter angle, and the interval center controls the shutter timing or syncronization. Bias lets you adjust for pyramidal blur that usually occurs in film cameras (varying amount of exposure during the entire frame exposure ).

Because of this none of the settings in VRay are wrong, they are all emulating a real physical camera. However, it is up to you to adjust the parameters to match your real life footage. If you want to have the blur centered around the object in the viewport you need to use a interval center of 0.0, the rest of the settings will have to be adjusted depending on the look you want.

[esi]

Hey egz,

First off thanks for the definitions for the bias, duration etc. We figured out most but you have cleared them up. I would suggest that more real world notation should be used such as sutter angle instead of duration or have both one in brackets. I personally am more familiar with camera/DOP language

Anyway, my last explanation was mud, so let me try again. everthing you said makes sense to me, and our main reason to ask these question was really revolving around the other settings. But since this has come up I must say that LW most accuratly represents how a shutter would see the image, ie it is open and then closed open etc. If the object has a constand rate of speed it would be exposed equally. So you would see it then some black then image and black etc. This is the physical part of it, the rest would be made up through persistance of vision. The object should in the real world, be at the leading edge of the motion blur, not in the middle.

Vray and others for that matter, look at it as how it travels throught the frame the same way I am sure, in other words it see where the object has traveled and puts the MB on the screen. What is the problem for me is that MAX is putting the object in the middle of it's trajectory. For instance, if we look at the middle frame of three frames, MAX, as you indicated, see's the distance traveled as .5 in front and .5 after (not talking rendering just distance traveled in a certain time frame or sub frame if you will). This means that MAX as you see it in the viewport represents the middle of the distance traveled in the time slice. This to me is a problem when it comes to MB.

MB should be behind the object travelled. Not in the front. That more accurately represents a real camera's exposure. So LW for what it is worth puts the MB behind the object that is traveling. MAX and others however do not, which is interesting in itself. I may be missing something here, but I have a hard time seeing passed this fundimental issue of MB being put in from of the object. I guess I should be clear here. The MB for the time segements is correct, but the object possition is wrong or switched around it also applies.

So simply, an obects that is moving in front of a none moving camera should have MB behind the leading object. so to be MAX has a basic problem here.

DFX Supervisor
Enigma Studios Inc

[egz]

I guess the key to the problem here is that the location of the object in the viewport is assumed to be the end of exposure for that frame, but in the renderer it is not... and you want it to be.

If VRay implemented negative values for interval center this would be possible with these values: Duration 0.5, Interval center -0.25. This would give you a 180° shutter angle, lightwave style. I guess negative interval center should be added to the wishlist for those that prefer it that way. More options never hurt anyone

A workaround to get the LightWave look is to render one frame early using the values: Duration 0.5, Interval center 0.75. What I mean by this is that the renderd image for frame 0 would give you the LW version of frame 1's blur, rendered frame 1 would give the LW version of frame 2's blur etc. So: start animating from frame 1, but render from frame 0.

Of course if you need a different shutter angle, remember to adjust the interval center as well, so that it equals (1.0-(duration/2))

[esi]

Hey there,

Well I guess if you are just doing CG a lot of this would not matter to much. However we do a lot of CG on Live action and this method would not work for us as we need to match the camera that was used on the day. I don't know 100% if this is the case, the bottom line is we did some CG stuff and due to some tech issues not of our making, we found a slipping effect when using anything but 0.0 on the interval to match the other method we used for another element. Vray did not work with this certain plugin, just in case you where wondering. We also had to match the look of LW as another studio was doing stuff in that. There is the problem.

Mind you we also had slipping issues with a CG on a live plate also. We will look into this more. But I really think that the bottom line is not really what Vray does but how the animation program is working here. Vray just renders what it see's, yes?

But I would strongly suggest a 0.0 as the default not .5

I am also going to bring this up with discreet and see what they say. As I said I just wanted to solve one problem and this entire issue came up and I like to know exactly what is going on. Now I am clear so thanks again for your debate.

Cheers

Mark R
DFX supervisor
Enigma Studios Inc