GLSLANG Spec 4.0 - OpenGL

4 Variables and Types
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, tessellation evaluation, and geometry output variables output per-vertex data and are declared
using the out, centroid out, or sample out storage qualifiers, or the deprecated varying storage qualifier.
It is an error to use patch out in a vertex, tessellation evaluation, or geometry shader. Output variables
can only be floating-point scalars, floating-point vectors, matrices, signed or unsigned integers or integer
vectors, or arrays or structures of any these.
Individual vertex, tessellation evaluation, 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;
sample out vec4 perSampleColor;
These can also appear in interface blocks, as described in 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.
Tessellation control shader output variables are may be used to output per-vertex and per-patch data. Pervertex
output variables are arrayed (see arrayed under 4.3.4 Inputs) and declared using out or centroid
out storage qualifiers. Per-patch output variables are declared using the patch out storage qualifier. Pervertex
and per-patch output variables can only be floating-point scalars, floating-point vectors, matrices,
signed or unsigned integers or integer vectors, or arrays or structures of any these. Since tessellation
control shaders produce an arrayed primitive comprising multiple vertices, each per-vertex output variable
(or output block, see interface blocks below) needs to be declared as an array. For example,
out float foo[]; // feeds next stage input “in float foo[]”
Each element of such an array corresponds to one vertex of the primitive being produced. Each array can
optionally have a size declared. The array size will be set by (or if provided must be consistent with) the
output layout declaration(s) establishing the number of vertices in the output patch, as described later in
section 4.3.8.2 “Output Layout Qualifiers”.
As described under the section 4.3.4 “Inputs” above, if a per-vertex output of the tessellation control
shader is itself an array with multiple values per vertex, it must appear in an output block (see interface
blocks below) in the tessellation control shader with a block instance name declared as an array.
Each tessellation control shader invocation has a corresponding output patch vertex, and may assign
values to per-vertex outputs only if they belong to that corresponding vertex. If a per-vertex output
variable is used as an l-value, it is an error if the expression indicating the vertex index is not the identifier
gl_InvocationID.
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