Vector Expressions

### 6.1.4 Vector Expressions

POV-Ray often requires you to specify a vector. A vector is a set of related float values. Vectors may be specified using literals, identifiers or functions which return vector values. You may also create very complex vector expressions from combinations of any of these using various familiar operators.

POV-Ray vectors may have from two to five components but the vast majority of vectors have three components. Unless specified otherwise, you should assume that the word "vector" means a three component vector. POV-Ray operates in a 3D x, y, z coordinate system and you will use three component vectors to specify x, y and z values. In some places POV-Ray needs only two coordinates. These are often specified by a 2D vector called an UV vector. Fractal objects use 4D vectors. Color expressions use 5D vectors but allow you to specify 3, 4 or 5 components and use default values for the unspecified components. Unless otherwise noted, all 2, 4 or 5 component vectors work just like 3D vectors but they have a different number of components.

The syntax for combining vector literals into vector expressions is almost identical to the rules for float expressions. In the syntax for vector expressions below, some of the syntax items are defined in the section for float expressions. See "Float Expressions" for those definitions. Detailed explanations of vector-specific issues are given in the following sub-sections.

    VECTOR:
        NUMERIC_TERM [SIGN NUMERIC_TERM]
    NUMERIC_TERM:
        NUMERIC_FACTOR [MULT NUMERIC_FACTOR]
    NUMERIC_FACTOR:
        VECTOR_LITERAL        |
        VECTOR_IDENTIFIER     |
        SIGN NUMERIC_FACTOR   |
        VECTOR_FUNCTION       |
        VECTOR_BUILT-IN_IDENT |
        ( FULL_EXPRESSION )   |
        ! NUMERIC_FACTOR      |
        FLOAT
    VECTOR_LITERAL:
        < FLOAT , FLOAT , FLOAT >
    VECTOR_FUNCTION:
        min_extent ( OBJECT_IDENTIFIER )         |
        max_extent ( OBJECT_IDENTIFIER )         |
        trace(OBJECT_IDENTIFIER, VECTOR, VECTOR, [VECTOR_IDENTIFIER] )|
        vaxis_rotate( VECTOR , VECTOR , FLOAT )  |
        vcross( VECTOR , VECTOR )                |
        vrotate( VECTOR , VECTOR )               |
        vnormalize( VECTOR )                     |
        vturbulence(FLOAT, FLOAT, FLOAT, VECTOR)
    VECTOR_BUILT-IN_IDENT:
         x | y | z | t | u | v

Note: VECTOR_IDENTIFIERS are identifiers previously declared to have vector values.

#### 6.1.4.1 Vector Literals

Vector literals consist of two to five float expressions that are bracketed by angle brackets `<` and `>`. The terms are separated by commas. For example here is a typical three component vector:

     < 1.0, 3.2, -5.4578 >

The commas between components are necessary to keep the program from thinking that the 2nd term is the single float expression `3.2-5.4578` and that there is no 3rd term. If you see an error message such as "Float expected but '>' found instead" then you probably have missed a comma.

Sometimes POV-Ray requires you to specify floats and vectors side-by-side. The rules for vector expressions allow for mixing of vectors with vectors or vectors with floats so commas are required separators whenever an ambiguity might arise. For example `<1,2,3>-4` evaluates as a mixed float and vector expression where 4 is subtracted from each component resulting in `<-3,-2,-1>`. However the comma in `<1,2,3>,-4` means this is a vector followed by a float.

Each component may be a full float expression. For example `<This+3,That/3,5*Other_Thing>` is a valid vector.

#### 6.1.4.2 Vector Identifiers

Vector identifiers may be declared to make scene files more readable and to parameterize scenes so that changing a single declaration changes many values. An identifier is declared as follows.

    VECTOR_DECLARATION:
        #declare IDENTIFIER = EXPRESSION; |
        #local IDENTIFIER = EXPRESSION;

Where IDENTIFIER is the name of the identifier up to 40 characters long and EXPRESSION is any valid expression which evaluates to a vector value.

Note: there should be a semi-colon after the expression in a vector declaration. If omitted, it generates a warning and some macros may not work properly. See " #declare vs. #local" for information on identifier scope.

Here are some examples....

