GLSL Spec

4 Variables and Types
evaluation shaders. As with other input variables, per-patch inputs must be declared using the same type
and qualification as per-patch outputs from the previous (tessellation control) shader stage.
Fragment shader inputs get per-fragment values, typically interpolated from a previous stage's outputs.
They are declared in fragment shaders with the in storage qualifier. The auxiliary storage qualifiers
centroid and sample can also be applied, as well as the interpolation qualifiers flat, noperspective, and
smooth. It is an error to use patch in a fragment shader. Fragment inputs can only be signed and
unsigned integers and integer vectors, floating point scalars, floating-point vectors, matrices, or arrays or
structures of these. Fragment shader inputs that are signed or unsigned integers, integer vectors, or any
double-precision floating-point type must be qualified with the interpolation qualifier flat.
Fragment inputs are declared as in the following examples:
in vec3 normal;
centroid in vec2 TexCoord;
invariant centroid in vec4 Color;
noperspective in float temperature;
flat in vec3 myColor;
noperspective centroid in vec2 myTexCoord;
The fragment shader inputs form an interface with the last active shader in the vertex processing pipeline.
For this interface, the last active shader stage output variables and fragment shader input variables of the
same name must match in type and qualification (other than out matching to in).
When an interface between shader stages is formed using shaders from two separate program objects, it is
not possible to detect mismatches between inputs and outputs when the programs are linked. When there
are mismatches between inputs and outputs on such interfaces, the values passed across the interface will
be partially or completely undefined. Shaders can ensure matches across such interfaces either by using
input and output layout qualifiers (sections 4.4.1 “Input Layout Qualifiers” and 4.4.2 “Output Layout
Qualifiers”) or by using identical input and output declarations of blocks or variables. Complete rules for
interface matching between programs are found in the "Shader Interface Matching" portion of section
2.14.PPO of the OpenGL Graphics System Specification.
4.3.5 Uniform
The uniform qualifier is used to declare global variables whose values are the same across the entire
primitive being processed. All uniform variables are read-only and are initialized externally either at link
time or through the API. The link time initial value is either the value of the variable's initializer, if
present, or 0 if no initializer is present. Opaque types cannot have initializers.
Example declarations are:
uniform vec4 lightPosition;
uniform vec3 color = vec3(0.7, 0.7, 0.2); // value assigned at link time
The uniform qualifier can be used with any of the basic data types, or when declaring a variable whose
type is a structure, or an array of any of these.
There is an implementation dependent limit on the amount of storage for uniforms that can be used for
each type of shader and if this is exceeded it will cause a compile-time or link-time error. Uniform
variables that are declared but not used do not count against this limit. The number of user-defined
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