[Sidefx-houdini-list] Feather/Fur for Bird

Ozgur Yılmaz eoyilmaz at gmail.com
Sat Jan 16 17:14:21 EST 2010


Hi Diana,

In our last project, I've used Houdini to create 3 pigeon characters
feathers. The characters were rigged in Maya.

You are right, the best way is to use Copy SOP with orient quaternions.
Instancing is not efficient, because, the base feather that is going to be
copied over the geometry generally has a very low number of polygons, so it
is not efficient to store all the matrices.

I'll try to summarize my approach in these steps:

*Exporting from Maya:*

   1. The characters are rigged and animated in Maya.
   2. The order of the objects when exporting is important, you should
   always export the objects in the same order or it will not work, so I stored
   the selection list in characters XFORM notes attribute (simple and quick
   solution), I've then used this attribute to automate and use the render farm
   for this export process
   3. Then converted them to bgeo.gz to reduce the disk usage in the file
   server
   4. I've also exported the T-Pose of the characters as OBJ models to be
   used as the Rest pose which is needed to have stabilized feathers on the
   characters over the frames

I think I need to write a bgeo exporter for Maya...

*Houdini Part*
We need to import the character sequences and then create the feathers...

*The Character:*

   1. Create a geometry node and give it a name like "bird_geo" or something
   ( '*_geo' is important cause later on it will be used in light linking)
   2. Load the bgeo.gz sequence (I'm going to call it Live Geometry or Live)
   and the T-Pose OBJ file (Rest Geometry) with two File nodes
   3. Attach a VOP SOP to the Live Geometry and pipe the Rest Geometry to
   the second input
   4. Store the Rest Geometry point positions in an vector attribute called
   'rest' in the Live geometry
   5. Do your material assignments and add a Null at the end of the graph
   and rename it RENDER
   6. I've also calculated the v (velocity) of each point by using a
   timeShift and a VOP SOP node, to have the motion blur of the geometry and
   the feathers

*
The Feathers:*
( The overall thing is like that: Import the geometry, move it to rest
position, create the orient quaternion and reorient the quaternion attribute
to the live geometry, then create the feathers with that orient values )

   1. create another geometry node, and go inside this node
   2. use an "Object Merge" node to merge the "bird_geo"s RENDERs output to
   this geometry
   3. create a VOP SOP and move the points to the rest position (the T-Pose)
   by connecting rest to P (we will calculate the orient quaternion in the rest
   pose and then reorient it to the live pose, this is the best approach to
   have stable feathers over time )
   4. create a comb node ( to have the featherDirection vectors)
   5. set override normal to an attribute called "featherDirection" and
   paint the directions that your feathers are going to point at
   6. create another VOP SOP and call it projFeatherDir_vopsop
   7. inside projFeatherDir_vopsop, what we simply need to do is to project
   the featherDirection vectors over the geometry, to have perfectly orthogonal
   vectors to the normal (N) vector of the geometry, you can simply do that by
   using to cross product nodes ( (NxF)XN will give you the projected vector )
   8. create attributes like inclanation, randomRotation, pScale ( I don't
   recall if it was pscale or pScale, it is a variable that Houdini understands
   by default)
   9. then create paint nodes for each of them ( paint_inclanation,
   paint_pScale ignore the randomRotation for now )
   10. I've used a simple expression (which I don't remember now) in a Point
   node to fill the randomRotation variable with random values and to randomize
   the pScale values
   11. create a measure node and let it calculate the area of the primitives
   ( it is going to be used in the scatter node later on )
   12. create a VOP SOP again and call it orient_vopsop and go inside this
   node
   13. to calculate the orient quaternion you basically need two vectors, N
   and the projFeatherDir vectors and the inclanation, randomRotation values
   14. as I remember I used a "lookAt" node at my latest setup to calculate
   the 3x3 rotation matrix that rotates the +Z axis of the feather to the
   projFeatherDir and aligns the +Y vector with N
   15. pipe the projFeatherDir to "to" input of the lookAt node and the N to
   the "up" input and set from to (0,0,0) (this is important!!!)
   16. this lookAt node will give you a 3x3 rotation matrix which is enough
   to rotate the feathers to match the projFeatherDir
   17. I've then created a constant node to create the x-axis ( vector 1,0,0
   )
   18. multiply the x-axis with this matrix from the output of the lookAt
   node to have the feathers x-axis in place
   19. create parameter nodes to get the inclanation and random rotation
   20. create two "rotate" nodes
   21. for the first rotate node, plug aligned x-axis to the axis and
   inclanation to the angle of the rotate node
   22. for the second rotate node, plug N to the axis and random rotation to
   the angle of the rotate node
   23. by multiplying all the three matrices in the right order ( lookAt ->
   inclanation -> randomRotation ) you should now have a final 3x3 matrix
   24. convert the matrix to a quaternion by using a "matrix3 to quaternion"
   node
   25. create a parameter node to export the quaternion with the name of
   "orient"
   26. go one up level ( out of the VOP SOP to the geometry )
   27. create an attributeReorient node
   28. put live geometry to the "input 1 : live geometry" plug and the
   orient_vopsop to the second plug
   29. select orient from the drop down menu inside the attributeReorient
   node
   30. create an attirbuteCopy node to copy the area, pScale and v
   attributes to the live geometry
   31. create a scatter node to have certain amount of points on the
   geometry
   32. deselect "scatter based on primitive area" and set "alternative
   attirbute" to area
   33. use "attribute bias" attribute to control the distribution of the
   points
   34. create a grid node, move it in the +Z axis to have one edge aligend
   with the x-axis, do whatever deformation you want on that grid
   35. create a copy node, and connect the grid to input 1, and the scatter
   to input 2
   36. celebrate yourself :)

