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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:
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.
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