[Sidefx-houdini-list] Expression syntax for matrices and vectors

Andy Nicholas andy at andynicholas.com
Tue Oct 30 12:48:55 EDT 2007


Haha! Fantastic. Thanks so much for taking the time to type all that out.
That's really cleared a lot of things up for me.

So I take it from the last bit you wrote, you could also do something like
this?:

vector v = vector4(<expr>,<expr>,<expr>,<expr>);

in which case, that solves the question about using the quattomatrix()
function. I'll try it out.

But is it me, or is this whole stripmatrix thing a little obtuse! It'd be
great if they could just add direct member access like they already do for
vectors. I've not starting using H9 yet, so maybe Python supports it.

(BTW, people should check out XSI's maths routines/objects for an example
of a really well structured (and documented) system.)

Thanks again for your help Mario :-)

All the best,

Andy


> On October 30, 2007 10:38 am, Andy Nicholas wrote:
>
>> i.e. this syntax (a trivial example):
>>
>> {
>>    float a = ch("sparevar_a");
>>    float b = ch("sparevar_b");
>>    return a*b;
>> }
>
> OK. That's the "expression language" -- part of "hscript", now replaced
> (or
> "superceded", I guess is a better term) with Python.
>
> Here's one way to extract elements or rows from a matrix using the
> expression
> language (either in-line like you're using it, or in a standalone custom
> expression function):
>
> Extracting the translation 3-vector
> {
> string m = stripmatrix(optransform("/obj/ground"));
> vector T = vector3(arg(m,12),arg(m,13),arg(m,14));
> return T;
> }
>
> A single element from the translation (x in this case)
> {
> string m = stripmatrix(optransform("/obj/ground"));
> vector T = vector3(arg(m,12),arg(m,13),arg(m,14));
> return T[0];
> }
>
> Arbitrary element extraction:
> {
> string m = stripmatrix(optransform("/obj/ground"));
> float m11 = arg(m,0);
> float m12 = arg(m,1);
> float m13 = arg(m,2);
> float m14 = arg(m,3);
> float m21 = arg(m,4);
> float m22 = arg(m,5);
> float m23 = arg(m,6);
> float m24 = arg(m,7);
> float m31 = arg(m,8);
> float m32 = arg(m,9);
> float m33 = arg(m,10);
> float m34 = arg(m,11);
> float m41 = arg(m,12);
> float m42 = arg(m,13);
> float m43 = arg(m,14);
> float m44 = arg(m,15);
> return m41;
> }
>
> And so on. Basically: stripmatrix() converts the usual bracketed syntax
> into a
> space-separated list of components that the arg() function can then
> extract
> elements from.
>
>>
>>
>> Actually, you've just uncovered a bigger confusion for me here. What are
>> expressions? Are they supposed to be HScript? If so, what's the
>> difference?
>
> Expressions are used inside parameters to calculate some value. The list
> of
> functions that can be used in these expressions is given by typing
> "exhelp"
> in the textport. The hscript commands are used to manipulate your houdini
> session via text commands, and the commands available are given when you
> type
> "help" in the textport.
>
>>
>> >>so not clear what you mean.
>>
>> Yes, you're right, I meant to type Matrix and Vector as lowercase. But
>> I've just figured that the only way (that I've found as yet) to create a
>> vector is to do this:
>>
>> vector v = vector("["+unitz_x+","+unitz_y+","+unitz_z+"]");
>
> vector v = vector3(<expr>,<expr>,<expr>);
>
>
> HTH. Cheers!
>
> --
> ......................................................................
> Mario Marengo                                  mailto:mario at axyzfx.com
> AXYZ ANIMATION                                 http://www.axyzfx.com
> ......................................................................
>





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