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Posts tagged ‘Transforms’

Multiplying matrices in MotionBuilder?

June 1, 2012

Charles

For the life of me I can’t figure this out – If I have a null and a cube at the origin, with the cube rotated and the null placed at [0,20,0] – Setting the nulls transform by multiplying it by that of the cubes should put it into the coordinate space of the cubes. (Essentially orbiting it about the cube)

This doesn’t appear to be the case though, as it appears to be doing the transformation in-place I.e. doing the transform with the first 3 rows and then adding the position part 4th row.

from pyfbsdk import *
box = FBFindModelByName('Cube')
null = FBFindModelByName('Null')

k = FBMatrix()
box.GetMatrix(k)

j = FBMatrix()
null.GetMatrix(j)

m = FBMatrix(j * k)
null.SetMatrix(m)

EDIT:

Just found out the FBMatrixMult – seems to do the job.

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Hiding Transforms in Layers of Abstraction

May 21, 2010

Charles

Don’t do this!

What you see is what you get – that is the rule you need to follow when it comes to getting and setting transforms. When an objects transform is actually an internal transform relative to an internal parent and not its physical one things get very complicated when trying to access them.

If at all possible (this applies to rigging too) when accessing data and there controls, don’t hide data/curves in other data/controls methods. Blending between IK-FK is a clearly defined system; you can see both systems and know when your blending between them.

A really, really indispensable PDF on BVH and motion capture formats.

September 16, 2009

Charles

I found this PDF on motion capture formats – funnily enough its called “Motion Capture File Formats Explained” and it is really essential if you’re trying to figure out why everything appears to work but doesn’t.

Motion Capture Formats Explained

Read on from page 16 if you like me, having built a correct matrix from the global data and offset find out that inaccuracies get past done the chain because of discrepancies in this very global matrix (due to the communicative problems of matrices). I will add this to my research page, and possibly keep a copy on my server for backup.

Matrices: Rules I try to remember, but tend to forget!

June 2, 2009

Charles

1. In 3d the product of matrix multiplication is always in world space.
2. An objects transform is a product of its target space multiplied by its reference space.

This second part is really vital to understand, (I dont think I’ve phased it myself well enough here even) But basically it means that if your transforming an object by the world, but the object is parented the difference between the object and its target space need to be multiplied by the objects local space.

I’ll try to describe this more with pics.

More on matrices: basics

May 30, 2009

Charles

Most object transforms in 3d software are matrices’ heres a rough breakdown of what they are.

A matrix in 3d is an axis defined by three vectors: X,Y and Z and fourth being it’s positional offset from an origin. The length of each axis from the origin defines the scale of that axis; 1.0 being 100%. The ‘identity’ matrix is an objects base transform e.g.

matrix3 [1,0,0] [0,1,0] [0,0,1] [0,0,0] – for the X, Y, Z axis’ and the positional offset from the origin.

So for instance if we wanted to scale an object by 200% along its X axis our matrix transform would look like this – matrix3 [2,0,0] [0,1,0] [0,0,1] [0,0,0]

Notice also that each axis is perpendicular to each other axis (90 degrees) – this is important is if it wasnt we would get skewing. Now each axis’ can point in any direction as long as the other two are perpendicular to it.

Transforms, matrices, ideas and the likes..

May 25, 2009

Charles

Sorry for the delay, pretty busy here at work essentially rebuilding some big pipeline tools, systems etc etc.

First up I’ve got a new idea for an animation controller which I’m sort of keeping under wraps. Basically rigs are hooked together by controllers which are animated, problem is most of these tend to be custom – what im planning is a very basic controller that does advanced transforms very well – on top of this customly defined controls can be added on top – which get sort of ‘wired’ into the system.

Secondly I’ve been trying to explain (hopefully) transforms over at CGTalk. A transform is basically a direction with a spin projected by an offset about a coordinate system. Now this sounds a little complex so over the next few posts i’ll explain them a little bit better hopefully with some interesting examples.