     #declare Here = <1,2,3>;
     #declare There = <3,4,5>;
     #declare Jump = <Foo*2,Bar-1,Bob/3>;
     #declare Route = There-Here;
     #declare Jump = Jump+<1,2,3>;

Note: you invoke a vector identifier by using its name without any angle brackets. As the last example shows, you can re-declare a vector identifier and may use previously declared values in that re-declaration. There are several built-in identifiers which POV-Ray declares for you. See section "Built-in Vector Identifiers" for details.

#### 6.1.4.3 Vector Operators

Vector literals, identifiers and functions may also be combined in expressions the same as float values. Operations are performed on a component-by-component basis. For example `<1,2,3> + <4,5,6>` evaluates the same as `<1+4,2+5,3+6>` or `<5,7,9>`. Other operations are done on a similar component-by-component basis. For example `(<1,2,3> = <3,2,1>)` evaluates to `<0,1,0>` because the middle components are equal but the others are not. Admittedly this isn't very useful but it's consistent with other vector operations.

Conditional expressions such as `(C ? A : B)` require that `C` is a float expression but `A` and `B` may be vector expressions. The result is that the entire conditional evaluates as a valid vector. For example if `Foo` and `Bar` are floats then `(Foo < Bar ? <1,2,3> : <5,6,7>)` evaluates as the vector `<1,2,3>` if `Foo` is less than `Bar` and evaluates as `<5,6,7>` otherwise.

You may use the dot operator to extract a single float component from a vector. Suppose the identifier `Spot` was previously defined as a vector. Then `Spot.x` is a float value that is the first component of this x, y, z vector. Similarly `Spot.y` and `Spot.z` reference the 2nd and 3rd components. If `Spot` was a two component UV vector you could use `Spot.u` and `Spot.v` to extract the first and second component. For a 4D vector use `.x`, `.y`, `.z`, and `.t` to extract each float component. The dot operator is also used in color expressions which are covered later.

#### 6.1.4.4 Operator Promotion

You may use a lone float expression to define a vector whose components are all the same. POV-Ray knows when it needs a vector of a particular type and will promote a float into a vector if need be. For example the POV-Ray `scale` statement requires a three component vector. If you specify `scale 5` then POV-Ray interprets this as `scale <5,5,5>` which means you want to scale by 5 in every direction.

Versions of POV-Ray prior to 3.0 only allowed such use of a float as a vector in various limited places such as `scale` and `turbulence`. However you may now use this trick anywhere. For example...

     box{0,1}  // Same as box{<0,0,0>,<1,1,1>}
     sphere{0,1} // Same as sphere{<0,0,0>,1}

When promoting a float into a vector of 2, 3, 4 or 5 components, all components are set to the float value, however when promoting a vector of a lower number of components into a higher order vector, all remaining components are set to zero. For example if POV-Ray expects a 4D vector and you specify `9` the result is `<9,9,9,9>` but if you specify `<7,6>` the result is `<7,6,0,0>`.

#### 6.1.4.5 Built-in Vector Identifiers

There are several built-in vector identifiers. You can use them to specify values or to create expressions but you cannot re-declare them to change their values. They are:

    VECTOR_BUILT-IN_IDENT:
        x | y | z | t | u | v

All built-in vector identifiers never change value. They are defined as though the following lines were at the start of every scene.

     #declare x = <1, 0, 0>;
     #declare y = <0, 1, 0>;
     #declare z = <0, 0, 1>;
     #declare t = <0, 0, 0, 1>;
     #declare u = <1, 0>;
     #declare v = <0, 1>;

The built-in vector identifiers `x`, `y`, and `z` provide much greater readability for your scene files when used in vector expressions. For example....

     plane { y, 1}    // The normal vector is obviously "y".
     plane { <0,1,0>, 1} // This is harder to read.
     translate 5*x    // Move 5 units in the "x" direction.
     translate <5,0,0>  // This is less obvious.

An expression like `5*x` evaluates to `5*<1,0,0>` or `<5,0,0>`.

Similarly `u` and `v` may be used in 2D vectors. When using 4D vectors you should use `x`, `y`, `z`, and `t` and POV-Ray will promote `x`, `y`, and `z` to 4D when used where 4D is required.

#### 6.1.4.6 Vector Functions

POV-Ray defines a variety of built-in functions for manipulating floats, vectors and strings. Function calls consist of a keyword which specifies the name of the function followed by a parameter list enclosed in parentheses. Parameters are separated by commas. For example:

     keyword(param1,param2)

The following are the functions which return vector values. They take one or more float, integer, vector, or string parameters. Assume that `A` and `B` are any valid expression that evaluates to a vector; and `F` is any float expression.

