OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Storage Qualifiers

Types can also have modifiers that affect their behavior. There are four modifiers defined in GLSL, as shown in Table 2.5, with their behaviors at global scope.

Table 2.5. GLSL Type Modifiers

const Storage Qualifier

Just as with “C”, const type modifier indicates that the variable is read-only. For example, the statement

const float Pi = 3.141529;

sets the variable Pi to an approximation of π. With the addition of the const modifier, it becomes an error to write to a variable after its declaration, so they must be initialized when declared.

in Storage Qualifier

The in modifier is used to qualify inputs into a shader stage. ...