copy sop or grouping/deleting points seems to destroy motion blur...

[Baldric]

Hello everyone,

 

I am currently working my way through one of cmivfx's tutorials - the one called "Houdini Mograph Title Design". In the hip file I have a copy sop that copies cubes onto points, which works fine, and everything rendered as expected as well. Until I decided to take things a bit further. One of the things I did was to delete some points that I didn't want to include in the copy procedure. I did this by procedurally creating groups for these points, and then deleting them before the copy sop.

Suddenly I have the strange problem that in those frames where I delete points the motion blur (geo blur / deformation blur) no longer works, whereas in the frames where the groups/deletes are not active it works fine. What on earth is going on here? Is this a known bug, or is it a common no-no to delete points before a copy sop?

At the moment I cannot upload the file - perhaps later - but I would really like to know if this is something that happens often, or if it sounds more like a bug.

 

Help is appreciated :-)

 

Thanks,

 

Doug

[Sierra62]

Are you deleting the points on specific frames, or are you just deleting the points?  Off hand I'm not sure what's causing your problem, but a quick fix for you would be to through down a trail sop and compute velocity.  This will update your velocity attribute for the motion blur.

[Baldric]

Thanks for your reply Sierra62!

 

I am actually deleting points over certain frameranges, depending on an animated value. What is happening is that I am creating boxes from a pointcloud inside the volume of one or more objects. I have created a set up that "dissolves" this object in 3d (using a cookie sop), which works well, but some of the boxes protrude through the dissolving geometry, so I decided to create groups for the points that are inside the geometry that is still intact, and for those points that outside the dissolve geometry. This way I can delete the protruding boxes, but keep the other ones. And this is were the problem occurs. As soon as I do that the motion blur does not seem to work for as long as I am deleting points...

 

Thanks,

 

Doug

Edited November 26, 2014 by Baldric

[Baldric]

Actually it seems like your trail sop suggestion did the trick... Am still curious about how stuff like this happens though...

Also, the velocity motion blur I get, using the trail sop - how does that compare in quality to the deformation motion blur I get without the trails sop? Are there any differences, things to watch out for?

 

Thanks,

 

Doug

[Sierra62]

The trail sop calculates the velocity of each point of the input object, so as it deforms it will calculate the correct velocity.  Where as using the copy sop, each point of the object will be given the velocity of the point its attached to. So if it the object on the particle deforms, the velocity attribute won't adjust for the deformation changes.

[Baldric]

Ok, so the real workaround for this problem seems to be to scale the points to 0 before the copy sop instead of deleting them. I was told it is because when the renderer samples the geometry, and points keeps changing it gets confused, producing the result I had. However with scaling to zero, the geometry is technically speaking still there, but will not show up in your renderer, the point count does not change, and it renders fine :-)

 

Don't know if anyone else have had this problem, but that did the trick for me. Using the trail sop also worked, but only gives a linear blur, which, honestly, shoud be enough for most cases...

 

EDIT: sorry, I didn't see your latest reply Sierra62. Thanks for the info! In this example the objects do not deform, but the path they take is such that a curved blur would look nicer. But I guess it is not really, really, necessary. I am trying to learn, however, and thus it was great how to find out how to deal with the problem if I should happen to see it again. :-)

 

Doug

Edited November 26, 2014 by Baldric