I think I had more nodes than I've explained here... And you can even add
more attributes to have automatic feather animation for wind simulation, to
rotate the feathers according to the curvature of the surface (use the
measure node to get the curvature), to have different material attributes
for every feather ( actually I had this in my setup ) etc. ....

The feathers shouldn't cast shadows, or they will look to hard rather than
soft. Adjust your lights to exclude feathers from shadow casting...

At the end I've exported the feather system as an asset, and created the
minimum amount of parameters to make it work with different characters.

*The Problems*

*Problem 1:* The interpolation of the quaternions: When you created the
scatter node, the quaternion values are interpolated from the primitive
points over the scattered points. If the primitive points' orient values are
so different (axis, angle, mainly the axis vector) than each other you
probably going to have feathers in strange orientations. To fix that I've
created a subdivide node and right after importing the live geometry and
divided the problematic areas. But even with this I had problematic
feathers.

*Problem 2:* Render Time and Memory Usage: I've used Mantra to render the
birds and feathers. Some how I couldn't render them quickly. I think it was
mostly related with the shading, because it was rendering super fast when
I've not used opacity maps. The render times for PAL resolution were around
10-15 minute for one character for fullbody renders, and around 5-6 minutes
for head close-ups. When you consider you have all the three characters side
by side in some shots, it was too much ( render farms have 8 core XEON cpus
with 10 gig of ram). And they were using too much memory around 6-7 gigs ( 3
ram + 3 virtual mem ). But, I've successfully reduced the memory usage by
writing down the final feathers to disk and cleaning the SOP cache and then
loading the feathers again in render time.

Here one of my collogue has added the commercial to vimeo:
http://www.vimeo.com/8671263

I hope it was clear enough... Like you, I couldn't find any good resource
giving a clue about creating a feather system. So I'm curious about how
others going to solve the problem of creating a feather system. And I've
used Houdini for the first time in my life, and there was no body around to
teach me, so I've probably did a lot of things wrong (especially the render
setup and the shaders)

Cheers...

E.Ozgur Yilmaz
Lead Technical Director
www.ozgurfx.com


On Wed, Jan 13, 2010 at 1:26 AM, <craigleehoffman at aol.com> wrote:

>
>
> Diana,
>
> Do you want all the feathers to be geometry (perhaps simple low-res proxy
> geometry only) seen in your interactive viewport so you can determine and
> fix interpenetration problems or animate them, etc.- or do you only need to
> see them at render time?  Are they large stiff type feathers like an eagle,
> or small fluffy/furry down-like feathers, etc.?
>
> There is no "one size fits all" solution for feathers- the solution needs
> to fit the design and needs of your project.  Houdini allows an enormous
> amount of flexibility and power for how to address something like feathers.
>
> Good luck,
> Craig
>
>
> -----Original Message-----
> From: Diana Li <r.diana.li at gmail.com>
> To: sidefx-houdini-list at sidefx.com
> Sent: Tue, Jan 12, 2010 3:16 pm
> Subject: [Sidefx-houdini-list] Feather/Fur for Bird
>
>
> Hi,
> I am working on a project involves feathers on a bird like creature.  I
> need
> ome help to see what's the best approach.  The creature will fly, but only
> n medium shots and long shots, and there is one close up shot of the head.
>  haven't found any tutorial on fur/feather system.  After researching, it
> eems that to create feather, it's adding fur on a tube like geometry, then
> opy/instance it to where you need it.  Does that sound right?  I never used
> ur, I'd really appreciate it if someone could point me in the right
> irection. thanks.
> diana
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