GLSL 3.30 - OpenGL

4 Variables and Types
If multiple shaders are linked together, then they will share a single global uniform name space, including
within a language as well as across languages. Hence, the types and initializers of uniform variables with
the same name must match across all shaders that are linked into a single program.
It is legal for some shaders to provide an initializer for a particular uniform variable, while another shader
does not, but all provided initializers must be equal.
4.3.6 Outputs
Shader output variables are declared with the out or centroid out storage qualifiers. They form the
output interface between the declaring shader and the subsequent stages of the OpenGL pipeline. Output
variables must be declared at global scope. During shader execution they will behave as normal
unqualified global variables. Their values are copied out to the subsequent pipeline stage on shader exit.
Only output variables that are read by the subsequent pipeline stage need to be written; it is allowed to
have superfluous declarations of output variables.
There is not an inout storage qualifier at global scope for declaring a single variable name as both input
and output to a shader. Output variables must be declared with different names than input variables.
However, nesting an input or output inside an interface block with an instance name allows the same
names with one referenced through a block instance name.
Vertex and geometry output variables output per-vertex data and are declared using the out storage
qualifier, the centroid out storage qualifier, or the deprecated varying storage qualifier. They can only be
float, floating-point vectors, matrices, signed or unsigned integers or integer vectors, or arrays or
structures of any these.
Individual vertex and geometry outputs are declared as in the following examples:
out vec3 normal;
centroid out vec2 TexCoord;
invariant centroid out vec4 Color;
noperspective out float temperature; // varying is deprecated
flat out vec3 myColor;
noperspective centroid out vec2 myTexCoord;
These can also appear in interface blocks, as described in the section 4.3.7 “Interface Blocks”. Interface
blocks allow simpler addition of arrays to the interface from vertex to geometry shader. They also allow a
fragment shader to have the same input interface as a geometry shader for a given vertex shader.
Fragment outputs output per-fragment data and are declared using the out storage qualifier. It is an error
to use centroid out in a fragment shader. Fragment outputs can only be float, floating-point vectors,
signed or unsigned integers or integer vectors, or arrays of any these. Matrices and structures cannot be
output. Fragment outputs are declared as in the following examples:
out vec4 FragmentColor;
out uint Luminosity;
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