`min_extent ( OBJECT_IDENTIFIER ), max_extent ( OBJECT_IDENTIFIER )`. The `min_extent` and `max_extent` return the minimum and maximum coordinates of a #declared object's bounding box (Corner1 and Corner2), in effect allowing you to find the dimensions and location of the object.

Note: this is not perfect, in some cases (such as CSG intersections and differences or isosurfaces) the bounding box does not represent the actual dimensions of the object.

Example:

    #declare Sphere =
    sphere {
      <0,0,0>, 1
      pigment { rgb <1,0,0> }
    }
    #declare Min = min_extent ( Sphere );
    #declare Max = max_extent ( Sphere );
    object { Sphere }
    box {
        Min, Max
        pigment { rgbf <1,1,1,0.5> }
    }

`trace(OBJECT_IDENTIFIER, A, B, [VECTOR_IDENTIFIER])`. `trace` helps you finding the exact location of a ray intersecting with an object's surface. It traces a ray beginning at the point `A` in the direction specified by the vector `B`. If the ray hits the specified object, this function returns the coordinate where the ray intersected the object. If not, it returns `<0,0,0>`. If a fourth parameter in the form of a vector identifier is provided, the normal of the object at the intersection point (not including any normal perturbations due to textures) is stored into that vector. If no intersection was found, the normal vector is reset to `<0,0,0>`.

Note: Checking the normal vector for `<0,0,0>` is the only reliable way to determine whether an intersection has actually occurred, intersections can and do occur anywhere, including at `<0,0,0>`.

Example:

    #declare MySphere = sphere { <0, 0, 0>, 1 }
    #declare Norm = <0, 0, 0>;
    #declare Start = <1, 1, 1>;
    #declare Inter=
      trace ( MySphere, Start, <0, 0, 0>-Start, Norm );
      object {
        MySphere
        texture {
          pigment { rgb 1}
        }
      }
      #if (vlength(Norm)!=0)
      cylinder {
        Inter, Inter+Norm, .1
        texture {
          pigment {color red 1}
        }
      }
     #end

`vaxis_rotate(A,B,F)` Rotate `A` about `B` by `F`. Given the x,y,z coordinates of a point in space designated by the vector `A`, rotate that point about an arbitrary axis defined by the vector `B`. Rotate it through an angle specified in degrees by the float value `F`. The result is a vector containing the new x,y,z coordinates of the point.

`vcross(A,B)` Cross product of `A` and `B`. Returns a vector that is the vector cross product of the two vectors. The resulting vector is perpendicular to the two original vectors and its length is equal to the area of the parallelogram defined by them. Or to put in an other way, the cross product can also be formulated as:`AxB = |A| * |B| * sin(angle(A,B)) * perpendicular_unit_vector(A,B)` So the length of the resulting vector is proportional to the sine of the angle between `A` and `B`. See the animated demo scene `VECT2.POV` for an illustration.

`vnormalize(A)` Normalize vector `A`. Returns a unit length vector that is the same direction as `A`. Formula is vnormalize(A)=A/vlength(A).

Note: `vnormalize(<0,0,0>)` will result in an error.

`vrotate(A,B)` Rotate `A` about origin by `B`. Given the x,y,z coordinates of a point in space designated by the vector `A`, rotate that point about the origin by an amount specified by the vector `B`. Rotate it about the x-axis by an angle specified in degrees by the float value `B.x`. Similarly `B.y` and `B.z` specify the amount to rotate in degrees about the y-axis and z-axis. The result is a vector containing the new x,y,z coordinates of the point.

`vturbulence(Lambda, Omega, Octaves, A)` Turbulence vector at A. Given the x,y,z coordinates of a point in space designated by the vector A, return the turbulence vector for that point based on the numbers given for Lambda, Omega and Octaves. For the meaning of the parameters, check out the Lambda, Omega and Octaves sections.
The amount of turbulence can be controlled by multiplying the turbulence vector by a multiple. The frequency at which the turbulence vector changes can be controlled by multiplying A with a multiple. The turbulence vector returned by the function can be added to the original point A to obtain a turbulated version of the point A. Example :
`#declare MyVector = MyVector + Amount*vturbulence( 2, 0.5, 6,MyVector*Frequency );`

See section "Float Functions" for other functions which are somewhat vector-related but which return floats. In addition to the above built-in functions, you may also define your own functions using the `#macro` directive. See the section "User Defined Macros" for more details.