Shutter alignment not yielding expected results

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Shutter alignment not yielding expected results

Postby scooternva » Tue Dec 10, 2019 5:25 am

scooternva Tue Dec 10, 2019 5:25 am
I don't know whether this is a bug or the expected behavior.

The test scene (attached) positions the camera at -35,0,0 in XYZ space and is rotated 90 degrees to point at the direction of travel of a glowing blue ball 5 meters in diameter that is traveling right-to-left in the scene. The ball is keyframed at frame 0 with a position of 0,0,-100 and at frame 10 with a position of 0,0,100. The ball's motion is linear; therefore, at frame 5 (midway between 0 and 10) the ball is at 0,0,0. In other words, with no motion blur applied, the ball should be dead center in the image.

That is not what I am seeing, depending on the "Shutter alignment" setting in the "MB" tab of the Render Target node. Only a "Shutter alignment" setting of "After" yields the expected result:

0_pct_motion_blur_shutter_alignment_before.png
0% motion blur; Shutter alignment: Before
0_pct_motion_blur_shutter_alignment_before.png (9.88 KiB) Viewed 3009 times

0_pct_motion_blur_shutter_alignment_symmetric.png
0% motion blur; Shutter alignment: Symmetric
0_pct_motion_blur_shutter_alignment_symmetric.png (9.33 KiB) Viewed 3009 times

0_pct_motion_blur_shutter_alignment_after.png
0% motion blur; Shutter alignment: After
0_pct_motion_blur_shutter_alignment_after.png (9.08 KiB) Viewed 3009 times

Also, no matter which "Shutter alignment" setting I choose, when motion blur is turned on I cannot get the blur to trail the object that is being blurred (think of a comet with a tail--that's the look I'm aiming for). OctaneRender seems to be ignoring the "Shutter Open" and "Shutter Efficiency" settings in LightWave; no matter what these are set at, the blur always seems to be in front of the moving object, which is completely wrong--the blur should be after the object. Making the "Shutter alignment" "Before", "Symmetric", or "After" only seems to be moving the position of the overall blurred object; it has no effect on where the blur is occurring on the object itself:

25_pct_motion_blur_shutter_alignment_before.png
25% motion blur; Shutter alignment: Before
25_pct_motion_blur_shutter_alignment_symmetric.png
25% motion blur; Shutter alignment: Symmetric
25_pct_motion_blur_shutter_alignment_after.png
25% motion blur; Shutter alignment: After


If I superimpose the non-blurred versions of the blue ball on top of the 25% blurred versions it's easier to visualize what I'm talking about. In all three cases, the non-blurred ball (which, remember, is moving right-to-left) is trailing the 25% blurred ball; the only thing that changes in each case is that the "Before" and "Symmetric" shutter angles visually put the object at the wrong location; only the "After" shutter angle positions the ball where it should be (0,0,0). And again, no matter what combination of settings I use, I cannot get the blur to trail the non-blurred part of the object; the blur is always preceding the non-blurred part of the object. It's as if the tail of the comet is in front of the comet!

25_pct_motion_blur_shutter_alignment_before_combined.png
25% motion blur superimposed on non-blurred image; Shutter alignment: Before

25_pct_motion_blur_shutter_alignment_symmetric_combined.png
25% motion blur superimposed on non-blurred image; Shutter alignment: Symmetric

25_pct_motion_blur_shutter_alignment_after_combined.png
25% motion blur superimposed on non-blurred image; Shutter alignment: After


Is this behavior expected? Are the "Shutter Open" and "Shutter Efficiency" settings in LightWave not supported? Comments elsewhere about how motion blur works seemed to imply that the "Shutter Open" setting would affect how the blur is rendered but that doesn't seem to be the case. I can set the "Shutter Open" setting anywhere from -100% to 100% and renders look exactly the same.
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Re: Shutter alignment not yielding expected results

Postby scooternva » Tue Dec 10, 2019 5:27 am

scooternva Tue Dec 10, 2019 5:27 am
motion_blur_test.zip
Example scene and object
(14.83 KiB) Downloaded 186 times

The example scene and object didn't upload for some reason; here they are.
Octane 2020.2.3.1 | LightWave 2019.1.5 (3134) | Win10 Pro 19041.1052 | 2 x Gigabyte AORUS RTX 2080 Ti Xtreme w/NVLink | Gigabyte AORUS Xtreme X399 | AMD Ryzen 2950X | 64 GB RAM | SSDs: Samsung 970 PRO 512 GB + Seagate FireCuda 510 2 TB | blog.gammans.zone
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Re: Shutter alignment not yielding expected results

Postby juanjgon » Tue Dec 10, 2019 11:51 am

juanjgon Tue Dec 10, 2019 11:51 am
Yes, I think that this is the expected behavior in Octane. The LightWave camera "Shutter Open" and "Shutter Efficiency" parameters are not supported.

