point sop to wrangle sop translation

[bonassus]

I'm new trying to learn the basics of using the Wrangle sop. As a test I used an expression from the expression cook book in the Houdini docs. So there is a line and a point sop. In the position fields of the point sop the expressions are:

X: cos(@ptnum *5)

Y:@ptnum/400

Z: sin(@ptnum *5)

this makes a spiral from the line. 

I want to do the same thing with the attribwrangle. So naively i tried:

@P.x = cos(@ptnum *5);

@P.y = @ptnum/400;

@P.z = sin(@ptnum *5);

What this did to the line was interesting. but was not the same as what the point sop did. 

I think if some one could explain how this is done it would help me a bit. 

thanks very much,

b

 

Edited January 9, 2017 by bonassus

[f1480187]

Expressions are always floats. @ptnum is an integer attribute in VEX, even if you omit type (which will be defaulted to f@foo for some random attributes). You should use floats to avoid integer arithmetic. Here is plain and stupid example:

3 / 2 == 1
3.0 / 2.0 == 1.5
3.0 / 2 == 1.5
3 / 2.0 == 1.5
(float) 3 / 2 == 1.5
3 / (float) 2 == 1.5

int_var / int_var == integer_result
int_var / float_var == float_result
float_var / int_var == float_result

 

Edited January 9, 2017 by f1480187

[bonassus]

f489187,

Thank you for your response. I did forget the values types. So I cast the values to float. Now I get something new but still not what the point sop is doing. see pics. 

thanks again

[f1480187]

1. You need to convert operands, not result.

cos(3 / 2) == cos(1)
cos(3.0 / 2) == cos(1.5)

 

2. In that case, it's more like a frequency problem (spiral swirls too much). Expression's sin() expect degrees, VEX's sin() working with radians. You need to convert expression's example to radians, since it was set up to output big values.

cos(radians(3.0 / 2))

 

point_to_wrangle.hipnc

Edited January 9, 2017 by f1480187

[bonassus]

Yes!

Radians not degrees, of course. Thank you for the point in the right direction. 

 

this did the trick.

@P.x = cos(radians(@ptnum *5));

@P.y = (float)@ptnum/400;

@P.z = sin(radians(@ptnum *5));