10.1.3 Calculator Engines

Calculator engines do exactly what it sounds like they do: they calculate things. They don't animate anything per se, but they are useful for controlling the behaviour of other nodes by allowing you to use simple mathematical expressions for transforming the values connected to the input fields into the desired value to be sent to the Calculator engine's output fields, and hence to any nodes connected to those output fields.

The important input fields are:

• a,b,c,d,e,f,g,h: floating-point input values (all initially 0 by default).
• A,B,C,D,E,F,G,H: 3D vector input values (all initially 0 0 0 by default).
• expression: a string representing the mathematical expression to calculate (initially "" by default).

There are also some temporary variables that you can use to store intermediate calculations:

• ta,tb,tc,td,te,tf,tg,th: floating-point temporary variables.
• tA,tB,tC,tD,tE,tF,tG,tH: 3D vector temporary variables.

The important output fields are:

• oa,ob,oc,od: floating-point output values.
• oA,oB,oC,oD: 3D vector output values.

Example

Here is a complete Inventor .iv file for an example that combines Calculator and TimeCounter engines to make a cone slide around a square path. Just cut and paste the text into a file named test.iv and then do ivview -q test.iv:

#Inventor V2.0 ascii

Separator {
    PerspectiveCamera {
        position    50 20 250
    }
    Translation {
        translation 0 0 0 =
	    Calculator {
		a    0 =
		    TimeCounter {
			min          0
			max	     399
			frequency    0.1
		    }
		    . output
		expression [
		    "ta=floor(a/100)",
		    "tb=a - ta*100",
		    "tc=(ta<1)?tb:(ta<2)?99:(ta<3)?(99 - tb):0",
		    "td=(ta<1)?0:(ta<2)?tb:(ta<3)?99:(99 - tb)",
		    "oA=vec3f(tc,0,td)"]
	    } . oA
    }
    Cone {
        bottomRadius 10
        height 20
    }
}

You should note several things about this example:

• This TimeCounter engine counts from 0 to 399 for a total of 400 steps. The square has 4 sides of length 100. This allows us to use the output field to generate a traversal of all four sides, as long as we do a little calculation to determine the current position.
• The a input field of the Calculator engine is connected to the output output field of the TimeCounter engine. This value is used to determine which side (numbered 0, 1, 2, 3) the cone is currently traversing (stored in ta) and the relative position along that side the cone should be (stored in tb). These are used to determine the coordinates of the transformation (stored in tc and td). Finally, the Calculator's output field oA is a vector containing the results of the calculations performed, and will be used to translate (move) the cone to the desired position.
• The expression field can consist of either a single string enclosed in double quotes ("), or a sequence of such strings separated by commas (,) and enclosed in square brackets ([,]).
• The extra white space placed around minus signs (-) may be necessary to avoid hitting a parsing bug in Open Inventor 2.0.
• No output variable (one whose name starts with "o") can appear on the right-hand side of an equation. You can only assign results to output variables. Temporary variables (ones whose name starts with "t") can appear on both the right- and left-hand side of an equation. Input variables can only appear on the right-hand side of an equation.

More Information

More information about Calculator engines is available in chapter 13 of Inventor Mentor. If you are going to use this type of engine, you are strongly advised to read the man page for SoCalculator. It describes the kinds of expressions you are allowed to use.