Thanks,
-Juanjo
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Re: Shutter alignment not yielding expected results

Postby scooternva » Tue Dec 10, 2019 1:40 pm

scooternva Tue Dec 10, 2019 1:40 pm
Oookaaay... but that's not what you implied in this thread, though. You suggested adjusting the "Shutter Open" parameter to tweak the appearance of motion blurring, but now you're telling us that this setting isn't really available after all.

This is disappointing. It just doesn't look right for the blurring to precede the object. Is there any way that we could get the "Shutter Open" and "Shutter Efficiency" settings supported in an upcoming OctaneRender release so that the look of motion blurring can be more controllable?

Thanks.
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Re: Shutter alignment not yielding expected results

Postby frankmci » Tue Dec 10, 2019 4:12 pm

frankmci Tue Dec 10, 2019 4:12 pm
scooternva wrote:This is disappointing. It just doesn't look right for the blurring to precede the object. Is there any way that we could get the "Shutter Open" and "Shutter Efficiency" settings supported in an upcoming OctaneRender release so that the look of motion blurring can be more controllable?
Thanks.


As I understand motion blur and film exposure, Octane is behaving correctly. The film doesn't know which direction an object is moving, it only knows the interval any given point has been exposed to light.

Say you have a sphere with a diameter of 10 units. It moves at a constant speed to the right. In the time the shutter is open, it moves ten units. Both the left and right edges of the sphere image have been exposed for the same amount of time. The blur should be uniform, left to right, across the entire object. It doesn't matter when the shutter opens, whether it's the beginning of the frame, the middle of the frame, or the end of the frame. Regardless of exactly when the shutter actually opens, relative to when you think of your frame, the shutter is open for the same amount of time, and the exposure will be uniform across that period of time. What changes is the position of the objects in that rendered frame, not the position of their blur.

The "Shutter Alignment" setting adjusts the timing of the shutter mechanism, relative to the time you choose to label your frame, that's all. It doesn't change the actual exposure.

In other words, if you take a picture of a real golf ball flying through the air, it will be blurred along both the positive and negative motion vectors, just as you are seeing. The following image is exactly what I would expect, and is accurate to how it works in the real world.
Attachments
Screen Shot 2019-12-10 at 11.31.13 AM.png
Same frame - Before, Centered, and After Shutter Alignment, behaving as expected
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Re: Shutter alignment not yielding expected results

Postby juanjgon » Tue Dec 10, 2019 9:29 pm

juanjgon Tue Dec 10, 2019 9:29 pm
Hi,

What I mean in this topic is that you could need to tune the "shutter open" parameter while rendering with the LightWave native engine to match the LightWave motion blur to the Octane one. This parameter only affects the LW rendering, not the Octane rendering.

I must agree with the idea that the film doesn't know which direction an object is moving. The "comet-like" motion blur is a weird effect in the real world, probably always created in post, or perhaps with a renderer that could modify the shutter efficiency using a curve along the frame time, or something like that. In any case, nothing related to the photorealistic rendering, at least IMHO.

Thanks,
-Juanjo
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Re: Shutter alignment not yielding expected results

Postby senorpablo » Thu Oct 15, 2020 4:37 am

senorpablo Thu Oct 15, 2020 4:37 am
juanjgon wrote:Hi,

What I mean in this topic is that you could need to tune the "shutter open" parameter while rendering with the LightWave native engine to match the LightWave motion blur to the Octane one. This parameter only affects the LW rendering, not the Octane rendering.

I must agree with the idea that the film doesn't know which direction an object is moving. The "comet-like" motion blur is a weird effect in the real world, probably always created in post, or perhaps with a renderer that could modify the shutter efficiency using a curve along the frame time, or something like that. In any case, nothing related to the photorealistic rendering, at least IMHO.

Thanks,
-Juanjo


Something does seem broken in the Octane implementation of shutter alignment. Shutter alignment set to before yields no blur at all. Shutter alignment should be the same as front or back curtain flash sync in cameras. Which is to control whether the flash is fired at the beginning of the exposure, or the end. Objects rendered with no motion blur should be sampled at the time in the frame based on the shutter alignment. In other words, with no motion blur, it should represent where the instantaneous image is grabbed in the duration of time represented by the animation. 1/24th of a second, 1/30th, etc.

I'm trying to get the same effect as the original poster. I have two animated, moving lights at the same position. One has motion blur, the other does not. If I set the shutter alignment to after, I get a motion trail in front(in the direction it's going) of the brighter light with no blur. If I set it to before, there's no blur at all. Symmetrical yields the same exact result as after.

The comet effect described above is possible with a real camera and a flash using front or rear curtain sync. The representation of the object frozen in space by the near instantaneous flash would either precede or follow the blurred representation of the object in motion. It should be possible to render the same thing if Octane had implemented the shutter alignment correctly. Even the naming, before and after, suggests the original author perhaps didn't understand the feature they were trying to implement. It should be labeled start and end rather than before or after.

Here's an example:

https://64ee9a43-a-0b51289a-s-sites.goo ... edirects=0
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