Getting Node Being Deformed

[isam]

i hope my question makes sense, but im building a deformer that works upon worldspace coordinates, and it requires that i have a dag path of the object being deformed, so how can i provide the dag path from the deformer node, i suspected from the iterator, but the iterator gets the dag path as input by default(built into the deformer node), so how can i know the input of that iterator?

thanks in advance

[joojaa]

Node design one on one:

Each node may not see anything beyound its code block, if it does you will pay for the consequences.

So how would you do it properly then, well you would simply connect the local space + the world space matrix in. Dont go and fetch it inside maya api functions because then maya wont know what order to evaluate warning you will get slower performance. In fact you shouldnt even attempt to try to fetch the dagpath.

See how the particle softbody connects in. Or how merge polygons works

See just because you have all the functions dont mean theya re meant for all situations.

[petershipkov]

Just override the compute method and use it instead the deform one.

[isam]

joojaa: thnx 4 the response, i canceled out fetching the dag node, but what resources do u recommend for the maya 4*4 transformation matrix, i looked around the documentation, and googled around but came out with nth really valuable, i need something to describe how exactly they work,
(in this case i wont be gettin much into the transformation matrices as ill just do the transformations, and convert thru substitution) but i'm just curious how exactly they work,

pshipkov: thanks 4 ur reply, but performance is an issue here, so i dont want to add up any more unnecessary calculations, thnx a lot tho

[joojaa]

They work just like any other matrix, they just encode the i j k vectors of your now space (and offset to the bases center). As such you just point matrix multiply the point positions with the world matrix attribute to get local space into world space.

Basic calculus. VERY IMPORTANT TO KNOW if you do any 3d.

[petershipkov]

Sorry for the off-topic comment, but ...
I cannot make any difference ( performance wise ) between deform vs compute.
Do you have an example that shows it ?

[joojaa]

all things being equal compute should be lighter than deform, by very very small margin. So suing compute is arguably better.

[isam]

i know how matrices mathematically work, and read about the transformation matrices n all, but i didnt figure this out: "As such you just point matrix multiply the point positions with the world matrix attribute to get local space into world space."

ok got it! if anyone's interested, u place the x y in the last line of the matrix, the rest of the matrix is the identity matrix,, and just multiply matrices

[joojaa]

QUOTE(isam @ 11/25/07, 12:29 PM) [snapback]276946[/snapback]
ok got it! if anyone's interested, u place the x y in the last line of the matrix, the rest of the matrix is the identity matrix,, and just multiply matrices

Please dont do that. A point vector is a 1x4 matrix all ready and works quite correctly in multiplication directly, No need to convert anything to a matrix. (waste of time)

the mPoint (read the docs of MPoint, it also offers the other way around, in wich case its trasposed) class directly multiplies with matrices.

so basically its just.

MPoint*MMatrix

(should be that way i never remember which way is which, usually i think that the notation in matrix math is inverse to natural reading order)

And that is called point matrix multiplication because it quite clearly is point times matrix. Basically the transformation matrix just packs the information of your coordinate system.

theres a related function to this which is

MVector*MMatrix

which does the same, almost (difference is that the last value of the 4 dimensional vector the last value of a point is 1 and a vector is 0 this means MPoint works like your matrix and is fully affected, whereas the Vector does only scale and rotate but does not offset), theres a 3rd class of 3d "vectors" the normal wich you shouldnt try to transform with a matrix it will fail at peculiar moments.

I suggest you read the lore of a TD section, chapter 1.1 basic computer graphics, of this document:

http://www.renderman.org/RMR/Books/sig02.course16.pdf.gz

it covers most of all the maths you NEED to know. And s very well written IMHO. Anyway the site was temporarily down. If it keeps being down i do have a copy of the pdf on my drive, so send me PM and i will upload it somewhere where you can get it from. You may find the document a invaluable aid in thinking.

PS: maya does provide a node that lets you do this too so your node dont necceserily need to do this itself.