- <registerName> and <len> specify the string name of a program
- register (such as "R3"), input attribute (such as "vertex.color"),
- an output attribute (such as "result.texcoord[0]") or a user-
- defined identifier. The current value of that variable is
- returned as four floats in <v>.
-
+ <registerName> and <len> specify the name a variable, input
+ attribute, temporary, or result register in the program string.
+ The current value of the named variable is returned as four
+ values in <v>. If <name> doesn't exist in the program string,
+ the error INVALID_OPERATION is generated.